(c) The University of Glasgow 2006
(c) The GRASP/AQUA Project, Glasgow University, 1998
\section[TyCoRep]{Type and Coercion - friends' interface}

Note [The Type-related module hierarchy]
  TyCon    imports Class, CoAxiom
  TyCoRep  imports Class, CoAxiom, TyCon
  TysPrim  imports TyCoRep ( including mkTyConTy )
  Kind     imports TysPrim ( mainly for primitive kinds )
  Type     imports Kind
  Coercion imports Type

-- We expose the relevant stuff from this module via the Type module
{-# OPTIONS_HADDOCK hide #-}
{-# LANGUAGE CPP, DeriveDataTypeable, DeriveFunctor, DeriveFoldable,
             DeriveTraversable, MultiWayIf #-}
{-# LANGUAGE ImplicitParams #-}

module TyCoRep (
        KindOrType, Kind,
        PredType, ThetaType,      -- Synonyms

        -- Coercions
        Coercion(..), LeftOrRight(..),
        UnivCoProvenance(..), CoercionHole(..),
        CoercionN, CoercionR, CoercionP, KindCoercion,

        -- Functions over types
        mkTyConTy, mkTyVarTy, mkTyVarTys,
        mkFunTy, mkFunTys, mkForAllTys,
        isLiftedTypeKind, isUnliftedTypeKind,
        isCoercionType, isRuntimeRepTy, isRuntimeRepVar,
        isRuntimeRepKindedTy, dropRuntimeRepArgs,

        -- Functions over binders
        binderType, delBinderVar, isInvisibleBinder, isVisibleBinder,
        isNamedBinder, isAnonBinder, delBinderVarFV,

        -- Functions over coercions

        -- Pretty-printing
        pprType, pprParendType, pprTypeApp, pprTvBndr, pprTvBndrs,
        pprTyThing, pprTyThingCategory, pprSigmaType,
        pprTheta, pprForAll, pprForAllImplicit, pprUserForAll,
        pprThetaArrowTy, pprClassPred,
        pprKind, pprParendKind, pprTyLit,
        TyPrec(..), maybeParen, pprTcAppCo, pprTcAppTy,
        pprPrefixApp, pprArrowChain, ppr_type,

        -- * Free variables
        tyCoVarsOfType, tyCoVarsOfTypeDSet, tyCoVarsOfTypes, tyCoVarsOfTypesDSet,
        tyCoFVsBndr, tyCoFVsOfType, tyCoVarsOfTypeList,
        tyCoFVsOfTypes, tyCoVarsOfTypesList,
        closeOverKindsDSet, closeOverKindsFV, closeOverKindsList,
        coVarsOfType, coVarsOfTypes,
        coVarsOfCo, coVarsOfCos,
        tyCoVarsOfCo, tyCoVarsOfCos,
        tyCoFVsOfCo, tyCoFVsOfCos,
        tyCoVarsOfCoList, tyCoVarsOfProv,

        -- * Substitutions
        TCvSubst(..), TvSubstEnv, CvSubstEnv,
        emptyTvSubstEnv, emptyCvSubstEnv, composeTCvSubstEnv, composeTCvSubst,
        emptyTCvSubst, mkEmptyTCvSubst, isEmptyTCvSubst,
        mkTCvSubst, mkTvSubst,
        getCvSubstEnv, getTCvInScope, getTCvSubstRangeFVs,
        isInScope, notElemTCvSubst,
        setTvSubstEnv, setCvSubstEnv, zapTCvSubst,
        extendTCvInScope, extendTCvInScopeList, extendTCvInScopeSet,
        extendCvSubst, extendCvSubstWithClone,
        extendTvSubst, extendTvSubstWithClone,
        extendTvSubstList, extendTvSubstAndInScope,
        unionTCvSubst, zipTyEnv, zipCoEnv, mkTyCoInScopeSet,
        zipTvSubst, zipCvSubst,

        substTyWith, substTyWithCoVars, substTysWith, substTysWithCoVars,
        substTy, substTyAddInScope,
        substTyUnchecked, substTysUnchecked, substThetaUnchecked,
        substTyWithBindersUnchecked, substTyWithUnchecked,
        substCoUnchecked, substCoWithUnchecked,
        substTyWithBinders, substTyWithInScope,
        substTys, substTheta,
        lookupTyVar, substTyVarBndr,
        substCo, substCos, substCoVar, substCoVars, lookupCoVar,
        substCoVarBndr, cloneTyVarBndr, cloneTyVarBndrs,
        substTyVar, substTyVars,
        substTyVarBndrCallback, substForAllCoBndrCallback,

        -- * Tidying type related things up for printing
        tidyType,      tidyTypes,
        tidyOpenType,  tidyOpenTypes,
        tidyTyCoVarBndr, tidyTyCoVarBndrs, tidyFreeTyCoVars,
        tidyOpenTyCoVar, tidyOpenTyCoVars,
        tidyCo, tidyCos,
        tidyTyBinder, tidyTyBinders
    ) where

#include "HsVersions.h"

import {-# SOURCE #-} DataCon( dataConTyCon, dataConFullSig
                              , dataConUnivTyBinders, dataConExTyBinders
                              , DataCon, filterEqSpec )
import {-# SOURCE #-} Type( isPredTy, isCoercionTy, mkAppTy
                          , tyCoVarsOfTypesWellScoped
                          , partitionInvisibles, coreView, typeKind
                          , eqType )
   -- Transitively pulls in a LOT of stuff, better to break the loop

import {-# SOURCE #-} Coercion
import {-# SOURCE #-} ConLike ( ConLike(..) )
import {-# SOURCE #-} TysWiredIn ( ptrRepLiftedTy )

-- friends:
import Var
import VarEnv
import VarSet
import Name hiding ( varName )
import BasicTypes
import TyCon
import Class
import CoAxiom
import FV

-- others
import PrelNames
import Binary
import Outputable
import DynFlags
import StaticFlags ( opt_PprStyle_Debug )
import FastString
import Pair
import UniqSupply
import Util
import UniqFM

-- libraries
import qualified Data.Data as Data hiding ( TyCon )
import Data.List
import Data.IORef ( IORef )   -- for CoercionHole
#if __GLASGOW_HASKELL__ > 710
import GHC.Stack (CallStack)

%*                                                                      *
\subsection{The data type}
%*                                                                      *

-- | The key representation of types within the compiler

-- If you edit this type, you may need to update the GHC formalism
-- See Note [GHC Formalism] in coreSyn/CoreLint.hs
data Type
  -- See Note [Non-trivial definitional equality]
  = TyVarTy Var -- ^ Vanilla type or kind variable (*never* a coercion variable)

  | AppTy         -- See Note [AppTy rep]
        Type            -- ^ Type application to something other than a 'TyCon'. Parameters:
                        --  1) Function: must /not/ be a 'TyConApp',
                        --     must be another 'AppTy', or 'TyVarTy'
                        --  2) Argument type

  | TyConApp      -- See Note [AppTy rep]
        [KindOrType]    -- ^ Application of a 'TyCon', including newtypes /and/ synonyms.
                        -- Invariant: saturated applications of 'FunTyCon' must
                        -- use 'FunTy' and saturated synonyms must use their own
                        -- constructors. However, /unsaturated/ 'FunTyCon's
                        -- do appear as 'TyConApp's.
                        -- Parameters:
                        -- 1) Type constructor being applied to.
                        -- 2) Type arguments. Might not have enough type arguments
                        --    here to saturate the constructor.
                        --    Even type synonyms are not necessarily saturated;
                        --    for example unsaturated type synonyms
                        --    can appear as the right hand side of a type synonym.

  | ForAllTy
        Type            -- ^ A Π type.
                        -- This includes arrow types, constructed with
                        -- @ForAllTy (Anon ...)@. See also Note [TyBinder].

  | LitTy TyLit     -- ^ Type literals are similar to type constructors.

  | CastTy
        KindCoercion  -- ^ A kind cast. The coercion is always nominal.
                      -- INVARIANT: The cast is never refl.
                      -- INVARIANT: The cast is "pushed down" as far as it
                      -- can go. See Note [Pushing down casts]

  | CoercionTy
        Coercion    -- ^ Injection of a Coercion into a type
                    -- This should only ever be used in the RHS of an AppTy,
                    -- in the list of a TyConApp, when applying a promoted
                    -- GADT data constructor

  deriving (Data.Data, Data.Typeable)

-- NOTE:  Other parts of the code assume that type literals do not contain
-- types or type variables.
data TyLit
  = NumTyLit Integer
  | StrTyLit FastString
  deriving (Eq, Ord, Data.Data, Data.Typeable)

-- | A 'TyBinder' represents an argument to a function. TyBinders can be dependent
-- ('Named') or nondependent ('Anon'). They may also be visible or not.
-- See also Note [TyBinder]
data TyBinder
  = Named TyVar VisibilityFlag  -- Always a TyVar (not CoVar or Id)
  | Anon Type   -- Visibility is determined by the type (Constraint vs. *)
    deriving (Data.Typeable, Data.Data)

-- | Is something required to appear in source Haskell ('Visible'),
-- permitted by request ('Specified') (visible type application), or
-- prohibited entirely from appearing in source Haskell ('Invisible')?
-- Examples in Note [VisibilityFlag]
data VisibilityFlag = Visible | Specified | Invisible
  deriving (Eq, Data.Typeable, Data.Data)

-- | Do these denote the same level of visibility? Except that
-- 'Specified' and 'Invisible' are considered the same. Used
-- for printing.
sameVis :: VisibilityFlag -> VisibilityFlag -> Bool
sameVis Visible Visible = True
sameVis Visible _       = False
sameVis _       Visible = False
sameVis _       _       = True

instance Binary VisibilityFlag where
  put_ bh Visible   = putByte bh 0
  put_ bh Specified = putByte bh 1
  put_ bh Invisible = putByte bh 2

  get bh = do
    h <- getByte bh
    case h of
      0 -> return Visible
      1 -> return Specified
      _ -> return Invisible

type KindOrType = Type -- See Note [Arguments to type constructors]

-- | The key type representing kinds in the compiler.
type Kind = Type

Note [TyBinder]
This represents the type of binders -- that is, the type of an argument
to a Pi-type. GHC Core currently supports two different Pi-types:
a non-dependent function, written with ->, and a dependent compile-time-only
polytype, written with forall. Both Pi-types classify terms/types that
take an argument. In other words, if `x` is either a function or a polytype,
`x arg` makes sense (for an appropriate `arg`). It is thus often convenient
to group Pi-types together. This is ForAllTy.

The two constructors for TyBinder sort out the two different possibilities.
`Named` builds a polytype, while `Anon` builds an ordinary function.
(ForAllTy (Anon arg) res used to be called FunTy arg res.)

Note [The kind invariant]
The kinds
   #          UnliftedTypeKind
   OpenKind   super-kind of *, #

can never appear under an arrow or type constructor in a kind; they
can only be at the top level of a kind.  It follows that primitive TyCons,
which have a naughty pseudo-kind
   State# :: * -> #
must always be saturated, so that we can never get a type whose kind
has a UnliftedTypeKind or ArgTypeKind underneath an arrow.

Nor can we abstract over a type variable with any of these kinds.

    k :: = kk | # | ArgKind | (#) | OpenKind
    kk :: = * | kk -> kk | T kk1 ... kkn

So a type variable can only be abstracted kk.

Note [AppTy rep]
Types of the form 'f a' must be of kind *, not #, so we are guaranteed
that they are represented by pointers.  The reason is that f must have
kind (kk -> kk) and kk cannot be unlifted; see Note [The kind invariant]
in TyCoRep.

Note [Arguments to type constructors]
Because of kind polymorphism, in addition to type application we now
have kind instantiation. We reuse the same notations to do so.

For example:

  Just (* -> *) Maybe
  Right * Nat Zero

are represented by:

  TyConApp (PromotedDataCon Just) [* -> *, Maybe]
  TyConApp (PromotedDataCon Right) [*, Nat, (PromotedDataCon Zero)]

Important note: Nat is used as a *kind* and not as a type. This can be
confusing, since type-level Nat and kind-level Nat are identical. We
use the kind of (PromotedDataCon Right) to know if its arguments are
kinds or types.

This kind instantiation only happens in TyConApp currently.

Note [Pushing down casts]
Suppose we have (a :: k1 -> *), (b :: k1), and (co :: * ~ q).
The type (a b |> co) is `eqType` to ((a |> co') b), where
co' = (->) <k1> co. Thus, to make this visible to functions
that inspect types, we always push down coercions, preferring
the second form. Note that this also applies to TyConApps!

Note [Non-trivial definitional equality]
Is Int |> <*> the same as Int? YES! In order to reduce headaches,
we decide that any reflexive casts in types are just ignored. More
generally, the `eqType` function, which defines Core's type equality
relation, ignores casts and coercion arguments, as long as the
two types have the same kind. This allows us to be a little sloppier
in keeping track of coercions, which is a good thing. It also means
that eqType does not depend on eqCoercion, which is also a good thing.

Why is this sensible? That is, why is something different than α-equivalence
appropriate for the implementation of eqType?

Anything smaller than ~ and homogeneous is an appropriate definition for
equality. The type safety of FC depends only on ~. Let's say η : τ ~ σ. Any
expression of type τ can be transmuted to one of type σ at any point by
casting. The same is true of types of type τ. So in some sense, τ and σ are

But let's be more precise. If we examine the typing rules of FC (say, those in
there are several places where the same metavariable is used in two different
premises to a rule. (For example, see Ty_App.) There is an implicit equality
check here. What definition of equality should we use? By convention, we use
α-equivalence. Take any rule with one (or more) of these implicit equality
checks. Then there is an admissible rule that uses ~ instead of the implicit
check, adding in casts as appropriate.

The only problem here is that ~ is heterogeneous. To make the kinds work out
in the admissible rule that uses ~, it is necessary to homogenize the
coercions. That is, if we have η : (τ : κ1) ~ (σ : κ2), then we don't use η;
we use η |> kind η, which is homogeneous.

The effect of this all is that eqType, the implementation of the implicit
equality check, can use any homogeneous relation that is smaller than ~, as
those rules must also be admissible.

What would go wrong if we insisted on the casts matching? See the beginning of
Section 8 in the unpublished paper above. Theoretically, nothing at all goes
wrong. But in practical terms, getting the coercions right proved to be
nightmarish. And types would explode: during kind-checking, we often produce
reflexive kind coercions. When we try to cast by these, mkCastTy just discards
them. But if we used an eqType that distinguished between Int and Int |> <*>,
then we couldn't discard -- the output of kind-checking would be enormous,
and we would need enormous casts with lots of CoherenceCo's to straighten
them out.

Would anything go wrong if eqType respected type families? No, not at all. But
that makes eqType rather hard to implement.

Thus, the guideline for eqType is that it should be the largest
easy-to-implement relation that is still smaller than ~ and homogeneous. The
precise choice of relation is somewhat incidental, as long as the smart
constructors and destructors in Type respect whatever relation is chosen.

Another helpful principle with eqType is this:

 ** If (t1 eqType t2) then I can replace t1 by t2 anywhere. **

This principle also tells us that eqType must relate only types with the
same kinds.

Note [VisibilityFlag]
All named binders are equipped with a visibility flag, which says
whether or not arguments for this binder should be visible (explicit)
in source Haskell. Historically, all named binders (that is, polytype
binders) have been Invisible. But now it's more complicated.

 Argument does not ever appear in source Haskell. With visible type
 application, only GHC-generated polytypes have Invisible binders.
 This exactly corresponds to "generalized" variables from the
 Visible Type Applications paper (ESOP'16).

 Example: f x = x
 `f` will be inferred to have type `forall a. a -> a`, where `a` is
 Invisible. Note that there is no type annotation for `f`.

 Printing: With -fprint-explicit-foralls, Invisible binders are written
 in braces. Otherwise, they are printed like Specified binders.

 The argument for this binder may appear in source Haskell only with
 visible type application. Otherwise, it is omitted.

 Example: id :: forall a. a -> a
 `a` is a Specified binder, because you can say `id @Int` in source Haskell.

 Example: const :: a -> b -> a
 Both `a` and `b` are Specified binders, even though they are not bound
 by an explicit forall.

 Printing: a list of Specified binders are put between `forall` and `.`:
 const :: forall a b. a -> b -> a

 The argument must be given. Visible binders come up only with TypeInType.

 Example: data Proxy k (a :: k) = P
 The kind of Proxy is forall k -> k -> *, where k is a Visible binder.

 Printing: As in the example above, Visible binders are put between `forall`
 and `->`. This syntax is not parsed (yet), however.

                Note [PredTy]

-- | A type of the form @p@ of kind @Constraint@ represents a value whose type is
-- the Haskell predicate @p@, where a predicate is what occurs before
-- the @=>@ in a Haskell type.
-- We use 'PredType' as documentation to mark those types that we guarantee to have
-- this kind.
-- It can be expanded into its representation, but:
-- * The type checker must treat it as opaque
-- * The rest of the compiler treats it as transparent
-- Consider these examples:
-- > f :: (Eq a) => a -> Int
-- > g :: (?x :: Int -> Int) => a -> Int
-- > h :: (r\l) => {r} => {l::Int | r}
-- Here the @Eq a@ and @?x :: Int -> Int@ and @r\l@ are all called \"predicates\"
type PredType = Type

-- | A collection of 'PredType's
type ThetaType = [PredType]

(We don't support TREX records yet, but the setup is designed
to expand to allow them.)

A Haskell qualified type, such as that for f,g,h above, is
represented using
        * a FunTy for the double arrow
        * with a type of kind Constraint as the function argument

The predicate really does turn into a real extra argument to the
function.  If the argument has type (p :: Constraint) then the predicate p is
represented by evidence of type p.

%*                                                                      *
            Simple constructors
%*                                                                      *

These functions are here so that they can be used by TysPrim,
which in turn is imported by Type

-- named with "Only" to prevent naive use of mkTyVarTy
mkTyVarTy  :: TyVar   -> Type
mkTyVarTy v = ASSERT2( isTyVar v, ppr v <+> dcolon <+> ppr (tyVarKind v) )
                  TyVarTy v

mkTyVarTys :: [TyVar] -> [Type]
mkTyVarTys = map mkTyVarTy -- a common use of mkTyVarTy

infixr 3 `mkFunTy`      -- Associates to the right
-- | Make an arrow type
mkFunTy :: Type -> Type -> Type
mkFunTy arg res = ForAllTy (Anon arg) res

-- | Make nested arrow types
mkFunTys :: [Type] -> Type -> Type
mkFunTys tys ty = foldr mkFunTy ty tys

-- | Wraps foralls over the type using the provided 'TyVar's from left to right
mkForAllTys :: [TyBinder] -> Type -> Type
mkForAllTys tyvars ty = foldr ForAllTy ty tyvars

-- | Does this type classify a core (unlifted) Coercion?
-- At either role nominal or reprsentational
--    (t1 ~# t2) or (t1 ~R# t2)
isCoercionType :: Type -> Bool
isCoercionType (TyConApp tc tys)
  | (tc `hasKey` eqPrimTyConKey) || (tc `hasKey` eqReprPrimTyConKey)
  , length tys == 4
  = True
isCoercionType _ = False

binderType :: TyBinder -> Type
binderType (Named v _) = varType v
binderType (Anon ty)   = ty

-- | Remove the binder's variable from the set, if the binder has
-- a variable.
delBinderVar :: VarSet -> TyBinder -> VarSet
delBinderVar vars (Named tv _) = vars `delVarSet` tv
delBinderVar vars (Anon {})    = vars

-- | Remove the binder's variable from the set, if the binder has
-- a variable.
delBinderVarFV :: TyBinder -> FV -> FV
delBinderVarFV (Named tv _) vars fv_cand in_scope acc = delFV tv vars fv_cand in_scope acc
delBinderVarFV (Anon {})    vars fv_cand in_scope acc = vars fv_cand in_scope acc

-- | Does this binder bind an invisible argument?
isInvisibleBinder :: TyBinder -> Bool
isInvisibleBinder (Named _ vis) = vis /= Visible
isInvisibleBinder (Anon ty)     = isPredTy ty

-- | Does this binder bind a visible argument?
isVisibleBinder :: TyBinder -> Bool
isVisibleBinder = not . isInvisibleBinder

isNamedBinder :: TyBinder -> Bool
isNamedBinder (Named {}) = True
isNamedBinder _          = False

isAnonBinder :: TyBinder -> Bool
isAnonBinder (Anon {}) = True
isAnonBinder _         = False

-- | Create the plain type constructor type which has been applied to no type arguments at all.
mkTyConTy :: TyCon -> Type
mkTyConTy tycon = TyConApp tycon []

Some basic functions, put here to break loops eg with the pretty printer

-- | This version considers Constraint to be distinct from *.
isLiftedTypeKind :: Kind -> Bool
isLiftedTypeKind ki | Just ki' <- coreView ki = isLiftedTypeKind ki'
isLiftedTypeKind (TyConApp tc [TyConApp ptr_rep []])
  =  tc      `hasKey` tYPETyConKey
  && ptr_rep `hasKey` ptrRepLiftedDataConKey
isLiftedTypeKind _                = False

isUnliftedTypeKind :: Kind -> Bool
isUnliftedTypeKind ki | Just ki' <- coreView ki = isUnliftedTypeKind ki'
isUnliftedTypeKind (TyConApp tc [TyConApp ptr_rep []])
  | tc       `hasKey` tYPETyConKey
  , ptr_rep  `hasKey` ptrRepLiftedDataConKey
  = False
isUnliftedTypeKind (TyConApp tc [arg])
  = tc `hasKey` tYPETyConKey && isEmptyVarSet (tyCoVarsOfType arg)
      -- all other possibilities are unlifted
isUnliftedTypeKind _ = False

-- | Is this the type 'RuntimeRep'?
isRuntimeRepTy :: Type -> Bool
isRuntimeRepTy ty | Just ty' <- coreView ty = isRuntimeRepTy ty'
isRuntimeRepTy (TyConApp tc []) = tc `hasKey` runtimeRepTyConKey
isRuntimeRepTy _ = False

-- | Is this a type of kind RuntimeRep? (e.g. PtrRep)
isRuntimeRepKindedTy :: Type -> Bool
isRuntimeRepKindedTy = isRuntimeRepTy . typeKind

-- | Is a tyvar of type 'RuntimeRep'?
isRuntimeRepVar :: TyVar -> Bool
isRuntimeRepVar = isRuntimeRepTy . tyVarKind

-- | Drops prefix of RuntimeRep constructors in 'TyConApp's. Useful for e.g.
-- dropping 'PtrRep arguments of unboxed tuple TyCon applications:
--   dropRuntimeRepArgs [ 'PtrRepLifted, 'PtrRepUnlifted
--                      , String, Int# ] == [String, Int#]
dropRuntimeRepArgs :: [Type] -> [Type]
dropRuntimeRepArgs = dropWhile isRuntimeRepKindedTy

%*                                                                      *
%*                                                                      *

-- | A 'Coercion' is concrete evidence of the equality/convertibility
-- of two types.

-- If you edit this type, you may need to update the GHC formalism
-- See Note [GHC Formalism] in coreSyn/CoreLint.hs
data Coercion
  -- Each constructor has a "role signature", indicating the way roles are
  -- propagated through coercions.
  --    -  P, N, and R stand for coercions of the given role
  --    -  e stands for a coercion of a specific unknown role
  --           (think "role polymorphism")
  --    -  "e" stands for an explicit role parameter indicating role e.
  --    -   _ stands for a parameter that is not a Role or Coercion.

  -- These ones mirror the shape of types
  = -- Refl :: "e" -> _ -> e
    Refl Role Type  -- See Note [Refl invariant]
          -- Invariant: applications of (Refl T) to a bunch of identity coercions
          --            always show up as Refl.
          -- For example  (Refl T) (Refl a) (Refl b) shows up as (Refl (T a b)).

          -- Applications of (Refl T) to some coercions, at least one of
          -- which is NOT the identity, show up as TyConAppCo.
          -- (They may not be fully saturated however.)
          -- ConAppCo coercions (like all coercions other than Refl)
          -- are NEVER the identity.

          -- Use (Refl Representational _), not (SubCo (Refl Nominal _))

  -- These ones simply lift the correspondingly-named
  -- Type constructors into Coercions

  -- TyConAppCo :: "e" -> _ -> ?? -> e
  -- See Note [TyConAppCo roles]
  | TyConAppCo Role TyCon [Coercion]    -- lift TyConApp
               -- The TyCon is never a synonym;
               -- we expand synonyms eagerly
               -- But it can be a type function

  | AppCo Coercion CoercionN             -- lift AppTy
          -- AppCo :: e -> N -> e

  -- See Note [Forall coercions]
  | ForAllCo TyVar KindCoercion Coercion
         -- ForAllCo :: _ -> N -> e -> e

  -- These are special
  | CoVarCo CoVar      -- :: _ -> (N or R)
                       -- result role depends on the tycon of the variable's type

    -- AxiomInstCo :: e -> _ -> [N] -> e
  | AxiomInstCo (CoAxiom Branched) BranchIndex [Coercion]
     -- See also [CoAxiom index]
     -- The coercion arguments always *precisely* saturate
     -- arity of (that branch of) the CoAxiom. If there are
     -- any left over, we use AppCo.
     -- See [Coercion axioms applied to coercions]

  | UnivCo UnivCoProvenance Role Type Type
      -- :: _ -> "e" -> _ -> _ -> e

  | SymCo Coercion             -- :: e -> e
  | TransCo Coercion Coercion  -- :: e -> e -> e

    -- The number coercions should match exactly the expectations
    -- of the CoAxiomRule (i.e., the rule is fully saturated).
  | AxiomRuleCo CoAxiomRule [Coercion]

  | NthCo  Int         Coercion     -- Zero-indexed; decomposes (T t0 ... tn)
    -- :: _ -> e -> ?? (inverse of TyConAppCo, see Note [TyConAppCo roles])
    -- Using NthCo on a ForAllCo gives an N coercion always
    -- See Note [NthCo and newtypes]

  | LRCo   LeftOrRight CoercionN     -- Decomposes (t_left t_right)
    -- :: _ -> N -> N
  | InstCo Coercion CoercionN
    -- :: e -> N -> e
    -- See Note [InstCo roles]

  -- Coherence applies a coercion to the left-hand type of another coercion
  -- See Note [Coherence]
  | CoherenceCo Coercion KindCoercion
     -- :: e -> N -> e

  -- Extract a kind coercion from a (heterogeneous) type coercion
  -- NB: all kind coercions are Nominal
  | KindCo Coercion
     -- :: e -> N

  | SubCo CoercionN                  -- Turns a ~N into a ~R
    -- :: N -> R

  deriving (Data.Data, Data.Typeable)

type CoercionN = Coercion       -- always nominal
type CoercionR = Coercion       -- always representational
type CoercionP = Coercion       -- always phantom
type KindCoercion = CoercionN   -- always nominal

-- If you edit this type, you may need to update the GHC formalism
-- See Note [GHC Formalism] in coreSyn/CoreLint.hs
data LeftOrRight = CLeft | CRight
                 deriving( Eq, Data.Data, Data.Typeable )

instance Binary LeftOrRight where
   put_ bh CLeft  = putByte bh 0
   put_ bh CRight = putByte bh 1

   get bh = do { h <- getByte bh
               ; case h of
                   0 -> return CLeft
                   _ -> return CRight }

pickLR :: LeftOrRight -> (a,a) -> a
pickLR CLeft  (l,_) = l
pickLR CRight (_,r) = r

Note [Refl invariant]
Invariant 1:

Coercions have the following invariant
     Refl is always lifted as far as possible.

You might think that a consequencs is:
     Every identity coercions has Refl at the root

But that's not quite true because of coercion variables.  Consider
     g         where g :: Int~Int
     Left h    where h :: Maybe Int ~ Maybe Int
etc.  So the consequence is only true of coercions that
have no coercion variables.

Note [Coercion axioms applied to coercions]
The reason coercion axioms can be applied to coercions and not just
types is to allow for better optimization.  There are some cases where
we need to be able to "push transitivity inside" an axiom in order to
expose further opportunities for optimization.

For example, suppose we have

  C a : t[a] ~ F a
  g   : b ~ c

and we want to optimize

  sym (C b) ; t[g] ; C c

which has the kind

  F b ~ F c

(stopping through t[b] and t[c] along the way).

We'd like to optimize this to just F g -- but how?  The key is
that we need to allow axioms to be instantiated by *coercions*,
not just by types.  Then we can (in certain cases) push
transitivity inside the axiom instantiations, and then react
opposite-polarity instantiations of the same axiom.  In this
case, e.g., we match t[g] against the LHS of (C c)'s kind, to
obtain the substitution  a |-> g  (note this operation is sort
of the dual of lifting!) and hence end up with

  C g : t[b] ~ F c

which indeed has the same kind as  t[g] ; C c.

Now we have

  sym (C b) ; C g

which can be optimized to F g.

Note [CoAxiom index]
A CoAxiom has 1 or more branches. Each branch has contains a list
of the free type variables in that branch, the LHS type patterns,
and the RHS type for that branch. When we apply an axiom to a list
of coercions, we must choose which branch of the axiom we wish to
use, as the different branches may have different numbers of free
type variables. (The number of type patterns is always the same
among branches, but that doesn't quite concern us here.)

The Int in the AxiomInstCo constructor is the 0-indexed number
of the chosen branch.

Note [Forall coercions]
Constructing coercions between forall-types can be a bit tricky,
because the kinds of the bound tyvars can be different.

The typing rule is:

  kind_co : k1 ~ k2
  tv1:k1 |- co : t1 ~ t2
  ForAllCo tv1 kind_co co : all tv1:k1. t1  ~
                            all tv1:k2. (t2[tv1 |-> tv1 |> sym kind_co])

First, the TyVar stored in a ForAllCo is really an optimisation: this field
should be a Name, as its kind is redundant. Thinking of the field as a Name
is helpful in understanding what a ForAllCo means.

The idea is that kind_co gives the two kinds of the tyvar. See how, in the
conclusion, tv1 is assigned kind k1 on the left but kind k2 on the right.

Of course, a type variable can't have different kinds at the same time. So,
we arbitrarily prefer the first kind when using tv1 in the inner coercion
co, which shows that t1 equals t2.

The last wrinkle is that we need to fix the kinds in the conclusion. In
t2, tv1 is assumed to have kind k1, but it has kind k2 in the conclusion of
the rule. So we do a kind-fixing substitution, replacing (tv1:k1) with
(tv1:k2) |> sym kind_co. This substitution is slightly bizarre, because it
mentions the same name with different kinds, but it *is* well-kinded, noting
that `(tv1:k2) |> sym kind_co` has kind k1.

This all really would work storing just a Name in the ForAllCo. But we can't
add Names to, e.g., VarSets, and there generally is just an impedence mismatch
in a bunch of places. So we use tv1. When we need tv2, we can use

Note [Coherence]
The Coherence typing rule is thus:

  g1 : s ~ t    s : k1    g2 : k1 ~ k2
  CoherenceCo g1 g2 : (s |> g2) ~ t

While this looks (and is) unsymmetric, a combination of other coercion
combinators can make the symmetric version.

For role information, see Note [Roles and kind coercions].

Note [Predicate coercions]
Suppose we have
   g :: a~b
How can we coerce between types
   ([c]~a) => [a] -> c
   ([c]~b) => [b] -> c
where the equality predicate *itself* differs?

Answer: we simply treat (~) as an ordinary type constructor, so these
types really look like

   ((~) [c] a) -> [a] -> c
   ((~) [c] b) -> [b] -> c

So the coercion between the two is obviously

   ((~) [c] g) -> [g] -> c

Another way to see this to say that we simply collapse predicates to
their representation type (see Type.coreView and Type.predTypeRep).

This collapse is done by mkPredCo; there is no PredCo constructor
in Coercion.  This is important because we need Nth to work on
predicates too:
    Nth 1 ((~) [c] g) = g
See Simplify.simplCoercionF, which generates such selections.

Note [Roles]
Roles are a solution to the GeneralizedNewtypeDeriving problem, articulated
in Trac #1496. The full story is in docs/core-spec/core-spec.pdf. Also, see

Here is one way to phrase the problem:

newtype Age = MkAge Int
type family F x
type instance F Age = Bool
type instance F Int = Char

This compiles down to:
axAge :: Age ~ Int
axF1 :: F Age ~ Bool
axF2 :: F Int ~ Char

Then, we can make:
(sym (axF1) ; F axAge ; axF2) :: Bool ~ Char


The solution is _roles_, as articulated in "Generative Type Abstraction and
Type-level Computation" (POPL 2010), available at

The specification for roles has evolved somewhat since that paper. For the
current full details, see the documentation in docs/core-spec. Here are some

We label every equality with a notion of type equivalence, of which there are
three options: Nominal, Representational, and Phantom. A ground type is
nominally equivalent only with itself. A newtype (which is considered a ground
type in Haskell) is representationally equivalent to its representation.
Anything is "phantomly" equivalent to anything else. We use "N", "R", and "P"
to denote the equivalences.

The axioms above would be:
axAge :: Age ~R Int
axF1 :: F Age ~N Bool
axF2 :: F Age ~N Char

Then, because transitivity applies only to coercions proving the same notion
of equivalence, the above construction is impossible.

However, there is still an escape hatch: we know that any two types that are
nominally equivalent are representationally equivalent as well. This is what
the form SubCo proves -- it "demotes" a nominal equivalence into a
representational equivalence. So, it would seem the following is possible:

sub (sym axF1) ; F axAge ; sub axF2 :: Bool ~R Char   -- WRONG

What saves us here is that the arguments to a type function F, lifted into a
coercion, *must* prove nominal equivalence. So, (F axAge) is ill-formed, and
we are safe.

Roles are attached to parameters to TyCons. When lifting a TyCon into a
coercion (through TyConAppCo), we need to ensure that the arguments to the
TyCon respect their roles. For example:

data T a b = MkT a (F b)

If we know that a1 ~R a2, then we know (T a1 b) ~R (T a2 b). But, if we know
that b1 ~R b2, we know nothing about (T a b1) and (T a b2)! This is because
the type function F branches on b's *name*, not representation. So, we say
that 'a' has role Representational and 'b' has role Nominal. The third role,
Phantom, is for parameters not used in the type's definition. Given the
following definition

data Q a = MkQ Int

the Phantom role allows us to say that (Q Bool) ~R (Q Char), because we
can construct the coercion Bool ~P Char (using UnivCo).

See the paper cited above for more examples and information.

Note [TyConAppCo roles]
The TyConAppCo constructor has a role parameter, indicating the role at
which the coercion proves equality. The choice of this parameter affects
the required roles of the arguments of the TyConAppCo. To help explain
it, assume the following definition:

  type instance F Int = Bool   -- Axiom axF : F Int ~N Bool
  newtype Age = MkAge Int      -- Axiom axAge : Age ~R Int
  data Foo a = MkFoo a         -- Role on Foo's parameter is Representational

TyConAppCo Nominal Foo axF : Foo (F Int) ~N Foo Bool
  For (TyConAppCo Nominal) all arguments must have role Nominal. Why?
  So that Foo Age ~N Foo Int does *not* hold.

TyConAppCo Representational Foo (SubCo axF) : Foo (F Int) ~R Foo Bool
TyConAppCo Representational Foo axAge       : Foo Age     ~R Foo Int
  For (TyConAppCo Representational), all arguments must have the roles
  corresponding to the result of tyConRoles on the TyCon. This is the
  whole point of having roles on the TyCon to begin with. So, we can
  have Foo Age ~R Foo Int, if Foo's parameter has role R.

  If a Representational TyConAppCo is over-saturated (which is otherwise fine),
  the spill-over arguments must all be at Nominal. This corresponds to the
  behavior for AppCo.

TyConAppCo Phantom Foo (UnivCo Phantom Int Bool) : Foo Int ~P Foo Bool
  All arguments must have role Phantom. This one isn't strictly
  necessary for soundness, but this choice removes ambiguity.

The rules here dictate the roles of the parameters to mkTyConAppCo
(should be checked by Lint).

Note [NthCo and newtypes]
Suppose we have

  newtype N a = MkN Int
  type role N representational

This yields axiom

  NTCo:N :: forall a. N a ~R Int

We can then build

  co :: forall a b. N a ~R N b
  co = NTCo:N a ; sym (NTCo:N b)

for any `a` and `b`. Because of the role annotation on N, if we use
NthCo, we'll get out a representational coercion. That is:

  NthCo 0 co :: forall a b. a ~R b

Yikes! Clearly, this is terrible. The solution is simple: forbid
NthCo to be used on newtypes if the internal coercion is representational.

This is not just some corner case discovered by a segfault somewhere;
it was discovered in the proof of soundness of roles and described
in the "Safe Coercions" paper (ICFP '14).

Note [InstCo roles]
Here is (essentially) the typing rule for InstCo:

g :: (forall a. t1) ~r (forall a. t2)
w :: s1 ~N s2
------------------------------- InstCo
InstCo g w :: (t1 [a |-> s1]) ~r (t2 [a |-> s2])

Note that the Coercion w *must* be nominal. This is necessary
because the variable a might be used in a "nominal position"
(that is, a place where role inference would require a nominal
role) in t1 or t2. If we allowed w to be representational, we
could get bogus equalities.

A more nuanced treatment might be able to relax this condition
somewhat, by checking if t1 and/or t2 use their bound variables
in nominal ways. If not, having w be representational is OK.

%*                                                                      *
%*                                                                      *

A UnivCo is a coercion whose proof does not directly express its role
and kind (indeed for some UnivCos, like UnsafeCoerceProv, there /is/
no proof).

The different kinds of UnivCo are described by UnivCoProvenance.  Really
each is entirely separate, but they all share the need to represent their
role and kind, which is done in the UnivCo constructor.


-- | For simplicity, we have just one UnivCo that represents a coercion from
-- some type to some other type, with (in general) no restrictions on the
-- type. The UnivCoProvenance specifies more exactly what the coercion really
-- is and why a program should (or shouldn't!) trust the coercion.
-- It is reasonable to consider each constructor of 'UnivCoProvenance'
-- as a totally independent coercion form; their only commonality is
-- that they don't tell you what types they coercion between. (That info
-- is in the 'UnivCo' constructor of 'Coercion'.
data UnivCoProvenance
  = UnsafeCoerceProv   -- ^ From @unsafeCoerce#@. These are unsound.

  | PhantomProv KindCoercion -- ^ See Note [Phantom coercions]. Only in Phantom
                             -- roled coercions

  | ProofIrrelProv KindCoercion  -- ^ From the fact that any two coercions are
                                 --   considered equivalent. See Note [ProofIrrelProv].
                                 -- Can be used in Nominal or Representational coercions

  | PluginProv String  -- ^ From a plugin, which asserts that this coercion
                       --   is sound. The string is for the use of the plugin.

  | HoleProv CoercionHole  -- ^ See Note [Coercion holes]
  deriving (Data.Data, Data.Typeable)

instance Outputable UnivCoProvenance where
  ppr UnsafeCoerceProv   = text "(unsafeCoerce#)"
  ppr (PhantomProv _)    = text "(phantom)"
  ppr (ProofIrrelProv _) = text "(proof irrel.)"
  ppr (PluginProv str)   = parens (text "plugin" <+> brackets (text str))
  ppr (HoleProv hole)    = parens (text "hole" <> ppr hole)

-- | A coercion to be filled in by the type-checker. See Note [Coercion holes]
data CoercionHole
  = CoercionHole { chUnique   :: Unique   -- ^ used only for debugging
                 , chCoercion :: IORef (Maybe Coercion)
  deriving (Data.Typeable)

instance Data.Data CoercionHole where
  -- don't traverse?
  toConstr _   = abstractConstr "CoercionHole"
  gunfold _ _  = error "gunfold"
  dataTypeOf _ = mkNoRepType "CoercionHole"

instance Outputable CoercionHole where
  ppr (CoercionHole u _) = braces (ppr u)

{- Note [Phantom coercions]
     data T a = T1 | T2
Then we have
     T s ~R T t
for any old s,t. The witness for this is (TyConAppCo T Rep co),
where (co :: s ~P t) is a phantom coercion built with PhantomProv.
The role of the UnivCo is always Phantom.  The Coercion stored is the
(nominal) kind coercion between the types
   kind(s) ~N kind (t)

Note [Coercion holes]
During typechecking, constraint solving for type classes works by
  - Generate an evidence Id,  d7 :: Num a
  - Wrap it in a Wanted constraint, [W] d7 :: Num a
  - Use the evidence Id where the evidence is needed
  - Solve the constraint later
  - When solved, add an enclosing let-binding  let d7 = .... in ....
    which actually binds d7 to the (Num a) evidence

For equality constraints we use a different strategy.  See Note [The
equality types story] in TysPrim for background on equality constraints.
  - For boxed equality constraints, (t1 ~N t2) and (t1 ~R t2), it's just
    like type classes above. (Indeed, boxed equality constraints *are* classes.)
  - But for /unboxed/ equality constraints (t1 ~R# t2) and (t1 ~N# t2)
    we use a different plan

For unboxed equalities:
  - Generate a CoercionHole, a mutable variable just like a unification
  - Wrap the CoercionHole in a Wanted constraint; see TcRnTypes.TcEvDest
  - Use the CoercionHole in a Coercion, via HoleProv
  - Solve the constraint later
  - When solved, fill in the CoercionHole by side effect, instead of
    doing the let-binding thing

The main reason for all this is that there may be no good place to let-bind
the evidence for unboxed equalities:
  - We emit constraints for kind coercions, to be used
    to cast a type's kind. These coercions then must be used in types. Because
    they might appear in a top-level type, there is no place to bind these
   (unlifted) coercions in the usual way.

  - A coercion for (forall a. t1) ~ forall a. t2) will look like
       forall a. (coercion for t1~t2)
    But the coercion for (t1~t2) may mention 'a', and we don't have let-bindings
    within coercions.  We could add them, but coercion holes are easier.

Other notes about HoleCo:

 * INVARIANT: CoercionHole and HoleProv are used only during type checking,
   and should never appear in Core. Just like unification variables; a Type
   can contain a TcTyVar, but only during type checking. If, one day, we
   use type-level information to separate out forms that can appear during
   type-checking vs forms that can appear in core proper, holes in Core will
   be ruled out.

 * The Unique carried with a coercion hole is used solely for debugging.

 * Coercion holes can be compared for equality only like other coercions:
   only by looking at the types coerced.

 * We don't use holes for other evidence because other evidence wants to
   be /shared/. But coercions are entirely erased, so there's little
   benefit to sharing.

Note [ProofIrrelProv]
A ProofIrrelProv is a coercion between coercions. For example:

  data G a where
    MkG :: G Bool

In core, we get

  G :: * -> *
  MkG :: forall (a :: *). (a ~ Bool) -> G a

Now, consider 'MkG -- that is, MkG used in a type -- and suppose we want
a proof that ('MkG co1 a1) ~ ('MkG co2 a2). This will have to be

  TyConAppCo Nominal MkG [co3, co4]
    co3 :: co1 ~ co2
    co4 :: a1 ~ a2

Note that
  co1 :: a1 ~ Bool
  co2 :: a2 ~ Bool

  co3 = UnivCo (ProofIrrelProv co5) Nominal (CoercionTy co1) (CoercionTy co2)
    co5 :: (a1 ~ Bool) ~ (a2 ~ Bool)
    co5 = TyConAppCo Nominal (~) [<*>, <*>, co4, <Bool>]

%*                                                                      *
                 Free variables of types and coercions
%*                                                                      *

-- | Returns free variables of a type, including kind variables as
-- a non-deterministic set. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfType :: Type -> TyCoVarSet
-- See Note [Free variables of types]
tyCoVarsOfType ty = fvVarSet $ tyCoFVsOfType ty

-- | `tyVarsOfType` that returns free variables of a type in a deterministic
-- set. For explanation of why using `VarSet` is not deterministic see
-- Note [Deterministic FV] in FV.
tyCoVarsOfTypeDSet :: Type -> DTyCoVarSet
-- See Note [Free variables of types]
tyCoVarsOfTypeDSet ty = fvDVarSet $ tyCoFVsOfType ty

-- | `tyVarsOfType` that returns free variables of a type in deterministic
-- order. For explanation of why using `VarSet` is not deterministic see
-- Note [Deterministic FV] in FV.
tyCoVarsOfTypeList :: Type -> [TyCoVar]
-- See Note [Free variables of types]
tyCoVarsOfTypeList ty = fvVarList $ tyCoFVsOfType ty

-- | The worker for `tyVarsOfType` and `tyVarsOfTypeList`.
-- The previous implementation used `unionVarSet` which is O(n+m) and can
-- make the function quadratic.
-- It's exported, so that it can be composed with
-- other functions that compute free variables.
-- See Note [FV naming conventions] in FV.
-- Eta-expanded because that makes it run faster (apparently)
tyCoFVsOfType :: Type -> FV
-- See Note [Free variables of types]
tyCoFVsOfType (TyVarTy v)        a b c = (unitFV v `unionFV` tyCoFVsOfType (tyVarKind v)) a b c
tyCoFVsOfType (TyConApp _ tys)   a b c = tyCoFVsOfTypes tys a b c
tyCoFVsOfType (LitTy {})         a b c = emptyFV a b c
tyCoFVsOfType (AppTy fun arg)    a b c = (tyCoFVsOfType fun `unionFV` tyCoFVsOfType arg) a b c
tyCoFVsOfType (ForAllTy bndr ty) a b c = tyCoFVsBndr bndr (tyCoFVsOfType ty)  a b c
tyCoFVsOfType (CastTy ty co)     a b c = (tyCoFVsOfType ty `unionFV` tyCoFVsOfCo co) a b c
tyCoFVsOfType (CoercionTy co)    a b c = tyCoFVsOfCo co a b c

tyCoFVsBndr :: TyBinder -> FV -> FV
-- Free vars of (forall b. <thing with fvs>)
tyCoFVsBndr bndr fvs = delBinderVarFV bndr fvs
                           `unionFV` tyCoFVsOfType (binderType bndr)

-- | Returns free variables of types, including kind variables as
-- a non-deterministic set. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfTypes :: [Type] -> TyCoVarSet
-- See Note [Free variables of types]
tyCoVarsOfTypes tys = fvVarSet $ tyCoFVsOfTypes tys

-- | Returns free variables of types, including kind variables as
-- a deterministic set. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfTypesDSet :: [Type] -> DTyCoVarSet
-- See Note [Free variables of types]
tyCoVarsOfTypesDSet tys = fvDVarSet $ tyCoFVsOfTypes tys

-- | Returns free variables of types, including kind variables as
-- a deterministically ordered list. For type synonyms it does /not/ expand the
-- synonym.
tyCoVarsOfTypesList :: [Type] -> [TyCoVar]
-- See Note [Free variables of types]
tyCoVarsOfTypesList tys = fvVarList $ tyCoFVsOfTypes tys

tyCoFVsOfTypes :: [Type] -> FV
-- See Note [Free variables of types]
tyCoFVsOfTypes (ty:tys) fv_cand in_scope acc = (tyCoFVsOfType ty `unionFV` tyCoFVsOfTypes tys) fv_cand in_scope acc
tyCoFVsOfTypes []       fv_cand in_scope acc = emptyFV fv_cand in_scope acc

tyCoVarsOfCo :: Coercion -> TyCoVarSet
-- See Note [Free variables of types]
tyCoVarsOfCo co = fvVarSet $ tyCoFVsOfCo co

-- | Get a deterministic set of the vars free in a coercion
tyCoVarsOfCoDSet :: Coercion -> DTyCoVarSet
-- See Note [Free variables of types]
tyCoVarsOfCoDSet co = fvDVarSet $ tyCoFVsOfCo co

tyCoVarsOfCoList :: Coercion -> [TyCoVar]
-- See Note [Free variables of types]
tyCoVarsOfCoList co = fvVarList $ tyCoFVsOfCo co

tyCoFVsOfCo :: Coercion -> FV
-- Extracts type and coercion variables from a coercion
-- See Note [Free variables of types]
tyCoFVsOfCo (Refl _ ty)         fv_cand in_scope acc = tyCoFVsOfType ty fv_cand in_scope acc
tyCoFVsOfCo (TyConAppCo _ _ cos) fv_cand in_scope acc = tyCoFVsOfCos cos fv_cand in_scope acc
tyCoFVsOfCo (AppCo co arg) fv_cand in_scope acc
  = (tyCoFVsOfCo co `unionFV` tyCoFVsOfCo arg) fv_cand in_scope acc
tyCoFVsOfCo (ForAllCo tv kind_co co) fv_cand in_scope acc
  = (delFV tv (tyCoFVsOfCo co) `unionFV` tyCoFVsOfCo kind_co) fv_cand in_scope acc
tyCoFVsOfCo (CoVarCo v) fv_cand in_scope acc
  = (unitFV v `unionFV` tyCoFVsOfType (varType v)) fv_cand in_scope acc
tyCoFVsOfCo (AxiomInstCo _ _ cos) fv_cand in_scope acc = tyCoFVsOfCos cos fv_cand in_scope acc
tyCoFVsOfCo (UnivCo p _ t1 t2) fv_cand in_scope acc
  = (tyCoFVsOfProv p `unionFV` tyCoFVsOfType t1
                         `unionFV` tyCoFVsOfType t2) fv_cand in_scope acc
tyCoFVsOfCo (SymCo co)          fv_cand in_scope acc = tyCoFVsOfCo co fv_cand in_scope acc
tyCoFVsOfCo (TransCo co1 co2)   fv_cand in_scope acc = (tyCoFVsOfCo co1 `unionFV` tyCoFVsOfCo co2) fv_cand in_scope acc
tyCoFVsOfCo (NthCo _ co)        fv_cand in_scope acc = tyCoFVsOfCo co fv_cand in_scope acc
tyCoFVsOfCo (LRCo _ co)         fv_cand in_scope acc = tyCoFVsOfCo co fv_cand in_scope acc
tyCoFVsOfCo (InstCo co arg)     fv_cand in_scope acc = (tyCoFVsOfCo co `unionFV` tyCoFVsOfCo arg) fv_cand in_scope acc
tyCoFVsOfCo (CoherenceCo c1 c2) fv_cand in_scope acc = (tyCoFVsOfCo c1 `unionFV` tyCoFVsOfCo c2) fv_cand in_scope acc
tyCoFVsOfCo (KindCo co)         fv_cand in_scope acc = tyCoFVsOfCo co fv_cand in_scope acc
tyCoFVsOfCo (SubCo co)          fv_cand in_scope acc = tyCoFVsOfCo co fv_cand in_scope acc
tyCoFVsOfCo (AxiomRuleCo _ cs)  fv_cand in_scope acc = tyCoFVsOfCos cs fv_cand in_scope acc

tyCoVarsOfProv :: UnivCoProvenance -> TyCoVarSet
tyCoVarsOfProv prov = fvVarSet $ tyCoFVsOfProv prov

tyCoFVsOfProv :: UnivCoProvenance -> FV
tyCoFVsOfProv UnsafeCoerceProv    fv_cand in_scope acc = emptyFV fv_cand in_scope acc
tyCoFVsOfProv (PhantomProv co)    fv_cand in_scope acc = tyCoFVsOfCo co fv_cand in_scope acc
tyCoFVsOfProv (ProofIrrelProv co) fv_cand in_scope acc = tyCoFVsOfCo co fv_cand in_scope acc
tyCoFVsOfProv (PluginProv _)      fv_cand in_scope acc = emptyFV fv_cand in_scope acc
tyCoFVsOfProv (HoleProv _)        fv_cand in_scope acc = emptyFV fv_cand in_scope acc

tyCoVarsOfCos :: [Coercion] -> TyCoVarSet
tyCoVarsOfCos cos = fvVarSet $ tyCoFVsOfCos cos

tyCoFVsOfCos :: [Coercion] -> FV
tyCoFVsOfCos []       fv_cand in_scope acc = emptyFV fv_cand in_scope acc
tyCoFVsOfCos (co:cos) fv_cand in_scope acc = (tyCoFVsOfCo co `unionFV` tyCoFVsOfCos cos) fv_cand in_scope acc

coVarsOfType :: Type -> CoVarSet
coVarsOfType (TyVarTy v)         = coVarsOfType (tyVarKind v)
coVarsOfType (TyConApp _ tys)    = coVarsOfTypes tys
coVarsOfType (LitTy {})          = emptyVarSet
coVarsOfType (AppTy fun arg)     = coVarsOfType fun `unionVarSet` coVarsOfType arg
coVarsOfType (ForAllTy bndr ty)
  = coVarsOfType ty `delBinderVar` bndr
    `unionVarSet` coVarsOfType (binderType bndr)
coVarsOfType (CastTy ty co)      = coVarsOfType ty `unionVarSet` coVarsOfCo co
coVarsOfType (CoercionTy co)     = coVarsOfCo co

coVarsOfTypes :: [Type] -> TyCoVarSet
coVarsOfTypes tys = mapUnionVarSet coVarsOfType tys

coVarsOfCo :: Coercion -> CoVarSet
-- Extract *coercion* variables only.  Tiresome to repeat the code, but easy.
coVarsOfCo (Refl _ ty)         = coVarsOfType ty
coVarsOfCo (TyConAppCo _ _ args) = coVarsOfCos args
coVarsOfCo (AppCo co arg)      = coVarsOfCo co `unionVarSet` coVarsOfCo arg
coVarsOfCo (ForAllCo tv kind_co co)
  = coVarsOfCo co `delVarSet` tv `unionVarSet` coVarsOfCo kind_co
coVarsOfCo (CoVarCo v)         = unitVarSet v `unionVarSet` coVarsOfType (varType v)
coVarsOfCo (AxiomInstCo _ _ args) = coVarsOfCos args
coVarsOfCo (UnivCo p _ t1 t2)  = coVarsOfProv p `unionVarSet` coVarsOfTypes [t1, t2]
coVarsOfCo (SymCo co)          = coVarsOfCo co
coVarsOfCo (TransCo co1 co2)   = coVarsOfCo co1 `unionVarSet` coVarsOfCo co2
coVarsOfCo (NthCo _ co)        = coVarsOfCo co
coVarsOfCo (LRCo _ co)         = coVarsOfCo co
coVarsOfCo (InstCo co arg)     = coVarsOfCo co `unionVarSet` coVarsOfCo arg
coVarsOfCo (CoherenceCo c1 c2) = coVarsOfCos [c1, c2]
coVarsOfCo (KindCo co)         = coVarsOfCo co
coVarsOfCo (SubCo co)          = coVarsOfCo co
coVarsOfCo (AxiomRuleCo _ cs)  = coVarsOfCos cs

coVarsOfProv :: UnivCoProvenance -> CoVarSet
coVarsOfProv UnsafeCoerceProv    = emptyVarSet
coVarsOfProv (PhantomProv co)    = coVarsOfCo co
coVarsOfProv (ProofIrrelProv co) = coVarsOfCo co
coVarsOfProv (PluginProv _)      = emptyVarSet
coVarsOfProv (HoleProv _)        = emptyVarSet

coVarsOfCos :: [Coercion] -> CoVarSet
coVarsOfCos cos = mapUnionVarSet coVarsOfCo cos

-- | Add the kind variables free in the kinds of the tyvars in the given set.
-- Returns a non-deterministic set.
closeOverKinds :: TyVarSet -> TyVarSet
closeOverKinds = fvVarSet . closeOverKindsFV . varSetElems

-- | Given a list of tyvars returns a deterministic FV computation that
-- returns the given tyvars with the kind variables free in the kinds of the
-- given tyvars.
closeOverKindsFV :: [TyVar] -> FV
closeOverKindsFV tvs =
  mapUnionFV (tyCoFVsOfType . tyVarKind) tvs `unionFV` mkFVs tvs

-- | Add the kind variables free in the kinds of the tyvars in the given set.
-- Returns a deterministically ordered list.
closeOverKindsList :: [TyVar] -> [TyVar]
closeOverKindsList tvs = fvVarList $ closeOverKindsFV tvs

-- | Add the kind variables free in the kinds of the tyvars in the given set.
-- Returns a deterministic set.
closeOverKindsDSet :: DTyVarSet -> DTyVarSet
closeOverKindsDSet = fvDVarSet . closeOverKindsFV . dVarSetElems

-- | Gets the free vars of a telescope, scoped over a given free var set.
tyCoVarsOfTelescope :: [Var] -> TyCoVarSet -> TyCoVarSet
tyCoVarsOfTelescope [] fvs = fvs
tyCoVarsOfTelescope (v:vs) fvs = tyCoVarsOfTelescope vs fvs
                                 `delVarSet` v
                                 `unionVarSet` tyCoVarsOfType (varType v)

%*                                                                      *
%*                                                                      *

Despite the fact that DataCon has to be imported via a hi-boot route,
this module seems the right place for TyThing, because it's needed for
funTyCon and all the types in TysPrim.

Note [ATyCon for classes]
Both classes and type constructors are represented in the type environment
as ATyCon.  You can tell the difference, and get to the class, with
   isClassTyCon :: TyCon -> Bool
   tyConClass_maybe :: TyCon -> Maybe Class
The Class and its associated TyCon have the same Name.

-- | A global typecheckable-thing, essentially anything that has a name.
-- Not to be confused with a 'TcTyThing', which is also a typecheckable
-- thing but in the *local* context.  See 'TcEnv' for how to retrieve
-- a 'TyThing' given a 'Name'.
data TyThing
  = AnId     Id
  | AConLike ConLike
  | ATyCon   TyCon       -- TyCons and classes; see Note [ATyCon for classes]
  | ACoAxiom (CoAxiom Branched)
  deriving (Eq, Ord)

instance Outputable TyThing where
  ppr = pprTyThing

pprTyThing :: TyThing -> SDoc
pprTyThing thing = pprTyThingCategory thing <+> quotes (ppr (getName thing))

pprTyThingCategory :: TyThing -> SDoc
pprTyThingCategory (ATyCon tc)
  | isClassTyCon tc = text "Class"
  | otherwise       = text "Type constructor"
pprTyThingCategory (ACoAxiom _) = text "Coercion axiom"
pprTyThingCategory (AnId   _)   = text "Identifier"
pprTyThingCategory (AConLike (RealDataCon _)) = text "Data constructor"
pprTyThingCategory (AConLike (PatSynCon _))  = text "Pattern synonym"

instance NamedThing TyThing where       -- Can't put this with the type
  getName (AnId id)     = getName id    -- decl, because the DataCon instance
  getName (ATyCon tc)   = getName tc    -- isn't visible there
  getName (ACoAxiom cc) = getName cc
  getName (AConLike cl) = getName cl

%*                                                                      *
      Data type defined here to avoid unnecessary mutual recursion
%*                                                                      *

-- | Type & coercion substitution
-- #tcvsubst_invariant#
-- The following invariants must hold of a 'TCvSubst':
-- 1. The in-scope set is needed /only/ to
-- guide the generation of fresh uniques
-- 2. In particular, the /kind/ of the type variables in
-- the in-scope set is not relevant
-- 3. The substitution is only applied ONCE! This is because
-- in general such application will not reach a fixed point.
data TCvSubst
  = TCvSubst InScopeSet -- The in-scope type and kind variables
             TvSubstEnv -- Substitutes both type and kind variables
             CvSubstEnv -- Substitutes coercion variables
        -- See Note [Apply Once]
        -- and Note [Extending the TvSubstEnv]
        -- and Note [Substituting types and coercions]
        -- and Note [The substitution invariant]

-- | A substitution of 'Type's for 'TyVar's
--                 and 'Kind's for 'KindVar's
type TvSubstEnv = TyVarEnv Type
        -- A TvSubstEnv is used both inside a TCvSubst (with the apply-once
        -- invariant discussed in Note [Apply Once]), and also independently
        -- in the middle of matching, and unification (see Types.Unify)
        -- So you have to look at the context to know if it's idempotent or
        -- apply-once or whatever

-- | A substitution of 'Coercion's for 'CoVar's
type CvSubstEnv = CoVarEnv Coercion

Note [Apply Once]
We use TCvSubsts to instantiate things, and we might instantiate
        forall a b. ty
\with the types
        [a, b], or [b, a].
So the substitution might go [a->b, b->a].  A similar situation arises in Core
when we find a beta redex like
        (/\ a /\ b -> e) b a
Then we also end up with a substitution that permutes type variables. Other
variations happen to; for example [a -> (a, b)].

        *** So a TCvSubst must be applied precisely once ***

A TCvSubst is not idempotent, but, unlike the non-idempotent substitution
we use during unifications, it must not be repeatedly applied.

Note [Extending the TvSubstEnv]
See #tcvsubst_invariant# for the invariants that must hold.

This invariant allows a short-cut when the subst envs are empty:
if the TvSubstEnv and CvSubstEnv are empty --- i.e. (isEmptyTCvSubst subst)
holds --- then (substTy subst ty) does nothing.

For example, consider:
        (/\a. /\b:(a~Int). ...b..) Int
We substitute Int for 'a'.  The Unique of 'b' does not change, but
nevertheless we add 'b' to the TvSubstEnv, because b's kind does change

This invariant has several crucial consequences:

* In substTyVarBndr, we need extend the TvSubstEnv
        - if the unique has changed
        - or if the kind has changed

* In substTyVar, we do not need to consult the in-scope set;
  the TvSubstEnv is enough

* In substTy, substTheta, we can short-circuit when the TvSubstEnv is empty

Note [Substituting types and coercions]
Types and coercions are mutually recursive, and either may have variables
"belonging" to the other. Thus, every time we wish to substitute in a
type, we may also need to substitute in a coercion, and vice versa.
However, the constructor used to create type variables is distinct from
that of coercion variables, so we carry two VarEnvs in a TCvSubst. Note
that it would be possible to use the CoercionTy constructor to combine
these environments, but that seems like a false economy.

Note that the TvSubstEnv should *never* map a CoVar (built with the Id
constructor) and the CvSubstEnv should *never* map a TyVar. Furthermore,
the range of the TvSubstEnv should *never* include a type headed with

Note [The substitution invariant]
When calling (substTy subst ty) it should be the case that
the in-scope set in the substitution is a superset of both:

  * The free vars of the range of the substitution
  * The free vars of ty minus the domain of the substitution

If we want to substitute [a -> ty1, b -> ty2] I used to
think it was enough to generate an in-scope set that includes
fv(ty1,ty2).  But that's not enough; we really should also take the
free vars of the type we are substituting into!  Example:
     (forall b. (a,b,x)) [a -> List b]
Then if we use the in-scope set {b}, there is a danger we will rename
the forall'd variable to 'x' by mistake, getting this:
     (forall x. (List b, x, x))

Breaking this invariant caused the bug from #11371.

emptyTvSubstEnv :: TvSubstEnv
emptyTvSubstEnv = emptyVarEnv

emptyCvSubstEnv :: CvSubstEnv
emptyCvSubstEnv = emptyVarEnv

composeTCvSubstEnv :: InScopeSet
                   -> (TvSubstEnv, CvSubstEnv)
                   -> (TvSubstEnv, CvSubstEnv)
                   -> (TvSubstEnv, CvSubstEnv)
-- ^ @(compose env1 env2)(x)@ is @env1(env2(x))@; i.e. apply @env2@ then @env1@.
-- It assumes that both are idempotent.
-- Typically, @env1@ is the refinement to a base substitution @env2@
composeTCvSubstEnv in_scope (tenv1, cenv1) (tenv2, cenv2)
  = ( tenv1 `plusVarEnv` mapVarEnv (substTy subst1) tenv2
    , cenv1 `plusVarEnv` mapVarEnv (substCo subst1) cenv2 )
        -- First apply env1 to the range of env2
        -- Then combine the two, making sure that env1 loses if
        -- both bind the same variable; that's why env1 is the
        --  *left* argument to plusVarEnv, because the right arg wins
    subst1 = TCvSubst in_scope tenv1 cenv1

-- | Composes two substitutions, applying the second one provided first,
-- like in function composition.
composeTCvSubst :: TCvSubst -> TCvSubst -> TCvSubst
composeTCvSubst (TCvSubst is1 tenv1 cenv1) (TCvSubst is2 tenv2 cenv2)
  = TCvSubst is3 tenv3 cenv3
    is3 = is1 `unionInScope` is2
    (tenv3, cenv3) = composeTCvSubstEnv is3 (tenv1, cenv1) (tenv2, cenv2)

emptyTCvSubst :: TCvSubst
emptyTCvSubst = TCvSubst emptyInScopeSet emptyTvSubstEnv emptyCvSubstEnv

mkEmptyTCvSubst :: InScopeSet -> TCvSubst
mkEmptyTCvSubst is = TCvSubst is emptyTvSubstEnv emptyCvSubstEnv

isEmptyTCvSubst :: TCvSubst -> Bool
         -- See Note [Extending the TvSubstEnv]
isEmptyTCvSubst (TCvSubst _ tenv cenv) = isEmptyVarEnv tenv && isEmptyVarEnv cenv

mkTCvSubst :: InScopeSet -> (TvSubstEnv, CvSubstEnv) -> TCvSubst
mkTCvSubst in_scope (tenv, cenv) = TCvSubst in_scope tenv cenv

mkTvSubst :: InScopeSet -> TvSubstEnv -> TCvSubst
-- ^ Mkae a TCvSubst with specified tyvar subst and empty covar subst
mkTvSubst in_scope tenv = TCvSubst in_scope tenv emptyCvSubstEnv

getTvSubstEnv :: TCvSubst -> TvSubstEnv
getTvSubstEnv (TCvSubst _ env _) = env

getCvSubstEnv :: TCvSubst -> CvSubstEnv
getCvSubstEnv (TCvSubst _ _ env) = env

getTCvInScope :: TCvSubst -> InScopeSet
getTCvInScope (TCvSubst in_scope _ _) = in_scope

-- | Returns the free variables of the types in the range of a substitution as
-- a non-deterministic set.
getTCvSubstRangeFVs :: TCvSubst -> VarSet
getTCvSubstRangeFVs (TCvSubst _ tenv cenv)
    = unionVarSet tenvFVs cenvFVs
    tenvFVs = tyCoVarsOfTypes $ varEnvElts tenv
    cenvFVs = tyCoVarsOfCos   $ varEnvElts cenv

isInScope :: Var -> TCvSubst -> Bool
isInScope v (TCvSubst in_scope _ _) = v `elemInScopeSet` in_scope

notElemTCvSubst :: Var -> TCvSubst -> Bool
notElemTCvSubst v (TCvSubst _ tenv cenv)
  | isTyVar v
  = not (v `elemVarEnv` tenv)
  | otherwise
  = not (v `elemVarEnv` cenv)

setTvSubstEnv :: TCvSubst -> TvSubstEnv -> TCvSubst
setTvSubstEnv (TCvSubst in_scope _ cenv) tenv = TCvSubst in_scope tenv cenv

setCvSubstEnv :: TCvSubst -> CvSubstEnv -> TCvSubst
setCvSubstEnv (TCvSubst in_scope tenv _) cenv = TCvSubst in_scope tenv cenv

zapTCvSubst :: TCvSubst -> TCvSubst
zapTCvSubst (TCvSubst in_scope _ _) = TCvSubst in_scope emptyVarEnv emptyVarEnv

extendTCvInScope :: TCvSubst -> Var -> TCvSubst
extendTCvInScope (TCvSubst in_scope tenv cenv) var
  = TCvSubst (extendInScopeSet in_scope var) tenv cenv

extendTCvInScopeList :: TCvSubst -> [Var] -> TCvSubst
extendTCvInScopeList (TCvSubst in_scope tenv cenv) vars
  = TCvSubst (extendInScopeSetList in_scope vars) tenv cenv

extendTCvInScopeSet :: TCvSubst -> VarSet -> TCvSubst
extendTCvInScopeSet (TCvSubst in_scope tenv cenv) vars
  = TCvSubst (extendInScopeSetSet in_scope vars) tenv cenv

extendTCvSubst :: TCvSubst -> TyCoVar -> Type -> TCvSubst
extendTCvSubst subst v ty
  | isTyVar v
  = extendTvSubst subst v ty
  | CoercionTy co <- ty
  = extendCvSubst subst v co
  | otherwise
  = pprPanic "extendTCvSubst" (ppr v <+> text "|->" <+> ppr ty)

extendTvSubst :: TCvSubst -> TyVar -> Type -> TCvSubst
extendTvSubst (TCvSubst in_scope tenv cenv) tv ty
  = TCvSubst in_scope (extendVarEnv tenv tv ty) cenv

extendTvSubstWithClone :: TCvSubst -> TyVar -> TyVar -> TCvSubst
-- Adds a new tv -> tv mapping, /and/ extends the in-scope set
extendTvSubstWithClone (TCvSubst in_scope tenv cenv) tv tv'
  = TCvSubst (extendInScopeSetSet in_scope new_in_scope)
             (extendVarEnv tenv tv (mkTyVarTy tv'))
    new_in_scope = tyCoVarsOfType (tyVarKind tv') `extendVarSet` tv'

extendCvSubst :: TCvSubst -> CoVar -> Coercion -> TCvSubst
extendCvSubst (TCvSubst in_scope tenv cenv) v co
  = TCvSubst in_scope tenv (extendVarEnv cenv v co)

extendCvSubstWithClone :: TCvSubst -> CoVar -> CoVar -> TCvSubst
extendCvSubstWithClone (TCvSubst in_scope tenv cenv) cv cv'
  = TCvSubst (extendInScopeSetSet in_scope new_in_scope)
             (extendVarEnv cenv cv (mkCoVarCo cv'))
    new_in_scope = tyCoVarsOfType (varType cv') `extendVarSet` cv'

extendTvSubstAndInScope :: TCvSubst -> TyVar -> Type -> TCvSubst
-- Also extends the in-scope set
extendTvSubstAndInScope (TCvSubst in_scope tenv cenv) tv ty
  = TCvSubst (in_scope `extendInScopeSetSet` tyCoVarsOfType ty)
             (extendVarEnv tenv tv ty)

extendTvSubstList :: TCvSubst -> [Var] -> [Type] -> TCvSubst
extendTvSubstList subst tvs tys
  = foldl2 extendTvSubst subst tvs tys

extendTvSubstBinder :: TCvSubst -> TyBinder -> Type -> TCvSubst
extendTvSubstBinder env (Anon {})    _  = env
extendTvSubstBinder env (Named tv _) ty = extendTvSubst env tv ty

unionTCvSubst :: TCvSubst -> TCvSubst -> TCvSubst
-- Works when the ranges are disjoint
unionTCvSubst (TCvSubst in_scope1 tenv1 cenv1) (TCvSubst in_scope2 tenv2 cenv2)
  = ASSERT( not (tenv1 `intersectsVarEnv` tenv2)
         && not (cenv1 `intersectsVarEnv` cenv2) )
    TCvSubst (in_scope1 `unionInScope` in_scope2)
             (tenv1     `plusVarEnv`   tenv2)
             (cenv1     `plusVarEnv`   cenv2)

-- mkTvSubstPrs and zipTvSubst generate the in-scope set from
-- the types given; but it's just a thunk so with a bit of luck
-- it'll never be evaluated

-- | Generates an in-scope set from the free variables in a list of types
-- and a list of coercions
mkTyCoInScopeSet :: [Type] -> [Coercion] -> InScopeSet
mkTyCoInScopeSet tys cos
  = mkInScopeSet (tyCoVarsOfTypes tys `unionVarSet` tyCoVarsOfCos cos)

-- | Generates the in-scope set for the 'TCvSubst' from the types in the incoming
-- environment. No CoVars, please!
zipTvSubst :: [TyVar] -> [Type] -> TCvSubst
zipTvSubst tvs tys
  | debugIsOn
  , not (all isTyVar tvs) || length tvs /= length tys
  = pprTrace "zipTvSubst" (ppr tvs $$ ppr tys) emptyTCvSubst
  | otherwise
  = mkTvSubst (mkInScopeSet (tyCoVarsOfTypes tys)) tenv
    tenv = zipTyEnv tvs tys

-- | Generates the in-scope set for the 'TCvSubst' from the types in the incoming
-- environment.  No TyVars, please!
zipCvSubst :: [CoVar] -> [Coercion] -> TCvSubst
zipCvSubst cvs cos
  | debugIsOn
  , not (all isCoVar cvs) || length cvs /= length cos
  = pprTrace "zipCvSubst" (ppr cvs $$ ppr cos) emptyTCvSubst
  | otherwise
  = TCvSubst (mkInScopeSet (tyCoVarsOfCos cos)) emptyTvSubstEnv cenv
    cenv = zipCoEnv cvs cos

-- | Create a TCvSubst combining the binders and types provided.
-- NB: It is specifically OK if the lists are of different lengths.
zipTyBinderSubst :: [TyBinder] -> [Type] -> TCvSubst
zipTyBinderSubst bndrs tys
  = mkTvSubst is tenv
    is = mkInScopeSet (tyCoVarsOfTypes tys)
    tenv = mkVarEnv [ (tv, ty) | (Named tv _, ty) <- zip bndrs tys ]

-- | Generates the in-scope set for the 'TCvSubst' from the types in the
-- incoming environment. No CoVars, please!
mkTvSubstPrs :: [(TyVar, Type)] -> TCvSubst
mkTvSubstPrs prs =
    ASSERT2( onlyTyVarsAndNoCoercionTy, text "prs" <+> ppr prs )
    mkTvSubst in_scope tenv
  where tenv = mkVarEnv prs
        in_scope = mkInScopeSet $ tyCoVarsOfTypes $ map snd prs
        onlyTyVarsAndNoCoercionTy =
          and [ isTyVar tv && not (isCoercionTy ty)
              | (tv, ty) <- prs ]

zipTyEnv :: [TyVar] -> [Type] -> TvSubstEnv
zipTyEnv tyvars tys
  = ASSERT( all (not . isCoercionTy) tys )
    mkVarEnv (zipEqual "zipTyEnv" tyvars tys)
        -- There used to be a special case for when
        --      ty == TyVarTy tv
        -- (a not-uncommon case) in which case the substitution was dropped.
        -- But the type-tidier changes the print-name of a type variable without
        -- changing the unique, and that led to a bug.   Why?  Pre-tidying, we had
        -- a type {Foo t}, where Foo is a one-method class.  So Foo is really a newtype.
        -- And it happened that t was the type variable of the class.  Post-tiding,
        -- it got turned into {Foo t2}.  The ext-core printer expanded this using
        -- sourceTypeRep, but that said "Oh, t == t2" because they have the same unique,
        -- and so generated a rep type mentioning t not t2.
        -- Simplest fix is to nuke the "optimisation"

zipCoEnv :: [CoVar] -> [Coercion] -> CvSubstEnv
zipCoEnv cvs cos = mkVarEnv (zipEqual "zipCoEnv" cvs cos)

instance Outputable TCvSubst where
  ppr (TCvSubst ins tenv cenv)
    = brackets $ sep[ text "TCvSubst",
                      nest 2 (text "In scope:" <+> ppr ins),
                      nest 2 (text "Type env:" <+> ppr tenv),
                      nest 2 (text "Co env:" <+> ppr cenv) ]

%*                                                                      *
                Performing type or kind substitutions
%*                                                                      *

Note [Sym and ForAllCo]
In OptCoercion, we try to push "sym" out to the leaves of a coercion. But,
how do we push sym into a ForAllCo? It's a little ugly.

Here is the typing rule:

h : k1 ~# k2
(tv : k1) |- g : ty1 ~# ty2
ForAllCo tv h g : (ForAllTy (tv : k1) ty1) ~#
                  (ForAllTy (tv : k2) (ty2[tv |-> tv |> sym h]))

Here is what we want:

ForAllCo tv h' g' : (ForAllTy (tv : k2) (ty2[tv |-> tv |> sym h])) ~#
                    (ForAllTy (tv : k1) ty1)

Because the kinds of the type variables to the right of the colon are the kinds
coerced by h', we know (h' : k2 ~# k1). Thus, (h' = sym h).

Now, we can rewrite ty1 to be (ty1[tv |-> tv |> sym h' |> h']). We thus want

ForAllCo tv h' g' :
  (ForAllTy (tv : k2) (ty2[tv |-> tv |> h'])) ~#
  (ForAllTy (tv : k1) (ty1[tv |-> tv |> h'][tv |-> tv |> sym h']))

We thus see that we want

g' : ty2[tv |-> tv |> h'] ~# ty1[tv |-> tv |> h']

and thus g' = sym (g[tv |-> tv |> h']).

Putting it all together, we get this:

sym (ForAllCo tv h g)
ForAllCo tv (sym h) (sym g[tv |-> tv |> sym h])


-- | Type substitution, see 'zipTvSubst'
substTyWith ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    [TyVar] -> [Type] -> Type -> Type
-- Works only if the domain of the substitution is a
-- superset of the type being substituted into
substTyWith tvs tys = ASSERT( length tvs == length tys )
                      substTy (zipTvSubst tvs tys)

-- | Type substitution, see 'zipTvSubst'. Disables sanity checks.
-- The problems that the sanity checks in substTy catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substTyUnchecked to
-- substTy and remove this function. Please don't use in new code.
substTyWithUnchecked :: [TyVar] -> [Type] -> Type -> Type
substTyWithUnchecked tvs tys
  = ASSERT( length tvs == length tys )
    substTyUnchecked (zipTvSubst tvs tys)

-- | Substitute tyvars within a type using a known 'InScopeSet'.
-- Pre-condition: the 'in_scope' set should satisfy Note [The substitution
-- invariant]; specifically it should include the free vars of 'tys',
-- and of 'ty' minus the domain of the subst.
substTyWithInScope :: InScopeSet -> [TyVar] -> [Type] -> Type -> Type
substTyWithInScope in_scope tvs tys ty =
  ASSERT( length tvs == length tys )
  substTy (mkTvSubst in_scope tenv) ty
  where tenv = zipTyEnv tvs tys

-- | Coercion substitution, see 'zipTvSubst'
substCoWith ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    [TyVar] -> [Type] -> Coercion -> Coercion
substCoWith tvs tys = ASSERT( length tvs == length tys )
                      substCo (zipTvSubst tvs tys)

-- | Coercion substitution, see 'zipTvSubst'. Disables sanity checks.
-- The problems that the sanity checks in substCo catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substCoUnchecked to
-- substCo and remove this function. Please don't use in new code.
substCoWithUnchecked :: [TyVar] -> [Type] -> Coercion -> Coercion
substCoWithUnchecked tvs tys
  = ASSERT( length tvs == length tys )
    substCoUnchecked (zipTvSubst tvs tys)

-- | Substitute covars within a type
substTyWithCoVars :: [CoVar] -> [Coercion] -> Type -> Type
substTyWithCoVars cvs cos = substTy (zipCvSubst cvs cos)

-- | Type substitution, see 'zipTvSubst'
substTysWith :: [TyVar] -> [Type] -> [Type] -> [Type]
substTysWith tvs tys = ASSERT( length tvs == length tys )
                       substTys (zipTvSubst tvs tys)

-- | Type substitution, see 'zipTvSubst'
substTysWithCoVars :: [CoVar] -> [Coercion] -> [Type] -> [Type]
substTysWithCoVars cvs cos = ASSERT( length cvs == length cos )
                             substTys (zipCvSubst cvs cos)

-- | Type substitution using 'Binder's. Anonymous binders
-- simply ignore their matching type.
substTyWithBinders ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    [TyBinder] -> [Type] -> Type -> Type
substTyWithBinders bndrs tys = ASSERT( length bndrs == length tys )
                               substTy (zipTyBinderSubst bndrs tys)

-- | Type substitution using 'Binder's disabling the sanity checks.
-- Anonymous binders simply ignore their matching type.
-- The problems that the sanity checks in substTy catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substTyUnchecked to
-- substTy and remove this function. Please don't use in new code.
substTyWithBindersUnchecked :: [TyBinder] -> [Type] -> Type -> Type
substTyWithBindersUnchecked bndrs tys
  = ASSERT( length bndrs == length tys )
    substTyUnchecked (zipTyBinderSubst bndrs tys)

-- | Substitute within a 'Type' after adding the free variables of the type
-- to the in-scope set. This is useful for the case when the free variables
-- aren't already in the in-scope set or easily available.
-- See also Note [The substitution invariant].
substTyAddInScope :: TCvSubst -> Type -> Type
substTyAddInScope subst ty =
  substTy (extendTCvInScopeSet subst $ tyCoVarsOfType ty) ty

-- | When calling `substTy` it should be the case that the in-scope set in
-- the substitution is a superset of the free vars of the range of the
-- substitution.
-- See also Note [The substitution invariant].
isValidTCvSubst :: TCvSubst -> Bool
isValidTCvSubst (TCvSubst in_scope tenv cenv) =
  (tenvFVs `varSetInScope` in_scope) &&
  (cenvFVs `varSetInScope` in_scope)
  tenvFVs = tyCoVarsOfTypes $ varEnvElts tenv
  cenvFVs = tyCoVarsOfCos $ varEnvElts cenv

-- | This checks if the substitution satisfies the invariant from
-- Note [The substitution invariant].
checkValidSubst ::
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    TCvSubst -> [Type] -> [Coercion] -> a -> a
checkValidSubst subst@(TCvSubst in_scope tenv cenv) tys cos a
  = ASSERT2( isValidTCvSubst subst,
             text "in_scope" <+> ppr in_scope $$
             text "tenv" <+> ppr tenv $$
             text "tenvFVs"
               <+> ppr (tyCoVarsOfTypes $ varEnvElts tenv) $$
             text "cenv" <+> ppr cenv $$
             text "cenvFVs"
               <+> ppr (tyCoVarsOfCos $ varEnvElts cenv) $$
             text "tys" <+> ppr tys $$
             text "cos" <+> ppr cos )
    ASSERT2( tysCosFVsInScope,
             text "in_scope" <+> ppr in_scope $$
             text "tenv" <+> ppr tenv $$
             text "cenv" <+> ppr cenv $$
             text "tys" <+> ppr tys $$
             text "cos" <+> ppr cos $$
             text "needInScope" <+> ppr needInScope )
  substDomain = varEnvKeys tenv ++ varEnvKeys cenv
  needInScope = (tyCoVarsOfTypes tys `unionVarSet` tyCoVarsOfCos cos)
                  `delListFromUFM_Directly` substDomain
  tysCosFVsInScope = needInScope `varSetInScope` in_scope

-- | Substitute within a 'Type'
-- The substitution has to satisfy the invariants described in
-- Note [The substitution invariant].
substTy ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    TCvSubst -> Type  -> Type
substTy subst ty
  | isEmptyTCvSubst subst = ty
  | otherwise = checkValidSubst subst [ty] [] $ subst_ty subst ty

-- | Substitute within a 'Type' disabling the sanity checks.
-- The problems that the sanity checks in substTy catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substTyUnchecked to
-- substTy and remove this function. Please don't use in new code.
substTyUnchecked :: TCvSubst -> Type -> Type
substTyUnchecked subst ty
                 | isEmptyTCvSubst subst = ty
                 | otherwise             = subst_ty subst ty

-- | Substitute within several 'Type's
-- The substitution has to satisfy the invariants described in
-- Note [The substitution invariant].
substTys ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    TCvSubst -> [Type] -> [Type]
substTys subst tys
  | isEmptyTCvSubst subst = tys
  | otherwise = checkValidSubst subst tys [] $ map (subst_ty subst) tys

-- | Substitute within several 'Type's disabling the sanity checks.
-- The problems that the sanity checks in substTys catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substTysUnchecked to
-- substTys and remove this function. Please don't use in new code.
substTysUnchecked :: TCvSubst -> [Type] -> [Type]
substTysUnchecked subst tys
                 | isEmptyTCvSubst subst = tys
                 | otherwise             = map (subst_ty subst) tys

-- | Substitute within a 'ThetaType'
-- The substitution has to satisfy the invariants described in
-- Note [The substitution invariant].
substTheta ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    TCvSubst -> ThetaType -> ThetaType
substTheta = substTys

-- | Substitute within a 'ThetaType' disabling the sanity checks.
-- The problems that the sanity checks in substTys catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substThetaUnchecked to
-- substTheta and remove this function. Please don't use in new code.
substThetaUnchecked :: TCvSubst -> ThetaType -> ThetaType
substThetaUnchecked = substTysUnchecked

subst_ty :: TCvSubst -> Type -> Type
-- subst_ty is the main workhorse for type substitution
-- Note that the in_scope set is poked only if we hit a forall
-- so it may often never be fully computed
subst_ty subst ty
   = go ty
    go (TyVarTy tv)      = substTyVar subst tv
    go (AppTy fun arg)   = mkAppTy (go fun) $! (go arg)
                -- The mkAppTy smart constructor is important
                -- we might be replacing (a Int), represented with App
                -- by [Int], represented with TyConApp
    go (TyConApp tc tys) = let args = map go tys
                           in  args `seqList` TyConApp tc args
    go (ForAllTy (Anon arg) res)
                         = (ForAllTy $! (Anon $! go arg)) $! go res
    go (ForAllTy (Named tv vis) ty)
                         = case substTyVarBndrUnchecked subst tv of
                             (subst', tv') ->
                               (ForAllTy $! ((Named $! tv') vis)) $!
                                            (subst_ty subst' ty)
    go (LitTy n)         = LitTy $! n
    go (CastTy ty co)    = (CastTy $! (go ty)) $! (subst_co subst co)
    go (CoercionTy co)   = CoercionTy $! (subst_co subst co)

substTyVar :: TCvSubst -> TyVar -> Type
substTyVar (TCvSubst _ tenv _) tv
  = ASSERT( isTyVar tv )
    case lookupVarEnv tenv tv of
      Just ty -> ty
      Nothing -> TyVarTy tv

substTyVars :: TCvSubst -> [TyVar] -> [Type]
substTyVars subst = map $ substTyVar subst

lookupTyVar :: TCvSubst -> TyVar  -> Maybe Type
        -- See Note [Extending the TCvSubst]
lookupTyVar (TCvSubst _ tenv _) tv
  = ASSERT( isTyVar tv )
    lookupVarEnv tenv tv

-- | Substitute within a 'Coercion'
-- The substitution has to satisfy the invariants described in
-- Note [The substitution invariant].
substCo ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    TCvSubst -> Coercion -> Coercion
substCo subst co
  | isEmptyTCvSubst subst = co
  | otherwise = checkValidSubst subst [] [co] $ subst_co subst co

-- | Substitute within a 'Coercion' disabling sanity checks.
-- The problems that the sanity checks in substCo catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substCoUnchecked to
-- substCo and remove this function. Please don't use in new code.
substCoUnchecked :: TCvSubst -> Coercion -> Coercion
substCoUnchecked subst co
  | isEmptyTCvSubst subst = co
  | otherwise = subst_co subst co

-- | Substitute within several 'Coercion's
-- The substitution has to satisfy the invariants described in
-- Note [The substitution invariant].
substCos ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    TCvSubst -> [Coercion] -> [Coercion]
substCos subst cos
  | isEmptyTCvSubst subst = cos
  | otherwise = checkValidSubst subst [] cos $ map (subst_co subst) cos

subst_co :: TCvSubst -> Coercion -> Coercion
subst_co subst co
  = go co
    go_ty :: Type -> Type
    go_ty = subst_ty subst

    go :: Coercion -> Coercion
    go (Refl r ty)           = mkReflCo r $! go_ty ty
    go (TyConAppCo r tc args)= let args' = map go args
                               in  args' `seqList` mkTyConAppCo r tc args'
    go (AppCo co arg)        = (mkAppCo $! go co) $! go arg
    go (ForAllCo tv kind_co co)
      = case substForAllCoBndrUnchecked subst tv kind_co of { (subst', tv', kind_co') ->
          ((mkForAllCo $! tv') $! kind_co') $! subst_co subst' co }
    go (CoVarCo cv)          = substCoVar subst cv
    go (AxiomInstCo con ind cos) = mkAxiomInstCo con ind $! map go cos
    go (UnivCo p r t1 t2)    = (((mkUnivCo $! go_prov p) $! r) $!
                                (go_ty t1)) $! (go_ty t2)
    go (SymCo co)            = mkSymCo $! (go co)
    go (TransCo co1 co2)     = (mkTransCo $! (go co1)) $! (go co2)
    go (NthCo d co)          = mkNthCo d $! (go co)
    go (LRCo lr co)          = mkLRCo lr $! (go co)
    go (InstCo co arg)       = (mkInstCo $! (go co)) $! go arg
    go (CoherenceCo co1 co2) = (mkCoherenceCo $! (go co1)) $! (go co2)
    go (KindCo co)           = mkKindCo $! (go co)
    go (SubCo co)            = mkSubCo $! (go co)
    go (AxiomRuleCo c cs)    = let cs1 = map go cs
                                in cs1 `seqList` AxiomRuleCo c cs1

    go_prov UnsafeCoerceProv     = UnsafeCoerceProv
    go_prov (PhantomProv kco)    = PhantomProv (go kco)
    go_prov (ProofIrrelProv kco) = ProofIrrelProv (go kco)
    go_prov p@(PluginProv _)     = p
    go_prov p@(HoleProv _)       = p
      -- NB: this last case is a little suspicious, but we need it. Originally,
      -- there was a panic here, but it triggered from deeplySkolemise. Because
      -- we only skolemise tyvars that are manually bound, this operation makes
      -- sense, even over a coercion with holes.

substForAllCoBndr :: TCvSubst -> TyVar -> Coercion -> (TCvSubst, TyVar, Coercion)
substForAllCoBndr subst
  = substForAllCoBndrCallback False (substCo subst) subst

-- | Like 'substForAllCoBndr', but disables sanity checks.
-- The problems that the sanity checks in substCo catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substCoUnchecked to
-- substCo and remove this function. Please don't use in new code.
substForAllCoBndrUnchecked :: TCvSubst -> TyVar -> Coercion -> (TCvSubst, TyVar, Coercion)
substForAllCoBndrUnchecked subst
  = substForAllCoBndrCallback False (substCoUnchecked subst) subst

-- See Note [Sym and ForAllCo]
substForAllCoBndrCallback :: Bool  -- apply sym to binder?
                          -> (Coercion -> Coercion)  -- transformation to kind co
                          -> TCvSubst -> TyVar -> Coercion
                          -> (TCvSubst, TyVar, Coercion)
substForAllCoBndrCallback sym sco (TCvSubst in_scope tenv cenv)
                          old_var old_kind_co
  = ( TCvSubst (in_scope `extendInScopeSet` new_var) new_env cenv
    , new_var, new_kind_co )
    new_env | no_change && not sym = delVarEnv tenv old_var
            | sym       = extendVarEnv tenv old_var $
                            TyVarTy new_var `CastTy` new_kind_co
            | otherwise = extendVarEnv tenv old_var (TyVarTy new_var)

    no_kind_change = isEmptyVarSet (tyCoVarsOfCo old_kind_co)
    no_change = no_kind_change && (new_var == old_var)

    new_kind_co | no_kind_change = old_kind_co
                | otherwise      = sco old_kind_co

    Pair new_ki1 _ = coercionKind new_kind_co

    new_var  = uniqAway in_scope (setTyVarKind old_var new_ki1)

substCoVar :: TCvSubst -> CoVar -> Coercion
substCoVar (TCvSubst _ _ cenv) cv
  = case lookupVarEnv cenv cv of
      Just co -> co
      Nothing -> CoVarCo cv

substCoVars :: TCvSubst -> [CoVar] -> [Coercion]
substCoVars subst cvs = map (substCoVar subst) cvs

lookupCoVar :: TCvSubst -> Var  -> Maybe Coercion
lookupCoVar (TCvSubst _ _ cenv) v = lookupVarEnv cenv v

substTyVarBndr ::
-- CallStack wasn't present in GHC 7.10.1, disable callstacks in stage 1
#if __GLASGOW_HASKELL__ > 710
    (?callStack :: CallStack) =>
    TCvSubst -> TyVar -> (TCvSubst, TyVar)
substTyVarBndr = substTyVarBndrCallback substTy

-- | Like 'substTyVarBndr' but disables sanity checks.
-- The problems that the sanity checks in substTy catch are described in
-- Note [The substitution invariant].
-- The goal of #11371 is to migrate all the calls of substTyUnchecked to
-- substTy and remove this function. Please don't use in new code.
substTyVarBndrUnchecked :: TCvSubst -> TyVar -> (TCvSubst, TyVar)
substTyVarBndrUnchecked = substTyVarBndrCallback substTyUnchecked

-- | Substitute a tyvar in a binding position, returning an
-- extended subst and a new tyvar.
substTyVarBndrCallback :: (TCvSubst -> Type -> Type)  -- ^ the subst function
                       -> TCvSubst -> TyVar -> (TCvSubst, TyVar)
substTyVarBndrCallback subst_fn subst@(TCvSubst in_scope tenv cenv) old_var
  = ASSERT2( _no_capture, pprTvBndr old_var $$ pprTvBndr new_var $$ ppr subst )
    ASSERT( isTyVar old_var )
    (TCvSubst (in_scope `extendInScopeSet` new_var) new_env cenv, new_var)
    new_env | no_change = delVarEnv tenv old_var
            | otherwise = extendVarEnv tenv old_var (TyVarTy new_var)

    _no_capture = not (new_var `elemVarSet` tyCoVarsOfTypes (varEnvElts tenv))
    -- Assertion check that we are not capturing something in the substitution

    old_ki = tyVarKind old_var
    no_kind_change = isEmptyVarSet (tyCoVarsOfType old_ki) -- verify that kind is closed
    no_change = no_kind_change && (new_var == old_var)
        -- no_change means that the new_var is identical in
        -- all respects to the old_var (same unique, same kind)
        -- See Note [Extending the TCvSubst]
        -- In that case we don't need to extend the substitution
        -- to map old to new.  But instead we must zap any
        -- current substitution for the variable. For example:
        --      (\x.e) with id_subst = [x |-> e']
        -- Here we must simply zap the substitution for x

    new_var | no_kind_change = uniqAway in_scope old_var
            | otherwise = uniqAway in_scope $
                          setTyVarKind old_var (subst_fn subst old_ki)
        -- The uniqAway part makes sure the new variable is not already in scope

substCoVarBndr :: TCvSubst -> CoVar -> (TCvSubst, CoVar)
substCoVarBndr = substCoVarBndrCallback False substTy

substCoVarBndrCallback :: Bool -- apply "sym" to the covar?
                       -> (TCvSubst -> Type -> Type)
                       -> TCvSubst -> CoVar -> (TCvSubst, CoVar)
substCoVarBndrCallback sym subst_fun subst@(TCvSubst in_scope tenv cenv) old_var
  = ASSERT( isCoVar old_var )
    (TCvSubst (in_scope `extendInScopeSet` new_var) tenv new_cenv, new_var)
    -- When we substitute (co :: t1 ~ t2) we may get the identity (co :: t ~ t)
    -- In that case, mkCoVarCo will return a ReflCoercion, and
    -- we want to substitute that (not new_var) for old_var
    new_co    = (if sym then mkSymCo else id) $ mkCoVarCo new_var
    no_kind_change = isEmptyVarSet (tyCoVarsOfTypes [t1, t2])
    no_change = new_var == old_var && not (isReflCo new_co) && no_kind_change

    new_cenv | no_change = delVarEnv cenv old_var
             | otherwise = extendVarEnv cenv old_var new_co

    new_var = uniqAway in_scope subst_old_var
    subst_old_var = mkCoVar (varName old_var) new_var_type

    (_, _, t1, t2, role) = coVarKindsTypesRole old_var
    t1' = subst_fun subst t1
    t2' = subst_fun subst t2
    new_var_type = uncurry (mkCoercionType role) (if sym then (t2', t1') else (t1', t2'))
                  -- It's important to do the substitution for coercions,
                  -- because they can have free type variables

cloneTyVarBndr :: TCvSubst -> TyVar -> Unique -> (TCvSubst, TyVar)
cloneTyVarBndr subst@(TCvSubst in_scope tv_env cv_env) tv uniq
  = ASSERT2( isTyVar tv, ppr tv )   -- I think it's only called on TyVars
    (TCvSubst (extendInScopeSet in_scope tv')
              (extendVarEnv tv_env tv (mkTyVarTy tv')) cv_env, tv')
    old_ki = tyVarKind tv
    no_kind_change = isEmptyVarSet (tyCoVarsOfType old_ki) -- verify that kind is closed

    tv1 | no_kind_change = tv
        | otherwise      = setTyVarKind tv (substTy subst old_ki)

    tv' = setVarUnique tv1 uniq

cloneTyVarBndrs :: TCvSubst -> [TyVar] -> UniqSupply -> (TCvSubst, [TyVar])
cloneTyVarBndrs subst []     _usupply = (subst, [])
cloneTyVarBndrs subst (t:ts)  usupply = (subst'', tv:tvs)
    (uniq, usupply') = takeUniqFromSupply usupply
    (subst' , tv )   = cloneTyVarBndr subst t uniq
    (subst'', tvs)   = cloneTyVarBndrs subst' ts usupply'

%*                                                                      *
                   Pretty-printing types

       Defined very early because of debug printing in assertions
%*                                                                      *

@pprType@ is the standard @Type@ printer; the overloaded @ppr@ function is
defined to use this.  @pprParendType@ is the same, except it puts
parens around the type, except for the atomic cases.  @pprParendType@
works just by setting the initial context precedence very high.

Note [Precedence in types]
We don't keep the fixity of type operators in the operator. So the pretty printer
operates the following precedene structre:
   Type constructor application   binds more tightly than
   Operator applications          which bind more tightly than
   Function arrow

So we might see  a :+: T b -> c
meaning          (a :+: (T b)) -> c

Maybe operator applications should bind a bit less tightly?

Anyway, that's the current story, and it is used consistently for Type and HsType

data TyPrec   -- See Note [Prededence in types]
  = TopPrec         -- No parens
  | FunPrec         -- Function args; no parens for tycon apps
  | TyOpPrec        -- Infix operator
  | TyConPrec       -- Tycon args; no parens for atomic
  deriving( Eq, Ord )

maybeParen :: TyPrec -> TyPrec -> SDoc -> SDoc
maybeParen ctxt_prec inner_prec pretty
  | ctxt_prec < inner_prec = pretty
  | otherwise              = parens pretty


Note [Defaulting RuntimeRep variables]

RuntimeRep variables are considered by many (most?) users to be little more than
syntactic noise. When the notion was introduced there was a signficant and
understandable push-back from those with pedagogy in mind, which argued that
RuntimeRep variables would throw a wrench into nearly any teach approach since
they appear in even the lowly ($) function's type,

    ($) :: forall (w :: RuntimeRep) a (b :: TYPE w). (a -> b) -> a -> b

which is significantly less readable than its non RuntimeRep-polymorphic type of

    ($) :: (a -> b) -> a -> b

Moreover, unboxed types don't appear all that often in run-of-the-mill Haskell
programs, so it makes little sense to make all users pay this syntactic

For this reason it was decided that we would hide RuntimeRep variables for now
(see #11549). We do this by defaulting all type variables of kind RuntimeRep to
PtrLiftedRep. This is done in a pass right before pretty-printing
(defaultRuntimeRepVars, controlled by -fprint-explicit-runtime-reps)

-- | Default 'RuntimeRep' variables to 'LiftedPtr'. e.g.
-- @
-- ($) :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r).
--        (a -> b) -> a -> b
-- @
-- turns in to,
-- @ ($) :: forall a (b :: *). (a -> b) -> a -> b @
-- We do this to prevent RuntimeRep variables from incurring a significant
-- syntactic overhead in otherwise simple type signatures (e.g. ($)). See
-- Note [Defaulting RuntimeRep variables] and #11549 for further discussion.
defaultRuntimeRepVars :: Type -> Type
defaultRuntimeRepVars = defaultRuntimeRepVars' emptyVarSet

defaultRuntimeRepVars' :: TyVarSet  -- ^ the binders which we should default
                       -> Type -> Type
-- TODO: Eventually we should just eliminate the Type pretty-printer
-- entirely and simply use IfaceType; this task is tracked as #11660.
defaultRuntimeRepVars' subs (ForAllTy (Named var vis) ty)
  | isRuntimeRepVar var                        =
    let subs' = extendVarSet subs var
    in defaultRuntimeRepVars' subs' ty
  | otherwise                                  =
    let var' = var { varType = defaultRuntimeRepVars' subs (varType var) }
    in ForAllTy (Named var' vis) (defaultRuntimeRepVars' subs ty)

defaultRuntimeRepVars' subs (ForAllTy (Anon kind) ty) =
    ForAllTy (Anon $ defaultRuntimeRepVars' subs kind)
             (defaultRuntimeRepVars' subs ty)

defaultRuntimeRepVars' subs (TyVarTy var)
  | var `elemVarSet` subs                      = ptrRepLiftedTy

defaultRuntimeRepVars' subs (TyConApp tc args) =
    TyConApp tc $ map (defaultRuntimeRepVars' subs) args

defaultRuntimeRepVars' subs (AppTy x y)        =
    defaultRuntimeRepVars' subs x `AppTy` defaultRuntimeRepVars' subs y

defaultRuntimeRepVars' subs (CastTy ty co)     =
    CastTy (defaultRuntimeRepVars' subs ty) co

defaultRuntimeRepVars' _    other              = other

eliminateRuntimeRep :: (Type -> SDoc) -> Type -> SDoc
eliminateRuntimeRep f ty = sdocWithDynFlags $ \dflags ->
    if gopt Opt_PrintExplicitRuntimeReps dflags
      then f ty
      else f (defaultRuntimeRepVars ty)

pprType, pprParendType :: Type -> SDoc
pprType       ty = eliminateRuntimeRep (ppr_type TopPrec) ty
pprParendType ty = eliminateRuntimeRep (ppr_type TyConPrec) ty

pprTyLit :: TyLit -> SDoc
pprTyLit = ppr_tylit TopPrec

pprKind, pprParendKind :: Kind -> SDoc
pprKind       = pprType
pprParendKind = pprParendType

pprClassPred :: Class -> [Type] -> SDoc
pprClassPred clas tys = pprTypeApp (classTyCon clas) tys

pprTheta :: ThetaType -> SDoc
pprTheta [pred] = ppr_type TopPrec pred     -- I'm in two minds about this
pprTheta theta  = parens (sep (punctuate comma (map (ppr_type TopPrec) theta)))

pprThetaArrowTy :: ThetaType -> SDoc
pprThetaArrowTy []     = empty
pprThetaArrowTy [pred] = ppr_type TyOpPrec pred <+> darrow
                         -- TyOpPrec:  Num a     => a -> a  does not need parens
                         --      bug   (a :~: b) => a -> b  currently does
                         -- Trac # 9658
pprThetaArrowTy preds  = parens (fsep (punctuate comma (map (ppr_type TopPrec) preds)))
                            <+> darrow
    -- Notice 'fsep' here rather that 'sep', so that
    -- type contexts don't get displayed in a giant column
    -- Rather than
    --  instance (Eq a,
    --            Eq b,
    --            Eq c,
    --            Eq d,
    --            Eq e,
    --            Eq f,
    --            Eq g,
    --            Eq h,
    --            Eq i,
    --            Eq j,
    --            Eq k,
    --            Eq l) =>
    --           Eq (a, b, c, d, e, f, g, h, i, j, k, l)
    -- we get
    --  instance (Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i,
    --            Eq j, Eq k, Eq l) =>
    --           Eq (a, b, c, d, e, f, g, h, i, j, k, l)

instance Outputable Type where
    ppr ty = pprType ty

instance Outputable TyLit where
   ppr = pprTyLit

        -- OK, here's the main printer

ppr_type :: TyPrec -> Type -> SDoc
ppr_type _ (TyVarTy tv)       = ppr_tvar tv

ppr_type p (TyConApp tc tys)  = pprTyTcApp p tc tys
ppr_type p (LitTy l)          = ppr_tylit p l
ppr_type p ty@(ForAllTy {})   = ppr_forall_type p ty

ppr_type p (AppTy t1 t2)
  = if_print_coercions
      (case split_app_tys t1 [t2] of
          (CastTy head _, args) -> ppr_type p (mk_app_tys head args)
          _                     -> ppr_app_ty)
    ppr_app_ty = maybeParen p TyConPrec $
                 ppr_type FunPrec t1 <+> ppr_type TyConPrec t2

    split_app_tys (AppTy ty1 ty2) args = split_app_tys ty1 (ty2:args)
    split_app_tys head            args = (head, args)

    mk_app_tys (TyConApp tc tys1) tys2 = TyConApp tc (tys1 ++ tys2)
    mk_app_tys ty1                tys2 = foldl AppTy ty1 tys2

ppr_type p (CastTy ty co)
  = if_print_coercions
      (parens (ppr_type TopPrec ty <+> text "|>" <+> ppr co))
      (ppr_type p ty)

ppr_type _ (CoercionTy co)
  = if_print_coercions
      (parens (ppr co))
      (text "<>")

ppr_forall_type :: TyPrec -> Type -> SDoc
ppr_forall_type p ty
  = maybeParen p FunPrec $
    sdocWithDynFlags $ \dflags ->
    ppr_sigma_type dflags True ty
    -- True <=> we always print the foralls on *nested* quantifiers
    -- Opt_PrintExplicitForalls only affects top-level quantifiers

ppr_tvar :: TyVar -> SDoc
ppr_tvar tv  -- Note [Infix type variables]
  = parenSymOcc (getOccName tv) (ppr tv)

ppr_tylit :: TyPrec -> TyLit -> SDoc
ppr_tylit _ tl =
  case tl of
    NumTyLit n -> integer n
    StrTyLit s -> text (show s)

if_print_coercions :: SDoc  -- if printing coercions
                   -> SDoc  -- otherwise
                   -> SDoc
if_print_coercions yes no
  = sdocWithDynFlags $ \dflags ->
    getPprStyle $ \style ->
    if gopt Opt_PrintExplicitCoercions dflags
         || dumpStyle style || debugStyle style
    then yes
    else no

ppr_sigma_type :: DynFlags
               -> Bool -- ^ True <=> Show the foralls unconditionally
               -> Type -> SDoc
-- Suppose we have (forall a. Show a => forall b. a -> b). When we're not
-- printing foralls, we want to drop both the (forall a) and the (forall b).
-- This logic does so.
ppr_sigma_type dflags False orig_ty
  | not (gopt Opt_PrintExplicitForalls dflags)
  , all (isEmptyVarSet . tyCoVarsOfType . binderType) named
      -- See Note [When to print foralls]
  = sep [ pprThetaArrowTy (map binderType ctxt)
        , pprArrowChain TopPrec (ppr_fun_tail tau) ]
    (invis_bndrs, tau) = split [] orig_ty
    (named, ctxt)      = partition isNamedBinder invis_bndrs

    split acc (ForAllTy bndr ty) | isInvisibleBinder bndr = split (bndr:acc) ty
    split acc ty                                          = (reverse acc, ty)

ppr_sigma_type _ _ ty
  = sep [ pprForAll bndrs
        , pprThetaArrowTy ctxt
        , pprArrowChain TopPrec (ppr_fun_tail tau) ]
    (bndrs, rho) = split1 [] ty
    (ctxt, tau)  = split2 [] rho

    split1 bndrs (ForAllTy bndr@(Named {}) ty) = split1 (bndr:bndrs) ty
    split1 bndrs ty                            = (reverse bndrs, ty)

    split2 ps (ForAllTy (Anon ty1) ty2) | isPredTy ty1 = split2 (ty1:ps) ty2
    split2 ps ty                                       = (reverse ps, ty)

    -- We don't want to lose synonyms, so we mustn't use splitFunTys here.
ppr_fun_tail :: Type -> [SDoc]
ppr_fun_tail (ForAllTy (Anon ty1) ty2)
  | not (isPredTy ty1) = ppr_type FunPrec ty1 : ppr_fun_tail ty2
ppr_fun_tail other_ty = [ppr_type TopPrec other_ty]

pprSigmaType :: Type -> SDoc
-- Prints a top-level type for the user; in particular
-- top-level foralls are omitted unless you use -fprint-explicit-foralls
pprSigmaType ty = sdocWithDynFlags $ \dflags ->
    eliminateRuntimeRep (ppr_sigma_type dflags False) ty

pprUserForAll :: [TyBinder] -> SDoc
-- Print a user-level forall; see Note [When to print foralls]
pprUserForAll bndrs
  = sdocWithDynFlags $ \dflags ->
    ppWhen (any bndr_has_kind_var bndrs || gopt Opt_PrintExplicitForalls dflags) $
    pprForAll bndrs
    bndr_has_kind_var bndr
      = not (isEmptyVarSet (tyCoVarsOfType (binderType bndr)))

pprForAllImplicit :: [TyVar] -> SDoc
pprForAllImplicit tvs = pprForAll (zipWith Named tvs (repeat Specified))

-- | Render the "forall ... ." or "forall ... ->" bit of a type.
-- Do not pass in anonymous binders!
pprForAll :: [TyBinder] -> SDoc
pprForAll [] = empty
pprForAll bndrs@(Named _ vis : _)
  = add_separator (forAllLit <+> doc) <+> pprForAll bndrs'
    (bndrs', doc) = ppr_tv_bndrs bndrs vis

    add_separator stuff = case vis of
                            Visible   -> stuff <+> arrow
                            _inv      -> stuff <>  dot
pprForAll bndrs = pprPanic "pprForAll: anonymous binder" (ppr bndrs)

pprTvBndrs :: [TyVar] -> SDoc
pprTvBndrs tvs = sep (map pprTvBndr tvs)

-- | Render the ... in @(forall ... .)@ or @(forall ... ->)@.
-- Returns both the list of not-yet-rendered binders and the doc.
-- No anonymous binders here!
ppr_tv_bndrs :: [TyBinder]
             -> VisibilityFlag  -- ^ visibility of the first binder in the list
             -> ([TyBinder], SDoc)
ppr_tv_bndrs all_bndrs@(Named tv vis : bndrs) vis1
  | vis `sameVis` vis1 = let (bndrs', doc) = ppr_tv_bndrs bndrs vis1
                             pp_tv = sdocWithDynFlags $ \dflags ->
                                     if Invisible == vis &&
                                        gopt Opt_PrintExplicitForalls dflags
                                     then braces (pprTvBndrNoParens tv)
                                     else pprTvBndr tv
                         (bndrs', pp_tv <+> doc)
  | otherwise   = (all_bndrs, empty)
ppr_tv_bndrs [] _ = ([], empty)
ppr_tv_bndrs bndrs _ = pprPanic "ppr_tv_bndrs: anonymous binder" (ppr bndrs)

pprTvBndr :: TyVar -> SDoc
pprTvBndr tv
  | isLiftedTypeKind kind = ppr_tvar tv
  | otherwise             = parens (ppr_tvar tv <+> dcolon <+> pprKind kind)
               kind = tyVarKind tv

pprTvBndrNoParens :: TyVar -> SDoc
pprTvBndrNoParens tv
  | isLiftedTypeKind kind = ppr_tvar tv
  | otherwise             = ppr_tvar tv <+> dcolon <+> pprKind kind
               kind = tyVarKind tv

instance Outputable TyBinder where
  ppr (Named v Visible)   = ppr v
  ppr (Named v Specified) = char '@' <> ppr v
  ppr (Named v Invisible) = braces (ppr v)
  ppr (Anon ty)       = text "[anon]" <+> ppr ty

instance Outputable VisibilityFlag where
  ppr Visible   = text "[vis]"
  ppr Specified = text "[spec]"
  ppr Invisible = text "[invis]"

instance Outputable Coercion where -- defined here to avoid orphans
  ppr = pprCo
instance Outputable LeftOrRight where
  ppr CLeft    = text "Left"
  ppr CRight   = text "Right"

Note [When to print foralls]
Mostly we want to print top-level foralls when (and only when) the user specifies
-fprint-explicit-foralls.  But when kind polymorphism is at work, that suppresses
too much information; see Trac #9018.

So I'm trying out this rule: print explicit foralls if
  a) User specifies -fprint-explicit-foralls, or
  b) Any of the quantified type variables has a kind
     that mentions a kind variable

This catches common situations, such as a type siguature
     f :: m a
which means
      f :: forall k. forall (m :: k->*) (a :: k). m a
We really want to see both the "forall k" and the kind signatures
on m and a.  The latter comes from pprTvBndr.

Note [Infix type variables]
With TypeOperators you can say

   f :: (a ~> b) -> b

and the (~>) is considered a type variable.  However, the type
pretty-printer in this module will just see (a ~> b) as

   App (App (TyVarTy "~>") (TyVarTy "a")) (TyVarTy "b")

So it'll print the type in prefix form.  To avoid confusion we must
remember to parenthesise the operator, thus

   (~>) a b -> b

See Trac #2766.

pprDataCons :: TyCon -> SDoc
pprDataCons = sepWithVBars . fmap pprDataConWithArgs . tyConDataCons
    sepWithVBars [] = empty
    sepWithVBars docs = sep (punctuate (space <> vbar) docs)

pprDataConWithArgs :: DataCon -> SDoc
pprDataConWithArgs dc = sep [forAllDoc, thetaDoc, ppr dc <+> argsDoc]
    (_univ_tvs, _ex_tvs, eq_spec, theta, arg_tys, _res_ty) = dataConFullSig dc
    univ_bndrs = dataConUnivTyBinders dc
    ex_bndrs   = dataConExTyBinders dc
    forAllDoc = pprUserForAll $ (filterEqSpec eq_spec univ_bndrs ++ ex_bndrs)
    thetaDoc  = pprThetaArrowTy theta
    argsDoc   = hsep (fmap pprParendType arg_tys)

pprTypeApp :: TyCon -> [Type] -> SDoc
pprTypeApp tc tys = pprTyTcApp TopPrec tc tys
        -- We have to use ppr on the TyCon (not its name)
        -- so that we get promotion quotes in the right place

pprTyTcApp :: TyPrec -> TyCon -> [Type] -> SDoc
-- Used for types only; so that we can make a
-- special case for type-level lists
pprTyTcApp p tc tys
  | tc `hasKey` ipClassKey
  , [LitTy (StrTyLit n),ty] <- tys
  = maybeParen p FunPrec $
    char '?' <> ftext n <> text "::" <> ppr_type TopPrec ty

  | tc `hasKey` consDataConKey
  , [_kind,ty1,ty2] <- tys
  = sdocWithDynFlags $ \dflags ->
    if gopt Opt_PrintExplicitKinds dflags then ppr_deflt
                                          else pprTyList p ty1 ty2

  | not opt_PprStyle_Debug
  , tc `hasKey` errorMessageTypeErrorFamKey
  = text "(TypeError ...)"   -- Suppress detail unles you _really_ want to see

  | tc `hasKey` tYPETyConKey
  , [TyConApp ptr_rep []] <- tys
  , ptr_rep `hasKey` ptrRepLiftedDataConKey
  = unicodeSyntax (char '★') (char '*')

  | tc `hasKey` tYPETyConKey
  , [TyConApp ptr_rep []] <- tys
  , ptr_rep `hasKey` ptrRepUnliftedDataConKey
  = char '#'

  | otherwise
  = ppr_deflt
    ppr_deflt = pprTcAppTy p ppr_type tc tys

pprTcAppTy :: TyPrec -> (TyPrec -> Type -> SDoc) -> TyCon -> [Type] -> SDoc
pprTcAppTy p pp tc tys
  = getPprStyle $ \style -> pprTcApp style id p pp tc tys

pprTcAppCo :: TyPrec -> (TyPrec -> Coercion -> SDoc)
           -> TyCon -> [Coercion] -> SDoc
pprTcAppCo p pp tc cos
  = getPprStyle $ \style ->
    pprTcApp style (pFst . coercionKind) p pp tc cos

pprTcApp :: PprStyle
         -> (a -> Type) -> TyPrec -> (TyPrec -> a -> SDoc) -> TyCon -> [a] -> SDoc
-- Used for both types and coercions, hence polymorphism
pprTcApp _ _ _ pp tc [ty]
  | tc `hasKey` listTyConKey = pprPromotionQuote tc <> brackets   (pp TopPrec ty)
  | tc `hasKey` parrTyConKey = pprPromotionQuote tc <> paBrackets (pp TopPrec ty)

pprTcApp style to_type p pp tc tys
  | not (debugStyle style)
  , Just sort <- tyConTuple_maybe tc
  , let arity = tyConArity tc
  , arity == length tys
  , let num_to_drop = case sort of UnboxedTuple -> arity `div` 2
                                   _            -> 0
  = pprTupleApp p pp tc sort (drop num_to_drop tys)

  | not (debugStyle style)
  , Just dc <- isPromotedDataCon_maybe tc
  , let dc_tc = dataConTyCon dc
  , Just tup_sort <- tyConTuple_maybe dc_tc
  , let arity = tyConArity dc_tc    -- E.g. 3 for (,,) k1 k2 k3 t1 t2 t3
        ty_args = drop arity tys    -- Drop the kind args
  , ty_args `lengthIs` arity        -- Result is saturated
  = pprPromotionQuote tc <>
    (tupleParens tup_sort $ pprWithCommas (pp TopPrec) ty_args)

  | otherwise
  = sdocWithDynFlags $ \dflags ->
    pprTcApp_help to_type p pp tc tys dflags style

pprTupleApp :: TyPrec -> (TyPrec -> a -> SDoc)
            -> TyCon -> TupleSort -> [a] -> SDoc
-- Print a saturated tuple
pprTupleApp p pp tc sort tys
  | null tys
  , ConstraintTuple <- sort
  = if opt_PprStyle_Debug then text "(%%)"
                          else maybeParen p FunPrec $
                               text "() :: Constraint"
  | otherwise
  = pprPromotionQuote tc <>
    tupleParens sort (pprWithCommas (pp TopPrec) tys)

pprTcApp_help :: (a -> Type) -> TyPrec -> (TyPrec -> a -> SDoc)
              -> TyCon -> [a] -> DynFlags -> PprStyle -> SDoc
-- This one has accss to the DynFlags
pprTcApp_help to_type p pp tc tys dflags style
  | not (isSymOcc (nameOccName tc_name)) -- Print prefix
  = pprPrefixApp p pp_tc (map (pp TyConPrec) tys_wo_kinds)

  | Just args <- mb_saturated_equality
  = print_equality args

  -- So we have an operator symbol of some kind

  | [ty1,ty2] <- tys_wo_kinds  -- Infix, two arguments;
                               -- we know nothing of precedence though
  = pprInfixApp p pp pp_tc ty1 ty2

  |  tc_name `hasKey` starKindTyConKey
  || tc_name `hasKey` unicodeStarKindTyConKey
  || tc_name `hasKey` unliftedTypeKindTyConKey
  = pp_tc   -- Do not wrap *, # in parens

  | otherwise  -- Unsaturated operator
  = pprPrefixApp p (parens (pp_tc)) (map (pp TyConPrec) tys_wo_kinds)
    tc_name      = tyConName tc
    pp_tc        = ppr tc
    tys_wo_kinds = suppressInvisibles to_type dflags tc tys

      | hetero_eq_tc
      , [k1, k2, t1, t2] <- tys
      = Just (k1, k2, t1, t2)
      | homo_eq_tc
      , [k, t1, t2] <- tys  -- we must have (~)
      = Just (k, k, t1, t2)
      | otherwise
      = Nothing

    homo_eq_tc   =  tc `hasKey` eqTyConKey             -- ~
    hetero_eq_tc =  tc `hasKey` eqPrimTyConKey         -- ~#
                 || tc `hasKey` eqReprPrimTyConKey     -- ~R#
                 || tc `hasKey` heqTyConKey            -- ~~

      -- This is all a bit ad-hoc, trying to print out the best representation
      -- of equalities. If you see a better design, go for it.

    print_equality (ki1, ki2, ty1, ty2)
      | print_eqs
      = ppr_infix_eq pp_tc

      | hetero_eq_tc
      , print_kinds || not (to_type ki1 `eqType` to_type ki2)
      = ppr_infix_eq $ if tc `hasKey` eqPrimTyConKey
                       then text "~~"
                       else pp_tc

      | otherwise
      = if tc `hasKey` eqReprPrimTyConKey
        then text "Coercible" <+> (sep [ pp TyConPrec ty1
                                       , pp TyConPrec ty2 ])
        else sep [pp TyOpPrec ty1, text "~", pp TyOpPrec ty2]

        ppr_infix_eq eq_op
           = sep [ parens (pp TyOpPrec ty1 <+> dcolon <+> pp TyOpPrec ki1)
                 , eq_op
                 , parens (pp TyOpPrec ty2 <+> dcolon <+> pp TyOpPrec ki2)]

    print_kinds = gopt Opt_PrintExplicitKinds dflags
    print_eqs   = gopt Opt_PrintEqualityRelations dflags ||
                  dumpStyle style ||
                  debugStyle style

-- | Given a 'TyCon',and the args to which it is applied,
-- suppress the args that are implicit
suppressInvisibles :: (a -> Type) -> DynFlags -> TyCon -> [a] -> [a]
suppressInvisibles to_type dflags tc xs
  | gopt Opt_PrintExplicitKinds dflags = xs
  | otherwise                          = snd $ partitionInvisibles tc to_type xs

pprTyList :: TyPrec -> Type -> Type -> SDoc
-- Given a type-level list (t1 ': t2), see if we can print
-- it in list notation [t1, ...].
pprTyList p ty1 ty2
  = case gather ty2 of
      (arg_tys, Nothing) -> char '\'' <> brackets (fsep (punctuate comma
                                            (map (ppr_type TopPrec) (ty1:arg_tys))))
      (arg_tys, Just tl) -> maybeParen p FunPrec $
                            hang (ppr_type FunPrec ty1)
                               2 (fsep [ colon <+> ppr_type FunPrec ty | ty <- arg_tys ++ [tl]])
    gather :: Type -> ([Type], Maybe Type)
     -- (gather ty) = (tys, Nothing) means ty is a list [t1, .., tn]
     --             = (tys, Just tl) means ty is of form t1:t2:...tn:tl
    gather (TyConApp tc tys)
      | tc `hasKey` consDataConKey
      , [_kind, ty1,ty2] <- tys
      , (args, tl) <- gather ty2
      = (ty1:args, tl)
      | tc `hasKey` nilDataConKey
      = ([], Nothing)
    gather ty = ([], Just ty)

pprInfixApp :: TyPrec -> (TyPrec -> a -> SDoc) -> SDoc -> a -> a -> SDoc
pprInfixApp p pp pp_tc ty1 ty2
  = maybeParen p TyOpPrec $
    sep [pp TyOpPrec ty1, pprInfixVar True pp_tc <+> pp TyOpPrec ty2]

pprPrefixApp :: TyPrec -> SDoc -> [SDoc] -> SDoc
pprPrefixApp p pp_fun pp_tys
  | null pp_tys = pp_fun
  | otherwise   = maybeParen p TyConPrec $
                  hang pp_fun 2 (sep pp_tys)
pprArrowChain :: TyPrec -> [SDoc] -> SDoc
-- pprArrowChain p [a,b,c]  generates   a -> b -> c
pprArrowChain _ []         = empty
pprArrowChain p (arg:args) = maybeParen p FunPrec $
                             sep [arg, sep (map (arrow <+>) args)]

%*                                                                      *
%*                                                                      *

-- | This tidies up a type for printing in an error message, or in
-- an interface file.
-- It doesn't change the uniques at all, just the print names.
tidyTyCoVarBndrs :: TidyEnv -> [TyCoVar] -> (TidyEnv, [TyCoVar])
tidyTyCoVarBndrs env tvs = mapAccumL tidyTyCoVarBndr env tvs

tidyTyCoVarBndr :: TidyEnv -> TyCoVar -> (TidyEnv, TyCoVar)
tidyTyCoVarBndr tidy_env@(occ_env, subst) tyvar
  = case tidyOccName occ_env occ1 of
      (tidy', occ') -> ((tidy', subst'), tyvar')
          subst' = extendVarEnv subst tyvar tyvar'
          tyvar' = setTyVarKind (setTyVarName tyvar name') kind'
          name'  = tidyNameOcc name occ'
          kind'  = tidyKind tidy_env (tyVarKind tyvar)
    name = tyVarName tyvar
    occ  = getOccName name
    -- System Names are for unification variables;
    -- when we tidy them we give them a trailing "0" (or 1 etc)
    -- so that they don't take precedence for the un-modified name
    -- Plus, indicating a unification variable in this way is a
    -- helpful clue for users
    occ1 | isSystemName name
         = if isTyVar tyvar
           then mkTyVarOcc (occNameString occ ++ "0")
           else mkVarOcc   (occNameString occ ++ "0")
         | otherwise         = occ

tidyTyBinder :: TidyEnv -> TyBinder -> (TidyEnv, TyBinder)
tidyTyBinder tidy_env (Named tv vis)
  = (tidy_env', Named tv' vis)
    (tidy_env', tv') = tidyTyCoVarBndr tidy_env tv
tidyTyBinder tidy_env (Anon ty)
  = (tidy_env, Anon $ tidyType tidy_env ty)

tidyTyBinders :: TidyEnv -> [TyBinder] -> (TidyEnv, [TyBinder])
tidyTyBinders = mapAccumL tidyTyBinder

tidyFreeTyCoVars :: TidyEnv -> [TyCoVar] -> TidyEnv
-- ^ Add the free 'TyVar's to the env in tidy form,
-- so that we can tidy the type they are free in
tidyFreeTyCoVars (full_occ_env, var_env) tyvars
  = fst (tidyOpenTyCoVars (full_occ_env, var_env) tyvars)

tidyOpenTyCoVars :: TidyEnv -> [TyCoVar] -> (TidyEnv, [TyCoVar])
tidyOpenTyCoVars env tyvars = mapAccumL tidyOpenTyCoVar env tyvars

tidyOpenTyCoVar :: TidyEnv -> TyCoVar -> (TidyEnv, TyCoVar)
-- ^ Treat a new 'TyCoVar' as a binder, and give it a fresh tidy name
-- using the environment if one has not already been allocated. See
-- also 'tidyTyCoVarBndr'
tidyOpenTyCoVar env@(_, subst) tyvar
  = case lookupVarEnv subst tyvar of
        Just tyvar' -> (env, tyvar')              -- Already substituted
        Nothing     ->
          let env' = tidyFreeTyCoVars env (tyCoVarsOfTypeList (tyVarKind tyvar))
          in tidyTyCoVarBndr env' tyvar  -- Treat it as a binder

tidyTyVarOcc :: TidyEnv -> TyVar -> TyVar
tidyTyVarOcc env@(_, subst) tv
  = case lookupVarEnv subst tv of
        Nothing  -> updateTyVarKind (tidyType env) tv
        Just tv' -> tv'

tidyTypes :: TidyEnv -> [Type] -> [Type]
tidyTypes env tys = map (tidyType env) tys

tidyType :: TidyEnv -> Type -> Type
tidyType _   (LitTy n)            = LitTy n
tidyType env (TyVarTy tv)         = TyVarTy (tidyTyVarOcc env tv)
tidyType env (TyConApp tycon tys) = let args = tidyTypes env tys
                                    in args `seqList` TyConApp tycon args
tidyType env (AppTy fun arg)      = (AppTy $! (tidyType env fun)) $! (tidyType env arg)
tidyType env (ForAllTy (Anon fun) arg)
  = (ForAllTy $! (Anon $! (tidyType env fun))) $! (tidyType env arg)
tidyType env (ForAllTy (Named tv vis) ty)
  = (ForAllTy $! ((Named $! tvp) $! vis)) $! (tidyType envp ty)
    (envp, tvp) = tidyTyCoVarBndr env tv
tidyType env (CastTy ty co)       = (CastTy $! tidyType env ty) $! (tidyCo env co)
tidyType env (CoercionTy co)      = CoercionTy $! (tidyCo env co)

-- | Grabs the free type variables, tidies them
-- and then uses 'tidyType' to work over the type itself
tidyOpenTypes :: TidyEnv -> [Type] -> (TidyEnv, [Type])
tidyOpenTypes env tys
  = (env', tidyTypes (trimmed_occ_env, var_env) tys)
    (env'@(_, var_env), tvs') = tidyOpenTyCoVars env $
                                tyCoVarsOfTypesWellScoped tys
    trimmed_occ_env = initTidyOccEnv (map getOccName tvs')
      -- The idea here was that we restrict the new TidyEnv to the
      -- _free_ vars of the types, so that we don't gratuitously rename
      -- the _bound_ variables of the types.

tidyOpenType :: TidyEnv -> Type -> (TidyEnv, Type)
tidyOpenType env ty = let (env', [ty']) = tidyOpenTypes env [ty] in
                      (env', ty')

-- | Calls 'tidyType' on a top-level type (i.e. with an empty tidying environment)
tidyTopType :: Type -> Type
tidyTopType ty = tidyType emptyTidyEnv ty

tidyOpenKind :: TidyEnv -> Kind -> (TidyEnv, Kind)
tidyOpenKind = tidyOpenType

tidyKind :: TidyEnv -> Kind -> Kind
tidyKind = tidyType

tidyCo :: TidyEnv -> Coercion -> Coercion
tidyCo env@(_, subst) co
  = go co
    go (Refl r ty)           = Refl r (tidyType env ty)
    go (TyConAppCo r tc cos) = let args = map go cos
                               in args `seqList` TyConAppCo r tc args
    go (AppCo co1 co2)       = (AppCo $! go co1) $! go co2
    go (ForAllCo tv h co)    = ((ForAllCo $! tvp) $! (go h)) $! (tidyCo envp co)
                               where (envp, tvp) = tidyTyCoVarBndr env tv
            -- the case above duplicates a bit of work in tidying h and the kind
            -- of tv. But the alternative is to use coercionKind, which seems worse.
    go (CoVarCo cv)          = case lookupVarEnv subst cv of
                                 Nothing  -> CoVarCo cv
                                 Just cv' -> CoVarCo cv'
    go (AxiomInstCo con ind cos) = let args = map go cos
                               in  args `seqList` AxiomInstCo con ind args
    go (UnivCo p r t1 t2)    = (((UnivCo $! (go_prov p)) $! r) $!
                                tidyType env t1) $! tidyType env t2
    go (SymCo co)            = SymCo $! go co
    go (TransCo co1 co2)     = (TransCo $! go co1) $! go co2
    go (NthCo d co)          = NthCo d $! go co
    go (LRCo lr co)          = LRCo lr $! go co
    go (InstCo co ty)        = (InstCo $! go co) $! go ty
    go (CoherenceCo co1 co2) = (CoherenceCo $! go co1) $! go co2
    go (KindCo co)           = KindCo $! go co
    go (SubCo co)            = SubCo $! go co
    go (AxiomRuleCo ax cos)  = let cos1 = tidyCos env cos
                               in cos1 `seqList` AxiomRuleCo ax cos1

    go_prov UnsafeCoerceProv    = UnsafeCoerceProv
    go_prov (PhantomProv co)    = PhantomProv (go co)
    go_prov (ProofIrrelProv co) = ProofIrrelProv (go co)
    go_prov p@(PluginProv _)    = p
    go_prov p@(HoleProv _)      = p

tidyCos :: TidyEnv -> [Coercion] -> [Coercion]
tidyCos env = map (tidyCo env)