| Copyright | 2009-2015 Edward Kmett 2012 Elliott Hird 2004 Oleg Kiselyov and Chung-chieh Shan |
|---|---|
| License | BSD3 |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Stability | experimental |
| Portability | non-portable |
| Safe Haskell | Trustworthy |
| Language | Haskell98 |
Data.Reflection
Contents
Description
Reifies arbitrary terms at the type level. Based on the Functional Pearl: Implicit Configurations paper by Oleg Kiselyov and Chung-chieh Shan.
http://okmij.org/ftp/Haskell/tr-15-04.pdf
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 , 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 Proxy t =
Proxyexamples/Monoid.hs).
In addition, a simpler API is offered for working with singleton values such as a system configuration, etc.
- class Reifies s a | s -> a where
- reify :: forall a r. a -> (forall s. Reifies s a => Proxy s -> r) -> r
- reifyNat :: forall r. Integer -> (forall n. KnownNat n => Proxy n -> r) -> r
- reifySymbol :: forall r. String -> (forall n. KnownSymbol n => Proxy n -> r) -> r
- reifyTypeable :: Typeable a => a -> (forall s. (Typeable s, Reifies s a) => Proxy s -> r) -> r
- class Given a where
- give :: forall a r. a -> (Given a => r) -> r
- int :: Int -> TypeQ
- nat :: Int -> TypeQ
- data Z
- data D n
- data SD n
- data PD n
- data ReifiedMonoid a = ReifiedMonoid {
- reifiedMappend :: a -> a -> a
- reifiedMempty :: a
- newtype ReflectedMonoid a s = ReflectedMonoid a
- 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
- foldBy :: Foldable t => (a -> a -> a) -> a -> t a -> a
- data ReifiedApplicative f = ReifiedApplicative {
- reifiedPure :: forall a. a -> f a
- reifiedAp :: forall a b. f (a -> b) -> f a -> f b
- newtype ReflectedApplicative f s a = ReflectedApplicative (f a)
- reifyApplicative :: (forall x. x -> f x) -> (forall x y. f (x -> y) -> f x -> f y) -> (forall s. Reifies s (ReifiedApplicative f) => t -> ReflectedApplicative f s a) -> t -> f a
- traverseBy :: Traversable t => (forall x. x -> f x) -> (forall x y. f (x -> y) -> f x -> f y) -> (a -> f b) -> t a -> f (t b)
- sequenceBy :: Traversable t => (forall x. x -> f x) -> (forall x y. f (x -> y) -> f x -> f y) -> t (f a) -> f (t a)
Reflection
class Reifies s a | s -> a where #
Minimal complete definition
Methods
Recover a value inside a reify context, given a proxy for its
reified type.
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.
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"
Given
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
Template Haskell reflection
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).
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
Reified Monoids
data ReifiedMonoid a #
Constructors
| ReifiedMonoid | |
Fields
| |
newtype ReflectedMonoid a s #
Constructors
| ReflectedMonoid a |
Instances
| Reifies k s (ReifiedMonoid a) => Monoid (ReflectedMonoid k a s) # | |
reifyMonoid :: (a -> a -> a) -> a -> (forall s. Reifies s (ReifiedMonoid a) => t -> ReflectedMonoid a s) -> t -> a #
Reified Applicatives
data ReifiedApplicative f #
Constructors
| ReifiedApplicative | |
Fields
| |
newtype ReflectedApplicative f s a #
Constructors
| ReflectedApplicative (f a) |
Instances
| Reifies k s (ReifiedApplicative f) => Functor (ReflectedApplicative k * f s) # | |
| Reifies k s (ReifiedApplicative f) => Applicative (ReflectedApplicative k * f s) # | |
reifyApplicative :: (forall x. x -> f x) -> (forall x y. f (x -> y) -> f x -> f y) -> (forall s. Reifies s (ReifiedApplicative f) => t -> ReflectedApplicative f s a) -> t -> f a #
traverseBy :: Traversable t => (forall x. x -> f x) -> (forall x y. f (x -> y) -> f x -> f y) -> (a -> f b) -> t a -> f (t b) #
Traverse a container using its Traversable instance using
explicitly provided Applicative operations. This is like traverse
where the Applicative instance can be manually specified.
sequenceBy :: Traversable t => (forall x. x -> f x) -> (forall x y. f (x -> y) -> f x -> f y) -> t (f a) -> f (t a) #
Sequence a container using its Traversable instance using
explicitly provided Applicative operations. This is like sequence
where the Applicative instance can be manually specified.