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langston-barrett / czz

Whole-program, Scheme-scriptable, multi-language, coverage-guided fuzzer
https://langston-barrett.github.io/czz/
MIT License
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husk: Don't use `Dyn1` for `Maybe` #128

Open langston-barrett opened 2 years ago

langston-barrett commented 2 years ago

For functors, the problem can be solved without wrapping data in the Dyn1 constructor.

{-# Language GADTs #-}

import Data.Dynamic
import Data.Type.Equality
import Type.Reflection

unwrap :: Functor f => TypeRep f -> Dynamic -> Maybe (f Dynamic)
unwrap rep (Dynamic (App rep' a) x) = fmap
    (\HRefl -> Dynamic a <$> x)
    (eqTypeRep rep rep')
unwrap _ _ = Nothing

or even

unwrap :: TypeRep f -> Dynamic -> (forall a. f a -> f a) -> Maybe Dynamic
unwrap rep (Dynamic r@(App rep' a) x) f =
  do HRefl <- eqTypeRep rep rep'
     pure (Dynamic r (f x))
unwrap _ _ _ = Nothing