Data.Reflection

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 reduces to using two combinators, reify and reflect.

 ghci> reify 6 (\p -> reflect p + reflect p) :: Int
 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).

Reifying any term at the type level

class Reified s where

Associated Types

type Reflected s

Methods

reflect :: p s -> Reflected s

reify :: a -> (forall s. (Reified s, Reflected s ~ a) => Proxy s -> w) -> w