Data.Reflection

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

http://www.cs.rutgers.edu/~ccshan/prepose/prepose.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 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).

Documentation

class Reifies s a | s -> a where

Methods

reflect :: proxy s -> a

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.