reflection-2.1.2: Reifies arbitrary terms into types that can be reflected back into terms

Copyright 2009-2015 Edward Kmett2012 Elliott Hird2004 Oleg Kiselyov and Chung-chieh Shan BSD3 Edward Kmett experimental non-portable Trustworthy Haskell98

Data.Reflection

Description

Reifies arbitrary terms at the type level. Based on the Functional Pearl: Implicit Configurations paper by Oleg Kiselyov and Chung-chieh Shan.

The approach from the paper was modified to work with Data.Proxy and to cheat by using knowledge of GHC's internal representations by Edward Kmett and Elliott Hird.

Usage comes down to two combinators, reify and reflect.

>>> reify 6 (\p -> reflect p + reflect p)
12


The argument passed along by reify is just a data Proxy t = Proxy, so all of the information needed to reconstruct your value has been moved to the type level. This enables it to be used when constructing instances (see examples/Monoid.hs).

In addition, a simpler API is offered for working with singleton values such as a system configuration, etc.

Synopsis

# Reflection

class Reifies s a | s -> a where #

Minimal complete definition

reflect

Methods

reflect :: proxy s -> a #

Recover a value inside a reify context, given a proxy for its reified type.

Instances

 # Methodsreflect :: proxy Integer -> a # # Methodsreflect :: proxy String -> a # # Methodsreflect :: proxy Int -> a # Reifies * n Int => Reifies * (PD n) Int # Methodsreflect :: proxy Int -> a # Reifies * n Int => Reifies * (SD n) Int # Methodsreflect :: proxy Int -> a # Reifies * n Int => Reifies * (D n) Int # Methodsreflect :: proxy Int -> a #

reify :: forall a r. a -> (forall s. Reifies s a => Proxy s -> r) -> r #

Reify a value at the type level, to be recovered with reflect.

reifyNat :: forall r. Integer -> (forall n. KnownNat n => Proxy n -> r) -> r #

This upgraded version of reify can be used to generate a KnownNat suitable for use with other APIs.

Available only on GHC 7.8+

>>> reifyNat 4 natVal
4

>>> reifyNat 4 reflect
4


reifySymbol :: forall r. String -> (forall n. KnownSymbol n => Proxy n -> r) -> r #

This upgraded version of reify can be used to generate a KnownSymbol suitable for use with other APIs.

Available only on GHC 7.8+

>>> reifySymbol "hello" symbolVal
"hello"

>>> reifySymbol "hello" reflect
"hello"


reifyTypeable :: Typeable a => a -> (forall s. (Typeable s, Reifies s a) => Proxy s -> r) -> r #

Reify a value at the type level in a Typeable-compatible fashion, to be recovered with reflect.

This can be necessary to work around the changes to Data.Typeable in GHC HEAD.

# Given

class Given a where #

This is a version of Reifies that allows for only a single value.

This is easier to work with than Reifies and permits extended defaulting, but it only offers a single reflected value of a given type at a time.

Minimal complete definition

given

Methods

given :: a #

Recover the value of a given type previously encoded with give.

give :: forall a r. a -> (Given a => r) -> r #

Reify a value into an instance to be recovered with given.

You should only give a single value for each type. If multiple instances are in scope, then the behavior is implementation defined.

int :: Int -> TypeQ #

This can be used to generate a template haskell splice for a type level version of a given int.

This does not use GHC TypeLits, instead it generates a numeric type by hand similar to the ones used in the "Functional Pearl: Implicit Configurations" paper by Oleg Kiselyov and Chung-Chieh Shan.

instance Num (Q Exp) provided in this package allows writing $(3) instead of $(int 3). Sometimes the two will produce the same representation (if compiled without the -DUSE_TYPE_LITS preprocessor directive).

nat :: Int -> TypeQ #

This is a restricted version of int that can only generate natural numbers. Attempting to generate a negative number results in a compile time error. Also the resulting sequence will consist entirely of Z, D, and SD constructors representing the number in zeroless binary.

# Useful compile time naturals

data Z #

Instances

 # Methodsreflect :: proxy Int -> a #

data D n #

Instances

 Reifies * n Int => Reifies * (D n) Int # Methodsreflect :: proxy Int -> a #

data SD n #

Instances

 Reifies * n Int => Reifies * (SD n) Int # Methodsreflect :: proxy Int -> a #

data PD n #

Instances

 Reifies * n Int => Reifies * (PD n) Int # Methodsreflect :: proxy Int -> a #

# Reified Monoids

data ReifiedMonoid a #

Constructors

 ReifiedMonoid FieldsreifiedMappend :: a -> a -> a reifiedMempty :: a

newtype ReflectedMonoid a s #

Constructors

 ReflectedMonoid a

Instances

 Reifies k s (ReifiedMonoid a) => Monoid (ReflectedMonoid k a s) # Methodsmempty :: ReflectedMonoid k a s #mappend :: ReflectedMonoid k a s -> ReflectedMonoid k a s -> ReflectedMonoid k a s #mconcat :: [ReflectedMonoid k a s] -> ReflectedMonoid k a s #

reifyMonoid :: (a -> a -> a) -> a -> (forall s. Reifies s (ReifiedMonoid a) => t -> ReflectedMonoid a s) -> t -> a #

foldMapBy :: Foldable t => (r -> r -> r) -> r -> (a -> r) -> t a -> r #

Fold a value using its Foldable instance using explicitly provided Monoid operations. This is like foldMap where the Monoid instance can be manually specified.

foldMapBy mappend mempty ≡ foldMap

>>> foldMapBy (+) 0 length ["hello","world"]
10


foldBy :: Foldable t => (a -> a -> a) -> a -> t a -> a #

Fold a value using its Foldable instance using explicitly provided Monoid operations. This is like fold where the Monoid instance can be manually specified.

foldBy mappend mempty ≡ fold

>>> foldBy (++) [] ["hello","world"]
"helloworld"


# Reified Applicatives

data ReifiedApplicative f #

Constructors

 ReifiedApplicative FieldsreifiedPure :: forall a. a -> f a reifiedAp :: forall a b. f (a -> b) -> f a -> f b

newtype ReflectedApplicative f s a #

Constructors

 ReflectedApplicative (f a)

Instances