Reference: http://stackoverflow.com/questions/17793466/black-magic-in-haskell-reflection

ReflectionTest.hs

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ScopedTypeVariables #-}

module ReflectionTest where

import Data.Proxy
import Unsafe.Coerce

data Config = Config
    { test :: Bool
    , dir  :: FilePath
    }

defaultConfig :: Config
defaultConfig = Config
    { test = False
    , dir  = "/Users/yuga"
    }

class Reifies s a | s -> a where
    reflect :: proxy s -> a

newtype Magic a r = Magic (forall (s :: *). Reifies s a => Proxy s -> r)

reify :: forall a r. a -> (forall (s :: *). Reifies s a => Proxy s -> r) -> r
reify a k = unsafeCoerce (Magic k :: Magic a r) (const a) Proxy

t :: FilePath
t = reify defaultConfig compose --(\p -> dir (reflect p) ++ "/test" )

compose :: Reifies s Config => Proxy s -> FilePath
compose p = dir (reflect p) ++ "/test"

-- Another implementation into which the above turns

newtype AnotherReifies s a = AnotherReifies { anotherReflect :: Proxy s -> a }

newtype AnotherMagic a r = AnotherMagic (forall s. AnotherReifies s a -> Proxy s -> r)

anotherReify :: forall a r. a -> (forall s. AnotherReifies s a -> Proxy s -> r) -> r
anotherReify a k = unsafeCoerce (AnotherMagic k :: AnotherMagic a r) (const a) Proxy

t2 :: FilePath
t2 = anotherReify defaultConfig compose2

compose2 :: AnotherReifies s Config -> Proxy s -> FilePath
compose2 dict p = dir (anotherReflect dict p) ++ "/test"

-- More another implementation

anotherReify2 :: forall a r. a -> (forall s. AnotherReifies s a -> Proxy s -> r) -> r
anotherReify2 a k =
    (unsafeCoerce :: (forall s. AnotherReifies s a -> Proxy s -> r) -> (Proxy s -> a) -> Proxy s -> r)
    k (const a) Proxy

t3 :: FilePath
t3 = anotherReify2 defaultConfig compose2