%
% (c) The University of Glasgow 2006
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
Handles @deriving@ clauses on @data@ declarations.
\begin{code}
module TcDeriv ( tcDeriving ) where
#include "HsVersions.h"
import HsSyn
import DynFlags
import TcRnMonad
import FamInst
import TcErrors( reportAllUnsolved )
import TcValidity( validDerivPred )
import TcEnv
import TcTyClsDecls( tcFamTyPats, tcAddDataFamInstCtxt )
import TcClassDcl( tcAddDeclCtxt )
import TcGenDeriv
import TcGenGenerics
import InstEnv
import Inst
import FamInstEnv
import TcHsType
import TcMType
import TcSimplify
import RnNames( extendGlobalRdrEnvRn )
import RnBinds
import RnEnv
import RnSource ( addTcgDUs )
import HscTypes
import Avail
import Unify( tcUnifyTy )
import Id( idType )
import Class
import Type
import Kind( isKind )
import ErrUtils
import MkId
import DataCon
import Maybes
import RdrName
import Name
import NameSet
import TyCon
import TcType
import Var
import VarSet
import PrelNames
import SrcLoc
import Util
import ListSetOps
import Outputable
import FastString
import Bag
import Pair
import Control.Monad
import Data.List
\end{code}
%************************************************************************
%* *
Overview
%* *
%************************************************************************
Overall plan
~~~~~~~~~~~~
1. Convert the decls (i.e. data/newtype deriving clauses,
plus standalone deriving) to [EarlyDerivSpec]
2. Infer the missing contexts for the InferTheta's
3. Add the derived bindings, generating InstInfos
\begin{code}
data DerivSpec theta = DS { ds_loc :: SrcSpan
, ds_name :: Name
, ds_tvs :: [TyVar]
, ds_theta :: theta
, ds_cls :: Class
, ds_tys :: [Type]
, ds_tc :: TyCon
, ds_tc_args :: [Type]
, ds_newtype :: Bool }
\end{code}
Example:
newtype instance T [a] = MkT (Tree a) deriving( C s )
==>
axiom T [a] = :RTList a
axiom :RTList a = Tree a
DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]]
, ds_tc = :RTList, ds_tc_args = [a]
, ds_newtype = True }
\begin{code}
type DerivContext = Maybe ThetaType
data PredOrigin = PredOrigin PredType CtOrigin
type ThetaOrigin = [PredOrigin]
mkPredOrigin :: CtOrigin -> PredType -> PredOrigin
mkPredOrigin origin pred = PredOrigin pred origin
mkThetaOrigin :: CtOrigin -> ThetaType -> ThetaOrigin
mkThetaOrigin origin = map (mkPredOrigin origin)
data EarlyDerivSpec = InferTheta (DerivSpec ThetaOrigin)
| GivenTheta (DerivSpec ThetaType)
forgetTheta :: EarlyDerivSpec -> DerivSpec ()
forgetTheta (InferTheta spec) = spec { ds_theta = () }
forgetTheta (GivenTheta spec) = spec { ds_theta = () }
earlyDSTyCon :: EarlyDerivSpec -> TyCon
earlyDSTyCon (InferTheta spec) = ds_tc spec
earlyDSTyCon (GivenTheta spec) = ds_tc spec
earlyDSLoc :: EarlyDerivSpec -> SrcSpan
earlyDSLoc (InferTheta spec) = ds_loc spec
earlyDSLoc (GivenTheta spec) = ds_loc spec
splitEarlyDerivSpec :: [EarlyDerivSpec] -> ([DerivSpec ThetaOrigin], [DerivSpec ThetaType])
splitEarlyDerivSpec [] = ([],[])
splitEarlyDerivSpec (InferTheta spec : specs) =
case splitEarlyDerivSpec specs of (is, gs) -> (spec : is, gs)
splitEarlyDerivSpec (GivenTheta spec : specs) =
case splitEarlyDerivSpec specs of (is, gs) -> (is, spec : gs)
pprDerivSpec :: Outputable theta => DerivSpec theta -> SDoc
pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
ds_cls = c, ds_tys = tys, ds_theta = rhs })
= parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
<+> equals <+> ppr rhs)
instance Outputable theta => Outputable (DerivSpec theta) where
ppr = pprDerivSpec
instance Outputable EarlyDerivSpec where
ppr (InferTheta spec) = ppr spec <+> ptext (sLit "(Infer)")
ppr (GivenTheta spec) = ppr spec <+> ptext (sLit "(Given)")
instance Outputable PredOrigin where
ppr (PredOrigin ty _) = ppr ty
\end{code}
Inferring missing contexts
~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
data T a b = C1 (Foo a) (Bar b)
| C2 Int (T b a)
| C3 (T a a)
deriving (Eq)
[NOTE: See end of these comments for what to do with
data (C a, D b) => T a b = ...
]
We want to come up with an instance declaration of the form
instance (Ping a, Pong b, ...) => Eq (T a b) where
x == y = ...
It is pretty easy, albeit tedious, to fill in the code "...". The
trick is to figure out what the context for the instance decl is,
namely @Ping@, @Pong@ and friends.
Let's call the context reqd for the T instance of class C at types
(a,b, ...) C (T a b). Thus:
Eq (T a b) = (Ping a, Pong b, ...)
Now we can get a (recursive) equation from the @data@ decl:
Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
u Eq (T b a) u Eq Int -- From C2
u Eq (T a a) -- From C3
Foo and Bar may have explicit instances for @Eq@, in which case we can
just substitute for them. Alternatively, either or both may have
their @Eq@ instances given by @deriving@ clauses, in which case they
form part of the system of equations.
Now all we need do is simplify and solve the equations, iterating to
find the least fixpoint. Notice that the order of the arguments can
switch around, as here in the recursive calls to T.
Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b.
We start with:
Eq (T a b) = {} -- The empty set
Next iteration:
Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
u Eq (T b a) u Eq Int -- From C2
u Eq (T a a) -- From C3
After simplification:
= Eq a u Ping b u {} u {} u {}
= Eq a u Ping b
Next iteration:
Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1
u Eq (T b a) u Eq Int -- From C2
u Eq (T a a) -- From C3
After simplification:
= Eq a u Ping b
u (Eq b u Ping a)
u (Eq a u Ping a)
= Eq a u Ping b u Eq b u Ping a
The next iteration gives the same result, so this is the fixpoint. We
need to make a canonical form of the RHS to ensure convergence. We do
this by simplifying the RHS to a form in which
- the classes constrain only tyvars
- the list is sorted by tyvar (major key) and then class (minor key)
- no duplicates, of course
So, here are the synonyms for the ``equation'' structures:
Note [Data decl contexts]
~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
data (RealFloat a) => Complex a = !a :+ !a deriving( Read )
We will need an instance decl like:
instance (Read a, RealFloat a) => Read (Complex a) where
...
The RealFloat in the context is because the read method for Complex is bound
to construct a Complex, and doing that requires that the argument type is
in RealFloat.
But this ain't true for Show, Eq, Ord, etc, since they don't construct
a Complex; they only take them apart.
Our approach: identify the offending classes, and add the data type
context to the instance decl. The "offending classes" are
Read, Enum?
FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that
pattern matching against a constructor from a data type with a context
gives rise to the constraints for that context -- or at least the thinned
version. So now all classes are "offending".
Note [Newtype deriving]
~~~~~~~~~~~~~~~~~~~~~~~
Consider this:
class C a b
instance C [a] Char
newtype T = T Char deriving( C [a] )
Notice the free 'a' in the deriving. We have to fill this out to
newtype T = T Char deriving( forall a. C [a] )
And then translate it to:
instance C [a] Char => C [a] T where ...
Note [Newtype deriving superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(See also Trac #1220 for an interesting exchange on newtype
deriving and superclasses.)
The 'tys' here come from the partial application in the deriving
clause. The last arg is the new instance type.
We must pass the superclasses; the newtype might be an instance
of them in a different way than the representation type
E.g. newtype Foo a = Foo a deriving( Show, Num, Eq )
Then the Show instance is not done via Coercible; it shows
Foo 3 as "Foo 3"
The Num instance is derived via Coercible, but the Show superclass
dictionary must the Show instance for Foo, *not* the Show dictionary
gotten from the Num dictionary. So we must build a whole new dictionary
not just use the Num one. The instance we want is something like:
instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where
(+) = ((+)@a)
...etc...
There may be a coercion needed which we get from the tycon for the newtype
when the dict is constructed in TcInstDcls.tcInstDecl2
Note [Unused constructors and deriving clauses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
See Trac #3221. Consider
data T = T1 | T2 deriving( Show )
Are T1 and T2 unused? Well, no: the deriving clause expands to mention
both of them. So we gather defs/uses from deriving just like anything else.
%************************************************************************
%* *
\subsection[TcDeriv-driver]{Top-level function for \tr{derivings}}
%* *
%************************************************************************
\begin{code}
tcDeriving :: [LTyClDecl Name]
-> [LInstDecl Name]
-> [LDerivDecl Name]
-> TcM (TcGblEnv, Bag (InstInfo Name), HsValBinds Name)
tcDeriving tycl_decls inst_decls deriv_decls
= recoverM (do { g <- getGblEnv
; return (g, emptyBag, emptyValBindsOut)}) $
do {
is_boot <- tcIsHsBoot
; traceTc "tcDeriving" (ppr is_boot)
; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
; traceTc "tcDeriving 1" (ppr early_specs)
; (commonAuxs, auxDerivStuff) <- commonAuxiliaries $ map forgetTheta early_specs
; overlap_flag <- getOverlapFlag
; let (infer_specs, given_specs) = splitEarlyDerivSpec early_specs
; insts1 <- mapM (genInst True overlap_flag commonAuxs) given_specs
; final_specs <- extendLocalInstEnv (map (iSpec . fstOf3) insts1) $
inferInstanceContexts overlap_flag infer_specs
; insts2 <- mapM (genInst False overlap_flag commonAuxs) final_specs
; let (inst_infos, deriv_stuff, maybe_fvs) = unzip3 (insts1 ++ insts2)
; loc <- getSrcSpanM
; let (binds, newTyCons, famInsts, extraInstances) =
genAuxBinds loc (unionManyBags (auxDerivStuff : deriv_stuff))
; (inst_info, rn_binds, rn_dus) <-
renameDeriv is_boot (inst_infos ++ (bagToList extraInstances)) binds
; dflags <- getDynFlags
; unless (isEmptyBag inst_info) $
liftIO (dumpIfSet_dyn dflags Opt_D_dump_deriv "Derived instances"
(ddump_deriving inst_info rn_binds newTyCons famInsts))
; let all_tycons = map ATyCon (bagToList newTyCons)
; gbl_env <- tcExtendGlobalEnv all_tycons $
tcExtendGlobalEnvImplicit (concatMap implicitTyThings all_tycons) $
tcExtendLocalFamInstEnv (bagToList famInsts) $
tcExtendLocalInstEnv (map iSpec (bagToList inst_info)) getGblEnv
; let all_dus = rn_dus `plusDU` usesOnly (mkFVs $ catMaybes maybe_fvs)
; return (addTcgDUs gbl_env all_dus, inst_info, rn_binds) }
where
ddump_deriving :: Bag (InstInfo Name) -> HsValBinds Name
-> Bag TyCon
-> Bag (FamInst)
-> SDoc
ddump_deriving inst_infos extra_binds repMetaTys repFamInsts
= hang (ptext (sLit "Derived instances:"))
2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") (bagToList inst_infos))
$$ ppr extra_binds)
$$ hangP "Generic representation:" (
hangP "Generated datatypes for meta-information:"
(vcat (map ppr (bagToList repMetaTys)))
$$ hangP "Representation types:"
(vcat (map pprRepTy (bagToList repFamInsts))))
hangP s x = text "" $$ hang (ptext (sLit s)) 2 x
pprRepTy :: FamInst -> SDoc
pprRepTy fi@(FamInst { fi_tys = lhs })
= ptext (sLit "type") <+> ppr (mkTyConApp (famInstTyCon fi) lhs) <+>
equals <+> ppr rhs
where rhs = famInstRHS fi
type CommonAuxiliary = MetaTyCons
type CommonAuxiliaries = [(TyCon, CommonAuxiliary)]
commonAuxiliaries :: [DerivSpec ()] -> TcM (CommonAuxiliaries, BagDerivStuff)
commonAuxiliaries = foldM snoc ([], emptyBag) where
snoc acc@(cas, stuff) (DS {ds_name = nm, ds_cls = cls, ds_tc = rep_tycon})
| getUnique cls `elem` [genClassKey, gen1ClassKey] =
extendComAux $ genGenericMetaTyCons rep_tycon (nameModule nm)
| otherwise = return acc
where extendComAux m
| any ((rep_tycon ==) . fst) cas = return acc
| otherwise = do (ca, new_stuff) <- m
return $ ((rep_tycon, ca) : cas, stuff `unionBags` new_stuff)
renameDeriv :: Bool
-> [InstInfo RdrName]
-> Bag (LHsBind RdrName, LSig RdrName)
-> TcM (Bag (InstInfo Name), HsValBinds Name, DefUses)
renameDeriv is_boot inst_infos bagBinds
| is_boot
= do { (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
; return ( listToBag rn_inst_infos
, emptyValBindsOut, usesOnly (plusFVs fvs)) }
| otherwise
= discardWarnings $
setXOptM Opt_EmptyCase $
setXOptM Opt_ScopedTypeVariables $
setXOptM Opt_KindSignatures $
do {
; (aux_binds, aux_sigs) <- mapAndUnzipBagM return bagBinds
; let aux_val_binds = ValBindsIn aux_binds (bagToList aux_sigs)
; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
; let bndrs = collectHsValBinders rn_aux_lhs
; envs <- extendGlobalRdrEnvRn (map Avail bndrs) emptyFsEnv ;
; setEnvs envs $
do { (rn_aux, dus_aux) <- rnValBindsRHS (TopSigCtxt (mkNameSet bndrs) False) rn_aux_lhs
; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
; return (listToBag rn_inst_infos, rn_aux,
dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }
where
rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
rn_inst_info
inst_info@(InstInfo { iSpec = inst
, iBinds = InstBindings
{ ib_binds = binds
, ib_pragmas = sigs
, ib_extensions = exts
, ib_standalone_deriving = sa } })
=
ASSERT( null sigs )
bindLocalNamesFV (map Var.varName tyvars) $
do { (rn_binds, fvs) <- rnMethodBinds (is_cls_nm inst) (\_ -> []) binds
; let binds' = InstBindings { ib_binds = rn_binds
, ib_pragmas = []
, ib_extensions = exts
, ib_standalone_deriving = sa }
; return (inst_info { iBinds = binds' }, fvs) }
where
(tyvars, _) = tcSplitForAllTys (idType (instanceDFunId inst))
\end{code}
Note [Newtype deriving and unused constructors]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this (see Trac #1954):
module Bug(P) where
newtype P a = MkP (IO a) deriving Monad
If you compile with -fwarn-unused-binds you do not expect the warning
"Defined but not used: data consructor MkP". Yet the newtype deriving
code does not explicitly mention MkP, but it should behave as if you
had written
instance Monad P where
return x = MkP (return x)
...etc...
So we want to signal a user of the data constructor 'MkP'.
This is the reason behind the (Maybe Name) part of the return type
of genInst.
%************************************************************************
%* *
From HsSyn to DerivSpec
%* *
%************************************************************************
@makeDerivSpecs@ fishes around to find the info about needed derived instances.
\begin{code}
makeDerivSpecs :: Bool
-> [LTyClDecl Name]
-> [LInstDecl Name]
-> [LDerivDecl Name]
-> TcM [EarlyDerivSpec]
makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
= do { eqns1 <- concatMapM (recoverM (return []) . deriveTyDecl) tycl_decls
; eqns2 <- concatMapM (recoverM (return []) . deriveInstDecl) inst_decls
; eqns3 <- mapAndRecoverM deriveStandalone deriv_decls
; let eqns = eqns1 ++ eqns2 ++ eqns3
; isAutoTypeable <- xoptM Opt_AutoDeriveTypeable
; let eqns4 = if isAutoTypeable then deriveTypeable tycl_decls eqns else []
; eqns4' <- mapAndRecoverM deriveStandalone eqns4
; let eqns' = eqns ++ eqns4'
; if is_boot then
do { unless (null eqns') (add_deriv_err (head eqns'))
; return [] }
else return eqns' }
where
deriveTypeable :: [LTyClDecl Name] -> [EarlyDerivSpec] -> [LDerivDecl Name]
deriveTypeable tys dss =
[ L l (DerivDecl (L l (HsAppTy (noLoc (HsTyVar typeableClassName))
(L l (HsTyVar (tcdName t))))))
| L l t <- tys
, not (isSynDecl t), not (isTypeFamilyDecl t)
, not (hasInstance (tcdName t) dss) ]
hasInstance :: Name -> [EarlyDerivSpec] -> Bool
hasInstance n = any (\ds -> n == tyConName (earlyDSTyCon ds))
add_deriv_err eqn
= setSrcSpan (earlyDSLoc eqn) $
addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
2 (ptext (sLit "Use an instance declaration instead")))
deriveTyDecl :: LTyClDecl Name -> TcM [EarlyDerivSpec]
deriveTyDecl (L _ decl@(DataDecl { tcdLName = L loc tc_name
, tcdDataDefn = HsDataDefn { dd_derivs = preds } }))
= tcAddDeclCtxt decl $
do { tc <- tcLookupTyCon tc_name
; let tvs = tyConTyVars tc
tys = mkTyVarTys tvs
pdcs :: [LDerivDecl Name]
pdcs = [ L loc (DerivDecl (L loc (HsAppTy (noLoc (HsTyVar typeableClassName))
(L loc (HsTyVar (tyConName pdc))))))
| Just pdc <- map promoteDataCon_maybe (tyConDataCons tc) ]
; isAutoTypeable <- xoptM Opt_AutoDeriveTypeable
; isDataKinds <- xoptM Opt_DataKinds
; prom_dcs_Typeable_instances <- if isAutoTypeable && isDataKinds
then mapM deriveStandalone pdcs
else return []
; other_instances <- case preds of
Just preds' -> mapM (deriveTyData tvs tc tys) preds'
Nothing -> return []
; return (prom_dcs_Typeable_instances ++ other_instances) }
deriveTyDecl _ = return []
deriveInstDecl :: LInstDecl Name -> TcM [EarlyDerivSpec]
deriveInstDecl (L _ (TyFamInstD {})) = return []
deriveInstDecl (L _ (DataFamInstD { dfid_inst = fam_inst }))
= deriveFamInst fam_inst
deriveInstDecl (L _ (ClsInstD { cid_inst = ClsInstDecl { cid_datafam_insts = fam_insts } }))
= concatMapM (deriveFamInst . unLoc) fam_insts
deriveFamInst :: DataFamInstDecl Name -> TcM [EarlyDerivSpec]
deriveFamInst decl@(DataFamInstDecl { dfid_tycon = L _ tc_name, dfid_pats = pats
, dfid_defn = HsDataDefn { dd_derivs = Just preds } })
= tcAddDataFamInstCtxt decl $
do { fam_tc <- tcLookupTyCon tc_name
; tcFamTyPats tc_name (tyConKind fam_tc) pats (\_ -> return ()) $
\ tvs' pats' _ ->
mapM (deriveTyData tvs' fam_tc pats') preds }
deriveFamInst _ = return []
deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
deriveStandalone (L loc (DerivDecl deriv_ty))
= setSrcSpan loc $
addErrCtxt (standaloneCtxt deriv_ty) $
do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
; (tvs, theta, cls, inst_tys) <- tcHsInstHead TcType.InstDeclCtxt deriv_ty
; traceTc "Standalone deriving;" $ vcat
[ text "tvs:" <+> ppr tvs
, text "theta:" <+> ppr theta
, text "cls:" <+> ppr cls
, text "tys:" <+> ppr inst_tys ]
; checkTc (not (null inst_tys)) derivingNullaryErr
; let cls_tys = take (length inst_tys 1) inst_tys
inst_ty = last inst_tys
; traceTc "Standalone deriving:" $ vcat
[ text "class:" <+> ppr cls
, text "class types:" <+> ppr cls_tys
, text "type:" <+> ppr inst_ty ]
; case tcSplitTyConApp_maybe inst_ty of
Just (tycon, tc_args)
| className cls == typeableClassName || isAlgTyCon tycon
-> mkEqnHelp tvs cls cls_tys tycon tc_args (Just theta)
_ ->
failWithTc $ derivingThingErr False cls cls_tys inst_ty $
ptext (sLit "The last argument of the instance must be a data or newtype application")
}
deriveTyData :: [TyVar] -> TyCon -> [Type]
-> LHsType Name
-> TcM EarlyDerivSpec
deriveTyData tvs tc tc_args (L loc deriv_pred)
= setSrcSpan loc $
do { (deriv_tvs, cls, cls_tys, cls_arg_kind)
<- tcExtendTyVarEnv tvs $
tcHsDeriv deriv_pred
; if className cls == typeableClassName
then derivePolyKindedTypeable cls cls_tys tvs tc tc_args
else do {
let (arg_kinds, _) = splitKindFunTys cls_arg_kind
n_args_to_drop = length arg_kinds
n_args_to_keep = tyConArity tc n_args_to_drop
args_to_drop = drop n_args_to_keep tc_args
tc_args_to_keep = take n_args_to_keep tc_args
inst_ty_kind = typeKind (mkTyConApp tc tc_args_to_keep)
dropped_tvs = tyVarsOfTypes args_to_drop
mb_match = tcUnifyTy inst_ty_kind cls_arg_kind
Just kind_subst = mb_match
(univ_kvs, univ_tvs) = partition isKindVar $ varSetElems $
mkVarSet deriv_tvs `unionVarSet`
tyVarsOfTypes tc_args_to_keep
univ_kvs' = filter (`notElemTvSubst` kind_subst) univ_kvs
(subst', univ_tvs') = mapAccumL substTyVarBndr kind_subst univ_tvs
final_tc_args = substTys subst' tc_args_to_keep
final_cls_tys = substTys subst' cls_tys
; traceTc "derivTyData1" (vcat [ pprTvBndrs tvs, ppr tc, ppr tc_args
, pprTvBndrs (varSetElems $ tyVarsOfTypes tc_args)
, ppr n_args_to_keep, ppr n_args_to_drop
, ppr inst_ty_kind, ppr cls_arg_kind, ppr mb_match ])
; checkTc (n_args_to_keep >= 0 && isJust mb_match)
(derivingKindErr tc cls cls_tys cls_arg_kind)
; traceTc "derivTyData2" (vcat [ ppr univ_tvs ])
; checkTc (allDistinctTyVars args_to_drop &&
not (any (`elemVarSet` dropped_tvs) univ_tvs))
(derivingEtaErr cls final_cls_tys (mkTyConApp tc final_tc_args))
; mkEqnHelp (univ_kvs' ++ univ_tvs')
cls final_cls_tys tc final_tc_args Nothing } }
derivePolyKindedTypeable :: Class -> [Type]
-> [TyVar] -> TyCon -> [Type]
-> TcM EarlyDerivSpec
derivePolyKindedTypeable cls cls_tys _tvs tc tc_args
= do { checkTc (isSingleton cls_tys) $
derivingThingErr False cls cls_tys (mkTyConApp tc tc_args)
(classArgsErr cls cls_tys)
; checkTc (allDistinctTyVars tc_args) $
derivingEtaErr cls cls_tys (mkTyConApp tc tc_kind_args)
; mkEqnHelp kind_vars cls cls_tys tc tc_kind_args Nothing }
where
kind_vars = kindVarsOnly tc_args
tc_kind_args = mkTyVarTys kind_vars
kindVarsOnly :: [Type] -> [KindVar]
kindVarsOnly [] = []
kindVarsOnly (t:ts) | Just v <- getTyVar_maybe t
, isKindVar v = v : kindVarsOnly ts
| otherwise = kindVarsOnly ts
\end{code}
Note [Unify kinds in deriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider (Trac #8534)
data T a b = MkT a deriving( Functor )
-- where Functor :: (*->*) -> Constraint
So T :: forall k. * -> k -> *. We want to get
instance Functor (T * (a:*)) where ...
Notice the '*' argument to T.
Moreover, as well as instantiating T's kind arguments, we may need to instantiate
C's kind args. Consider (Trac #8865):
newtype T a b = MkT (Either a b) deriving( Category )
where
Category :: forall k. (k -> k -> *) -> Constraint
We need to generate the instance
insatnce Category * (Either a) where ...
Notice the '*' argument to Cagegory.
So we need to
* drop arguments from (T a b) to match the number of
arrows in the (last argument of the) class;
* and then *unify* kind of the remaining type against the
expected kind, to figure out how to instantiate C's and T's
kind arguments.
In the two examples,
* we unify kind-of( T k (a:k) ) ~ kind-of( Functor )
i.e. (k -> *) ~ (* -> *) to find k:=*.
yielding k:=*
* we unify kind-of( Either ) ~ kind-of( Category )
i.e. (* -> * -> *) ~ (k -> k -> k)
yielding k:=*
Now we get a kind substition. We then need to:
1. Remove the substituted-out kind variables from the quantifed kind vars
2. Apply the substitution to the kinds of quantified *type* vars
(and extend the substitution to reflect this change)
3. Apply that extended substitution to the non-dropped args (types and
kinds) of the type and class
Forgetting step (2) caused Trac #8893:
data V a = V [a] deriving Functor
data P (x::k->*) (a:k) = P (x a) deriving Functor
data C (x::k->*) (a:k) = C (V (P x a)) deriving Functor
When deriving Functor for P, we unify k to *, but we then want
an instance $df :: forall (x:*->*). Functor x => Functor (P * (x:*->*))
and similarly for C. Notice the modifed kind of x, both at binding
and occurrence sites.
\begin{code}
mkEqnHelp :: [TyVar]
-> Class -> [Type]
-> TyCon -> [Type]
-> DerivContext
-> TcRn EarlyDerivSpec
mkEqnHelp tvs cls cls_tys tycon tc_args mtheta
| className cls `elem` oldTypeableClassNames
= do { dflags <- getDynFlags
; case checkOldTypeableConditions (dflags, tycon, tc_args) of
Just err -> bale_out err
Nothing -> mkOldTypeableEqn tvs cls tycon tc_args mtheta }
| className cls == typeableClassName
= do { dflags <- getDynFlags
; case checkTypeableConditions (dflags, tycon, tc_args) of
Just err -> bale_out err
Nothing -> mkPolyKindedTypeableEqn tvs cls tycon tc_args mtheta }
| otherwise
= do { (rep_tc, rep_tc_args) <- lookup_data_fam tycon tc_args
; rdr_env <- getGlobalRdrEnv
; let data_con_names = map dataConName (tyConDataCons rep_tc)
hidden_data_cons = not (isWiredInName (tyConName rep_tc)) &&
(isAbstractTyCon rep_tc ||
any not_in_scope data_con_names)
not_in_scope dc = null (lookupGRE_Name rdr_env dc)
data_con_rdrs = [ mkRdrQual (is_as (is_decl imp_spec)) occ
| dc_name <- data_con_names
, let occ = nameOccName dc_name
gres = lookupGRE_Name rdr_env dc_name
, not (null gres)
, Imported (imp_spec:_) <- [gre_prov (head gres)] ]
; addUsedRdrNames data_con_rdrs
; unless (isNothing mtheta || not hidden_data_cons)
(bale_out (derivingHiddenErr tycon))
; dflags <- getDynFlags
; if isDataTyCon rep_tc then
mkDataTypeEqn dflags tvs cls cls_tys
tycon tc_args rep_tc rep_tc_args mtheta
else
mkNewTypeEqn dflags tvs cls cls_tys
tycon tc_args rep_tc rep_tc_args mtheta }
where
bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
lookup_data_fam :: TyCon -> [Type] -> TcM (TyCon, [Type])
lookup_data_fam tycon tys
| not (isFamilyTyCon tycon)
= return (tycon, tys)
| otherwise
= ASSERT( isAlgTyCon tycon )
do { maybeFamInst <- tcLookupFamInst tycon tys
; case maybeFamInst of
Nothing -> bale_out (ptext (sLit "No family instance for")
<+> quotes (pprTypeApp tycon tys))
Just (FamInstMatch { fim_instance = famInst
, fim_tys = tys })
-> let tycon' = dataFamInstRepTyCon famInst
in return (tycon', tys) }
\end{code}
Note [Looking up family instances for deriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tcLookupFamInstExact is an auxiliary lookup wrapper which requires
that looked-up family instances exist. If called with a vanilla
tycon, the old type application is simply returned.
If we have
data instance F () = ... deriving Eq
data instance F () = ... deriving Eq
then tcLookupFamInstExact will be confused by the two matches;
but that can't happen because tcInstDecls1 doesn't call tcDeriving
if there are any overlaps.
There are two other things that might go wrong with the lookup.
First, we might see a standalone deriving clause
deriving Eq (F ())
when there is no data instance F () in scope.
Note that it's OK to have
data instance F [a] = ...
deriving Eq (F [(a,b)])
where the match is not exact; the same holds for ordinary data types
with standalone deriving declarations.
Note [Deriving, type families, and partial applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When there are no type families, it's quite easy:
newtype S a = MkS [a]
-- :CoS :: S ~ [] -- Eta-reduced
instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a)
instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S
When type familes are involved it's trickier:
data family T a b
newtype instance T Int a = MkT [a] deriving( Eq, Monad )
-- :RT is the representation type for (T Int a)
-- :Co:RT :: :RT ~ [] -- Eta-reduced!
-- :CoF:RT a :: T Int a ~ :RT a -- Also eta-reduced!
instance Eq [a] => Eq (T Int a) -- easy by coercion
-- d1 :: Eq [a]
-- d2 :: Eq (T Int a) = d1 |> Eq (sym (:Co:RT a ; :coF:RT a))
instance Monad [] => Monad (T Int) -- only if we can eta reduce???
-- d1 :: Monad []
-- d2 :: Monad (T Int) = d1 |> Monad (sym (:Co:RT ; :coF:RT))
Note the need for the eta-reduced rule axioms. After all, we can
write it out
instance Monad [] => Monad (T Int) -- only if we can eta reduce???
return x = MkT [x]
... etc ...
See Note [Eta reduction for data family axioms] in TcInstDcls.
%************************************************************************
%* *
Deriving data types
%* *
%************************************************************************
\begin{code}
mkDataTypeEqn :: DynFlags
-> [Var]
-> Class
-> [Type]
-> TyCon
-> [Type]
-> TyCon
-> [Type]
-> DerivContext
-> TcRn EarlyDerivSpec
mkDataTypeEqn dflags tvs cls cls_tys
tycon tc_args rep_tc rep_tc_args mtheta
= case checkSideConditions dflags mtheta cls cls_tys rep_tc rep_tc_args of
CanDerive -> go_for_it
NonDerivableClass -> bale_out (nonStdErr cls)
DerivableClassError msg -> bale_out msg
where
go_for_it = mk_data_eqn tvs cls tycon tc_args rep_tc rep_tc_args mtheta
bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)
mk_data_eqn :: [TyVar] -> Class
-> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
-> TcM EarlyDerivSpec
mk_data_eqn tvs cls tycon tc_args rep_tc rep_tc_args mtheta
= do loc <- getSrcSpanM
dfun_name <- new_dfun_name cls tycon
case mtheta of
Nothing -> do
inferred_constraints <- inferConstraints cls inst_tys rep_tc rep_tc_args
return $ InferTheta $ DS
{ ds_loc = loc
, ds_name = dfun_name, ds_tvs = tvs
, ds_cls = cls, ds_tys = inst_tys
, ds_tc = rep_tc, ds_tc_args = rep_tc_args
, ds_theta = inferred_constraints
, ds_newtype = False }
Just theta -> do
return $ GivenTheta $ DS
{ ds_loc = loc
, ds_name = dfun_name, ds_tvs = tvs
, ds_cls = cls, ds_tys = inst_tys
, ds_tc = rep_tc, ds_tc_args = rep_tc_args
, ds_theta = theta
, ds_newtype = False }
where
inst_tys = [mkTyConApp tycon tc_args]
mkOldTypeableEqn :: [TyVar] -> Class
-> TyCon -> [TcType] -> DerivContext
-> TcM EarlyDerivSpec
mkOldTypeableEqn tvs cls tycon tc_args mtheta
| isNothing mtheta
= do { checkTc (cls `hasKey` oldTypeableClassKey)
(ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
; real_cls <- tcLookupClass (oldTypeableClassNames `getNth` tyConArity tycon)
; mkOldTypeableEqn tvs real_cls tycon [] (Just []) }
| otherwise
= do { checkTc (null tc_args)
(ptext (sLit "Derived Typeable instance must be of form (Typeable")
<> int (tyConArity tycon) <+> ppr tycon <> rparen)
; dfun_name <- new_dfun_name cls tycon
; loc <- getSrcSpanM
; return (GivenTheta $
DS { ds_loc = loc, ds_name = dfun_name, ds_tvs = []
, ds_cls = cls, ds_tys = [mkTyConApp tycon []]
, ds_tc = tycon, ds_tc_args = []
, ds_theta = mtheta `orElse` [], ds_newtype = False }) }
mkPolyKindedTypeableEqn :: [TyVar] -> Class
-> TyCon -> [TcType] -> DerivContext
-> TcM EarlyDerivSpec
mkPolyKindedTypeableEqn tvs cls tycon tc_args mtheta
= do {
polykinds <- xoptM Opt_PolyKinds
; checkTc (all is_kind_var tc_args) (mk_msg polykinds)
; dfun_name <- new_dfun_name cls tycon
; loc <- getSrcSpanM
; let tc_app = mkTyConApp tycon tc_args
; return (GivenTheta $
DS { ds_loc = loc, ds_name = dfun_name
, ds_tvs = filter isKindVar tvs, ds_cls = cls
, ds_tys = typeKind tc_app : [tc_app]
, ds_tc = tycon, ds_tc_args = tc_args
, ds_theta = mtheta `orElse` []
, ds_newtype = False }) }
where
is_kind_var tc_arg = case tcGetTyVar_maybe tc_arg of
Just v -> isKindVar v
Nothing -> False
mk_msg polykinds | not polykinds
, all isKind tc_args
= hang (ptext (sLit "To make a Typeable instance of poly-kinded")
<+> quotes (ppr tycon) <> comma)
2 (ptext (sLit "use XPolyKinds"))
| otherwise
= ptext (sLit "Derived Typeable instance must be of form")
<+> parens (ptext (sLit "Typeable") <+> ppr tycon)
inferConstraints :: Class -> [TcType]
-> TyCon -> [TcType]
-> TcM ThetaOrigin
inferConstraints cls inst_tys rep_tc rep_tc_args
| cls `hasKey` genClassKey
= return []
| cls `hasKey` gen1ClassKey
= ASSERT(length rep_tc_tvs > 0)
do { functorClass <- tcLookupClass functorClassName
; return (con_arg_constraints functorClass (get_gen1_constrained_tys last_tv)) }
| otherwise
= ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
do { traceTc "inferConstraints" (vcat [ppr cls <+> ppr inst_tys, ppr arg_constraints])
; return (stupid_constraints ++ extra_constraints
++ sc_constraints
++ arg_constraints) }
where
arg_constraints = con_arg_constraints cls get_std_constrained_tys
con_arg_constraints cls' get_constrained_tys
= [ mkPredOrigin (DerivOriginDC data_con arg_n) (mkClassPred cls' [inner_ty])
| data_con <- tyConDataCons rep_tc
, (arg_n, arg_ty) <- ASSERT( isVanillaDataCon data_con )
zip [1..] $
dataConInstOrigArgTys data_con all_rep_tc_args
, not (isUnLiftedType arg_ty)
, inner_ty <- get_constrained_tys arg_ty ]
is_functor_like = getUnique cls `elem` functorLikeClassKeys
get_std_constrained_tys :: Type -> [Type]
get_std_constrained_tys ty
| is_functor_like = deepSubtypesContaining last_tv ty
| otherwise = [ty]
rep_tc_tvs = tyConTyVars rep_tc
last_tv = last rep_tc_tvs
all_rep_tc_args | cls `hasKey` gen1ClassKey || is_functor_like
= rep_tc_args ++ [mkTyVarTy last_tv]
| otherwise = rep_tc_args
sc_constraints = mkThetaOrigin DerivOrigin $
substTheta (zipOpenTvSubst (classTyVars cls) inst_tys) (classSCTheta cls)
stupid_constraints = mkThetaOrigin DerivOrigin $
substTheta subst (tyConStupidTheta rep_tc)
subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args
extra_constraints
| cls `hasKey` dataClassKey
, all (isLiftedTypeKind . typeKind) rep_tc_args
= [mkPredOrigin DerivOrigin (mkClassPred cls [ty]) | ty <- rep_tc_args]
| otherwise
= []
\end{code}
Note [Getting base classes]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Functor and Typeable are defined in package 'base', and that is not available
when compiling 'ghc-prim'. So we must be careful that 'deriving' for stuff in
ghc-prim does not use Functor or Typeable implicitly via these lookups.
Note [Deriving and unboxed types]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We have some special hacks to support things like
data T = MkT Int# deriving ( Show )
Specifically, we use TcGenDeriv.box_if_necy to box the Int# into an Int
(which we know how to show). It's a bit ad hoc.
\begin{code}
data DerivStatus = CanDerive
| DerivableClassError SDoc
| NonDerivableClass
checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType]
-> TyCon -> [Type]
-> DerivStatus
checkSideConditions dflags mtheta cls cls_tys rep_tc rep_tc_args
| Just cond <- sideConditions mtheta cls
= case (cond (dflags, rep_tc, rep_tc_args)) of
Just err -> DerivableClassError err
Nothing | null cls_tys -> CanDerive
| otherwise -> DerivableClassError (classArgsErr cls cls_tys)
| otherwise = NonDerivableClass
classArgsErr :: Class -> [Type] -> SDoc
classArgsErr cls cls_tys = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")
checkTypeableConditions, checkOldTypeableConditions :: Condition
checkTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_TypeableOK
checkOldTypeableConditions = checkFlag Opt_DeriveDataTypeable `andCond` cond_oldTypeableOK
nonStdErr :: Class -> SDoc
nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")
sideConditions :: DerivContext -> Class -> Maybe Condition
sideConditions mtheta cls
| cls_key == eqClassKey = Just (cond_std `andCond` cond_args cls)
| cls_key == ordClassKey = Just (cond_std `andCond` cond_args cls)
| cls_key == showClassKey = Just (cond_std `andCond` cond_args cls)
| cls_key == readClassKey = Just (cond_std `andCond` cond_args cls)
| cls_key == enumClassKey = Just (cond_std `andCond` cond_isEnumeration)
| cls_key == ixClassKey = Just (cond_std `andCond` cond_enumOrProduct cls)
| cls_key == boundedClassKey = Just (cond_std `andCond` cond_enumOrProduct cls)
| cls_key == dataClassKey = Just (checkFlag Opt_DeriveDataTypeable `andCond`
cond_std `andCond` cond_args cls)
| cls_key == functorClassKey = Just (checkFlag Opt_DeriveFunctor `andCond`
cond_functorOK True)
| cls_key == foldableClassKey = Just (checkFlag Opt_DeriveFoldable `andCond`
cond_functorOK False)
| cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
cond_functorOK False)
| cls_key == genClassKey = Just (cond_RepresentableOk `andCond`
checkFlag Opt_DeriveGeneric)
| cls_key == gen1ClassKey = Just (cond_Representable1Ok `andCond`
checkFlag Opt_DeriveGeneric)
| otherwise = Nothing
where
cls_key = getUnique cls
cond_std = cond_stdOK mtheta
type Condition = (DynFlags, TyCon, [Type]) -> Maybe SDoc
orCond :: Condition -> Condition -> Condition
orCond c1 c2 tc
= case c1 tc of
Nothing -> Nothing
Just x -> case c2 tc of
Nothing -> Nothing
Just y -> Just (x $$ ptext (sLit " or") $$ y)
andCond :: Condition -> Condition -> Condition
andCond c1 c2 tc = case c1 tc of
Nothing -> c2 tc
Just x -> Just x
cond_stdOK :: DerivContext -> Condition
cond_stdOK (Just _) _
= Nothing
cond_stdOK Nothing (_, rep_tc, _)
| null data_cons = Just (no_cons_why rep_tc $$ suggestion)
| not (null con_whys) = Just (vcat con_whys $$ suggestion)
| otherwise = Nothing
where
suggestion = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
data_cons = tyConDataCons rep_tc
con_whys = mapCatMaybes check_con data_cons
check_con :: DataCon -> Maybe SDoc
check_con con
| isVanillaDataCon con
, all isTauTy (dataConOrigArgTys con) = Nothing
| otherwise = Just (badCon con (ptext (sLit "must have a Haskell-98 type")))
no_cons_why :: TyCon -> SDoc
no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
ptext (sLit "must have at least one data constructor")
cond_RepresentableOk :: Condition
cond_RepresentableOk (_, tc, tc_args) = canDoGenerics tc tc_args
cond_Representable1Ok :: Condition
cond_Representable1Ok (_, tc, tc_args) = canDoGenerics1 tc tc_args
cond_enumOrProduct :: Class -> Condition
cond_enumOrProduct cls = cond_isEnumeration `orCond`
(cond_isProduct `andCond` cond_args cls)
cond_args :: Class -> Condition
cond_args cls (_, tc, _)
= case bad_args of
[] -> Nothing
(ty:_) -> Just (hang (ptext (sLit "Don't know how to derive") <+> quotes (ppr cls))
2 (ptext (sLit "for type") <+> quotes (ppr ty)))
where
bad_args = [ arg_ty | con <- tyConDataCons tc
, arg_ty <- dataConOrigArgTys con
, isUnLiftedType arg_ty
, not (ok_ty arg_ty) ]
cls_key = classKey cls
ok_ty arg_ty
| cls_key == eqClassKey = check_in arg_ty ordOpTbl
| cls_key == ordClassKey = check_in arg_ty ordOpTbl
| cls_key == showClassKey = check_in arg_ty boxConTbl
| otherwise = False
check_in :: Type -> [(Type,a)] -> Bool
check_in arg_ty tbl = any (eqType arg_ty . fst) tbl
cond_isEnumeration :: Condition
cond_isEnumeration (_, rep_tc, _)
| isEnumerationTyCon rep_tc = Nothing
| otherwise = Just why
where
why = sep [ quotes (pprSourceTyCon rep_tc) <+>
ptext (sLit "must be an enumeration type")
, ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
cond_isProduct :: Condition
cond_isProduct (_, rep_tc, _)
| isProductTyCon rep_tc = Nothing
| otherwise = Just why
where
why = quotes (pprSourceTyCon rep_tc) <+>
ptext (sLit "must have precisely one constructor")
cond_oldTypeableOK :: Condition
cond_oldTypeableOK (_, tc, _)
| tyConArity tc > 7 = Just too_many
| not (all (isSubOpenTypeKind . tyVarKind) (tyConTyVars tc))
= Just bad_kind
| otherwise = Nothing
where
too_many = quotes (pprSourceTyCon tc) <+>
ptext (sLit "must have 7 or fewer arguments")
bad_kind = quotes (pprSourceTyCon tc) <+>
ptext (sLit "must only have arguments of kind `*'")
cond_TypeableOK :: Condition
cond_TypeableOK (_, tc, tc_args)
| isDataFamilyTyCon tc && not (null tc_args)
= Just no_families
| otherwise
= Nothing
where
no_families = sep [ ptext (sLit "Deriving Typeable is not allowed for family instances;")
, ptext (sLit "derive Typeable for")
<+> quotes (pprSourceTyCon tc)
<+> ptext (sLit "alone") ]
functorLikeClassKeys :: [Unique]
functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]
cond_functorOK :: Bool -> Condition
cond_functorOK allowFunctions (_, rep_tc, _)
| null tc_tvs
= Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
<+> ptext (sLit "must have some type parameters"))
| not (null bad_stupid_theta)
= Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
<+> ptext (sLit "must not have a class context") <+> pprTheta bad_stupid_theta)
| otherwise
= msum (map check_con data_cons)
where
tc_tvs = tyConTyVars rep_tc
Just (_, last_tv) = snocView tc_tvs
bad_stupid_theta = filter is_bad (tyConStupidTheta rep_tc)
is_bad pred = last_tv `elemVarSet` tyVarsOfType pred
data_cons = tyConDataCons rep_tc
check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)
check_vanilla :: DataCon -> Maybe SDoc
check_vanilla con | isVanillaDataCon con = Nothing
| otherwise = Just (badCon con existential)
ft_check :: DataCon -> FFoldType (Maybe SDoc)
ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
, ft_co_var = Just (badCon con covariant)
, ft_fun = \x y -> if allowFunctions then x `mplus` y
else Just (badCon con functions)
, ft_tup = \_ xs -> msum xs
, ft_ty_app = \_ x -> x
, ft_bad_app = Just (badCon con wrong_arg)
, ft_forall = \_ x -> x }
existential = ptext (sLit "must not have existential arguments")
covariant = ptext (sLit "must not use the type variable in a function argument")
functions = ptext (sLit "must not contain function types")
wrong_arg = ptext (sLit "must use the type variable only as the last argument of a data type")
allDistinctTyVars :: [KindOrType] -> Bool
allDistinctTyVars tkvs = go emptyVarSet tkvs
where
go _ [] = True
go so_far (ty : tys)
= case getTyVar_maybe ty of
Nothing -> False
Just tv | tv `elemVarSet` so_far -> False
| otherwise -> go (so_far `extendVarSet` tv) tys
checkFlag :: ExtensionFlag -> Condition
checkFlag flag (dflags, _, _)
| xopt flag dflags = Nothing
| otherwise = Just why
where
why = ptext (sLit "You need ") <> text flag_str
<+> ptext (sLit "to derive an instance for this class")
flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
[s] -> s
other -> pprPanic "checkFlag" (ppr other)
std_class_via_coercible :: Class -> Bool
std_class_via_coercible clas
= classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
non_coercible_class :: Class -> Bool
non_coercible_class cls
= classKey cls `elem` ([ readClassKey, showClassKey, dataClassKey
, genClassKey, gen1ClassKey, typeableClassKey
, traversableClassKey ]
++ oldTypeableClassKeys)
oldTypeableClassKeys :: [Unique]
oldTypeableClassKeys = map getUnique oldTypeableClassNames
new_dfun_name :: Class -> TyCon -> TcM Name
new_dfun_name clas tycon
= do { loc <- getSrcSpanM
; newDFunName clas [mkTyConApp tycon []] loc }
badCon :: DataCon -> SDoc -> SDoc
badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg
\end{code}
Note [Superclasses of derived instance]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In general, a derived instance decl needs the superclasses of the derived
class too. So if we have
data T a = ...deriving( Ord )
then the initial context for Ord (T a) should include Eq (T a). Often this is
redundant; we'll also generate an Ord constraint for each constructor argument,
and that will probably generate enough constraints to make the Eq (T a) constraint
be satisfied too. But not always; consider:
data S a = S
instance Eq (S a)
instance Ord (S a)
data T a = MkT (S a) deriving( Ord )
instance Num a => Eq (T a)
The derived instance for (Ord (T a)) must have a (Num a) constraint!
Similarly consider:
data T a = MkT deriving( Data, Typeable )
Here there *is* no argument field, but we must nevertheless generate
a context for the Data instances:
instance Typable a => Data (T a) where ...
%************************************************************************
%* *
Deriving newtypes
%* *
%************************************************************************
\begin{code}
mkNewTypeEqn :: DynFlags -> [Var] -> Class
-> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
-> DerivContext
-> TcRn EarlyDerivSpec
mkNewTypeEqn dflags tvs
cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
| ASSERT( length cls_tys + 1 == classArity cls )
might_derive_via_coercible && (newtype_deriving || std_class_via_coercible cls)
= do traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
dfun_name <- new_dfun_name cls tycon
loc <- getSrcSpanM
case mtheta of
Just theta -> return $ GivenTheta $ DS
{ ds_loc = loc
, ds_name = dfun_name, ds_tvs = varSetElemsKvsFirst dfun_tvs
, ds_cls = cls, ds_tys = inst_tys
, ds_tc = rep_tycon, ds_tc_args = rep_tc_args
, ds_theta = theta
, ds_newtype = True }
Nothing -> return $ InferTheta $ DS
{ ds_loc = loc
, ds_name = dfun_name, ds_tvs = varSetElemsKvsFirst dfun_tvs
, ds_cls = cls, ds_tys = inst_tys
, ds_tc = rep_tycon, ds_tc_args = rep_tc_args
, ds_theta = all_preds
, ds_newtype = True }
| otherwise
= case checkSideConditions dflags mtheta cls cls_tys rep_tycon rep_tc_args of
CanDerive -> go_for_it
DerivableClassError msg
| might_derive_via_coercible -> bale_out (msg $$ suggest_nd)
| otherwise -> bale_out msg
NonDerivableClass
| newtype_deriving -> bale_out cant_derive_err
| might_derive_via_coercible -> bale_out (non_std $$ suggest_nd)
| otherwise -> bale_out non_std
where
newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
go_for_it = mk_data_eqn tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
bale_out msg = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)
non_std = nonStdErr cls
suggest_nd = ptext (sLit "Try GeneralizedNewtypeDeriving for GHC's newtype-deriving extension")
nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
rep_tys = cls_tys ++ [rep_inst_ty]
rep_pred = mkClassPred cls rep_tys
rep_pred_o = mkPredOrigin DerivOrigin rep_pred
cls_tyvars = classTyVars cls
dfun_tvs = tyVarsOfTypes inst_tys
inst_ty = mkTyConApp tycon tc_args
inst_tys = cls_tys ++ [inst_ty]
sc_theta =
mkThetaOrigin DerivOrigin $
substTheta (zipOpenTvSubst cls_tyvars inst_tys) (classSCTheta cls)
coercible_constraints =
[ let (Pair t1 t2) = mkCoerceClassMethEqn cls (varSetElemsKvsFirst dfun_tvs) inst_tys rep_inst_ty meth
in mkPredOrigin (DerivOriginCoerce meth t1 t2) (mkCoerciblePred t1 t2)
| meth <- classMethods cls ]
all_preds = rep_pred_o : coercible_constraints ++ sc_theta
might_derive_via_coercible
= not (non_coercible_class cls)
&& eta_ok
&& ats_ok
eta_ok = nt_eta_arity <= length rep_tc_args
ats_ok = null (classATs cls)
cant_derive_err
= vcat [ ppUnless eta_ok eta_msg
, ppUnless ats_ok ats_msg ]
eta_msg = ptext (sLit "cannot eta-reduce the representation type enough")
ats_msg = ptext (sLit "the class has associated types")
\end{code}
Note [Recursive newtypes]
~~~~~~~~~~~~~~~~~~~~~~~~~
Newtype deriving works fine, even if the newtype is recursive.
e.g. newtype S1 = S1 [T1 ()]
newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad )
Remember, too, that type families are currently (conservatively) given
a recursive flag, so this also allows newtype deriving to work
for type famillies.
We used to exclude recursive types, because we had a rather simple
minded way of generating the instance decl:
newtype A = MkA [A]
instance Eq [A] => Eq A -- Makes typechecker loop!
But now we require a simple context, so it's ok.
Note [Determining whether newtype-deriving is appropriate]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we see
newtype NT = MkNT Foo
deriving C
we have to decide how to perform the deriving. Do we do newtype deriving,
or do we do normal deriving? In general, we prefer to do newtype deriving
wherever possible. So, we try newtype deriving unless there's a glaring
reason not to.
Note that newtype deriving might fail, even after we commit to it. This
is because the derived instance uses `coerce`, which must satisfy its
`Coercible` constraint. This is different than other deriving scenarios,
where we're sure that the resulting instance will type-check.
%************************************************************************
%* *
\subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations}
%* *
%************************************************************************
A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv)
terms, which is the final correct RHS for the corresponding original
equation.
\begin{itemize}
\item
Each (k,TyVarTy tv) in a solution constrains only a type
variable, tv.
\item
The (k,TyVarTy tv) pairs in a solution are canonically
ordered by sorting on type varible, tv, (major key) and then class, k,
(minor key)
\end{itemize}
\begin{code}
inferInstanceContexts :: OverlapFlag -> [DerivSpec ThetaOrigin] -> TcM [DerivSpec ThetaType]
inferInstanceContexts _ [] = return []
inferInstanceContexts oflag infer_specs
= do { traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
; iterate_deriv 1 initial_solutions }
where
initial_solutions :: [ThetaType]
initial_solutions = [ [] | _ <- infer_specs ]
iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec ThetaType]
iterate_deriv n current_solns
| n > 20
= pprPanic "solveDerivEqns: probable loop"
(vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
| otherwise
= do {
inst_specs <- zipWithM (mkInstance oflag) current_solns infer_specs
; new_solns <- checkNoErrs $
extendLocalInstEnv inst_specs $
mapM gen_soln infer_specs
; if (current_solns `eqSolution` new_solns) then
return [ spec { ds_theta = soln }
| (spec, soln) <- zip infer_specs current_solns ]
else
iterate_deriv (n+1) new_solns }
eqSolution = eqListBy (eqListBy eqType)
gen_soln :: DerivSpec ThetaOrigin -> TcM ThetaType
gen_soln (DS { ds_loc = loc, ds_tvs = tyvars
, ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
= setSrcSpan loc $
addErrCtxt (derivInstCtxt the_pred) $
do { theta <- simplifyDeriv the_pred tyvars deriv_rhs
; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
; return (sortBy cmpType theta) }
where
the_pred = mkClassPred clas inst_tys
mkInstance :: OverlapFlag -> ThetaType -> DerivSpec theta -> TcM ClsInst
mkInstance overlap_flag theta
(DS { ds_name = dfun_name
, ds_tvs = tvs, ds_cls = clas, ds_tys = tys })
= do { (subst, tvs') <- tcInstSkolTyVars tvs
; return (mkLocalInstance dfun overlap_flag tvs' clas (substTys subst tys)) }
where
dfun = mkDictFunId dfun_name tvs theta clas tys
extendLocalInstEnv :: [ClsInst] -> TcM a -> TcM a
extendLocalInstEnv dfuns thing_inside
= do { env <- getGblEnv
; let inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
env' = env { tcg_inst_env = inst_env' }
; setGblEnv env' thing_inside }
\end{code}
***********************************************************************************
* *
* Simplify derived constraints
* *
***********************************************************************************
\begin{code}
simplifyDeriv :: PredType
-> [TyVar]
-> ThetaOrigin
-> TcM ThetaType
simplifyDeriv pred tvs theta
= do { (skol_subst, tvs_skols) <- tcInstSkolTyVars tvs
; let subst_skol = zipTopTvSubst tvs_skols $ map mkTyVarTy tvs
skol_set = mkVarSet tvs_skols
doc = ptext (sLit "deriving") <+> parens (ppr pred)
; wanted <- mapM (\(PredOrigin t o) -> newFlatWanted o (substTy skol_subst t)) theta
; traceTc "simplifyDeriv" $
vcat [ pprTvBndrs tvs $$ ppr theta $$ ppr wanted, doc ]
; (residual_wanted, _ev_binds1)
<- solveWantedsTcM (mkFlatWC wanted)
; let (good, bad) = partitionBagWith get_good (wc_flat residual_wanted)
get_good :: Ct -> Either PredType Ct
get_good ct | validDerivPred skol_set p
, isWantedCt ct = Left p
| otherwise = Right ct
where p = ctPred ct
; reportAllUnsolved (residual_wanted { wc_flat = bad })
; let min_theta = mkMinimalBySCs (bagToList good)
; return (substTheta subst_skol min_theta) }
\end{code}
Note [Overlap and deriving]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider some overlapping instances:
data Show a => Show [a] where ..
data Show [Char] where ...
Now a data type with deriving:
data T a = MkT [a] deriving( Show )
We want to get the derived instance
instance Show [a] => Show (T a) where...
and NOT
instance Show a => Show (T a) where...
so that the (Show (T Char)) instance does the Right Thing
It's very like the situation when we're inferring the type
of a function
f x = show [x]
and we want to infer
f :: Show [a] => a -> String
BOTTOM LINE: use vanilla, non-overlappable skolems when inferring
the context for the derived instance.
Hence tcInstSkolTyVars not tcInstSuperSkolTyVars
Note [Exotic derived instance contexts]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a 'derived' instance declaration, we *infer* the context. It's a
bit unclear what rules we should apply for this; the Haskell report is
silent. Obviously, constraints like (Eq a) are fine, but what about
data T f a = MkT (f a) deriving( Eq )
where we'd get an Eq (f a) constraint. That's probably fine too.
One could go further: consider
data T a b c = MkT (Foo a b c) deriving( Eq )
instance (C Int a, Eq b, Eq c) => Eq (Foo a b c)
Notice that this instance (just) satisfies the Paterson termination
conditions. Then we *could* derive an instance decl like this:
instance (C Int a, Eq b, Eq c) => Eq (T a b c)
even though there is no instance for (C Int a), because there just
*might* be an instance for, say, (C Int Bool) at a site where we
need the equality instance for T's.
However, this seems pretty exotic, and it's quite tricky to allow
this, and yet give sensible error messages in the (much more common)
case where we really want that instance decl for C.
So for now we simply require that the derived instance context
should have only type-variable constraints.
Here is another example:
data Fix f = In (f (Fix f)) deriving( Eq )
Here, if we are prepared to allow -XUndecidableInstances we
could derive the instance
instance Eq (f (Fix f)) => Eq (Fix f)
but this is so delicate that I don't think it should happen inside
'deriving'. If you want this, write it yourself!
NB: if you want to lift this condition, make sure you still meet the
termination conditions! If not, the deriving mechanism generates
larger and larger constraints. Example:
data Succ a = S a
data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
Note the lack of a Show instance for Succ. First we'll generate
instance (Show (Succ a), Show a) => Show (Seq a)
and then
instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
and so on. Instead we want to complain of no instance for (Show (Succ a)).
The bottom line
~~~~~~~~~~~~~~~
Allow constraints which consist only of type variables, with no repeats.
%************************************************************************
%* *
\subsection[TcDeriv-normal-binds]{Bindings for the various classes}
%* *
%************************************************************************
After all the trouble to figure out the required context for the
derived instance declarations, all that's left is to chug along to
produce them. They will then be shoved into @tcInstDecls2@, which
will do all its usual business.
There are lots of possibilities for code to generate. Here are
various general remarks.
PRINCIPLES:
\begin{itemize}
\item
We want derived instances of @Eq@ and @Ord@ (both v common) to be
``you-couldn't-do-better-by-hand'' efficient.
\item
Deriving @Show@---also pretty common--- should also be reasonable good code.
\item
Deriving for the other classes isn't that common or that big a deal.
\end{itemize}
PRAGMATICS:
\begin{itemize}
\item
Deriving @Ord@ is done mostly with the 1.3 @compare@ method.
\item
Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too.
\item
We {\em normally} generate code only for the non-defaulted methods;
there are some exceptions for @Eq@ and (especially) @Ord@...
\item
Sometimes we use a @_con2tag_@ function, which returns a data
constructor's numeric (@Int#@) tag. These are generated by
@gen_tag_n_con_binds@, and the heuristic for deciding if one of
these is around is given by @hasCon2TagFun@.
The examples under the different sections below will make this
clearer.
\item
Much less often (really just for deriving @Ix@), we use a
@_tag2con_@ function. See the examples.
\item
We use the renamer!!! Reason: we're supposed to be
producing @LHsBinds Name@ for the methods, but that means
producing correctly-uniquified code on the fly. This is entirely
possible (the @TcM@ monad has a @UniqueSupply@), but it is painful.
So, instead, we produce @MonoBinds RdrName@ then heave 'em through
the renamer. What a great hack!
\end{itemize}
\begin{code}
genInst :: Bool
-> OverlapFlag
-> CommonAuxiliaries
-> DerivSpec ThetaType -> TcM (InstInfo RdrName, BagDerivStuff, Maybe Name)
genInst standalone_deriv oflag comauxs
spec@(DS { ds_tvs = tvs, ds_tc = rep_tycon, ds_tc_args = rep_tc_args
, ds_theta = theta, ds_newtype = is_newtype, ds_tys = tys
, ds_name = name, ds_cls = clas, ds_loc = loc })
| is_newtype
= do { inst_spec <- mkInstance oflag theta spec
; traceTc "genInst/is_newtype" (vcat [ppr loc, ppr clas, ppr tvs, ppr tys, ppr rhs_ty])
; return ( InstInfo
{ iSpec = inst_spec
, iBinds = InstBindings
{ ib_binds = gen_Newtype_binds loc clas tvs tys rhs_ty
, ib_pragmas = []
, ib_extensions = [ Opt_ImpredicativeTypes
, Opt_RankNTypes ]
, ib_standalone_deriving = standalone_deriv } }
, emptyBag
, Just $ getName $ head $ tyConDataCons rep_tycon ) }
| otherwise
= do { fix_env <- getFixityEnv
; (meth_binds, deriv_stuff) <- genDerivStuff (getSrcSpan name)
fix_env clas name rep_tycon
(lookup rep_tycon comauxs)
; inst_spec <- mkInstance oflag theta spec
; let inst_info = InstInfo { iSpec = inst_spec
, iBinds = InstBindings
{ ib_binds = meth_binds
, ib_pragmas = []
, ib_extensions = []
, ib_standalone_deriving = standalone_deriv } }
; return ( inst_info, deriv_stuff, Nothing ) }
where
rhs_ty = newTyConInstRhs rep_tycon rep_tc_args
genDerivStuff :: SrcSpan -> FixityEnv -> Class -> Name -> TyCon
-> Maybe CommonAuxiliary
-> TcM (LHsBinds RdrName, BagDerivStuff)
genDerivStuff loc fix_env clas name tycon comaux_maybe
| className clas `elem` oldTypeableClassNames
= do dflags <- getDynFlags
return (gen_old_Typeable_binds dflags loc tycon, emptyBag)
| className clas == typeableClassName
= do dflags <- getDynFlags
return (gen_Typeable_binds dflags loc tycon, emptyBag)
| ck `elem` [genClassKey, gen1ClassKey]
= let gk = if ck == genClassKey then Gen0 else Gen1
Just metaTyCons = comaux_maybe
in do
(binds, faminst) <- gen_Generic_binds gk tycon metaTyCons (nameModule name)
return (binds, DerivFamInst faminst `consBag` emptyBag)
| otherwise
= do dflags <- getDynFlags
case assocMaybe (gen_list dflags) (getUnique clas) of
Just gen_fn -> return (gen_fn loc tycon)
Nothing -> pprPanic "genDerivStuff: bad derived class" (ppr clas)
where
ck = classKey clas
gen_list :: DynFlags
-> [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, BagDerivStuff))]
gen_list dflags
= [(eqClassKey, gen_Eq_binds)
,(ordClassKey, gen_Ord_binds)
,(enumClassKey, gen_Enum_binds)
,(boundedClassKey, gen_Bounded_binds)
,(ixClassKey, gen_Ix_binds)
,(showClassKey, gen_Show_binds fix_env)
,(readClassKey, gen_Read_binds fix_env)
,(dataClassKey, gen_Data_binds dflags)
,(functorClassKey, gen_Functor_binds)
,(foldableClassKey, gen_Foldable_binds)
,(traversableClassKey, gen_Traversable_binds)
]
\end{code}
%************************************************************************
%* *
\subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?}
%* *
%************************************************************************
\begin{code}
derivingNullaryErr :: MsgDoc
derivingNullaryErr = ptext (sLit "Cannot derive instances for nullary classes")
derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> MsgDoc
derivingKindErr tc cls cls_tys cls_kind
= hang (ptext (sLit "Cannot derive well-kinded instance of form")
<+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
2 (ptext (sLit "Class") <+> quotes (ppr cls)
<+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))
derivingEtaErr :: Class -> [Type] -> Type -> MsgDoc
derivingEtaErr cls cls_tys inst_ty
= sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
nest 2 (ptext (sLit "instance (...) =>")
<+> pprClassPred cls (cls_tys ++ [inst_ty]))]
derivingThingErr :: Bool -> Class -> [Type] -> Type -> MsgDoc -> MsgDoc
derivingThingErr newtype_deriving clas tys ty why
= sep [(hang (ptext (sLit "Can't make a derived instance of"))
2 (quotes (ppr pred))
$$ nest 2 extra) <> colon,
nest 2 why]
where
extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
| otherwise = empty
pred = mkClassPred clas (tys ++ [ty])
derivingHiddenErr :: TyCon -> SDoc
derivingHiddenErr tc
= hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
2 (ptext (sLit "so you cannot derive an instance for it"))
standaloneCtxt :: LHsType Name -> SDoc
standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
2 (quotes (ppr ty))
derivInstCtxt :: PredType -> MsgDoc
derivInstCtxt pred
= ptext (sLit "When deriving the instance for") <+> parens (ppr pred)
\end{code}