I have updated the reflection package on hackage to use an idea for avoiding dummy arguments posted to the Haskell cafe mailing list by Bertram Felgenhauer, which adapts nicely to the case of handling Reflection. The reflection package implements the ideas from the Functional Pearl: Implicit Configurations paper by Oleg Kiselyov and Chung-chieh Shan.
Now, you no longer need to use big scary undefineds throughout your code and can instead program with implicit configurations more naturally, using Applicative and Monad sugar.
*Data.Reflection> reify (+)
(reflect < *> pure 1 < *> (reflect < *> pure 2 < *> pure 3))
> 6
The Monad in question just replaces the lambda with a phantom type parameter, enabling the compiler to more readily notice that no instance can actually even try to use the value of the type parameter.
An example from the old API can be seen on the Haskell cafe.
This example can be made appreciably less scary now!
{-# LANGUAGE
MultiParamTypeClasses,
FlexibleInstances, Rank2Types,
FlexibleContexts, UndecidableInstances #-}
import Control.Applicative
import Data.Reflection
import Data.Monoid
import Data.Tagged
newtype M s a = M a
instance Reifies s (a,a → a → a) ⇒ Monoid (M s a) where
mempty = tagMonoid $ fst < $> reflect
a `mappend` b = tagMonoid $
snd < $> reflect < *> monoidTag a < *> monoidTag b
monoidTag :: M s a → Tagged s a
monoidTag (M a) = Tagged a
tagMonoid :: Tagged s a → M s a
tagMonoid (Tagged a) = M a
withMonoid :: a → (a → a → a) →
(∀s. Reifies s (a, a → a → a) ⇒ M s w) → w
withMonoid e op m = reify (e,op) (monoidTag m)
And with that we can cram a Monoid dictionary -- or any other -- with whatever methods we want and our safety is assured by parametricity due to the rank 2 type, just like with the ST monad.
*> withMonoid 0 (+) (M 5 `mappend` M 4 `mappend` mempty)
9
[Edit: factored Tagged out into Data.Tagged in a separate package, and modified reflection to use that instead, with an appropriate version bump to satisfy the package versioning policy]
