Created
October 27, 2022 08:08
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Generalising / DRYing functions which apply to GADTs
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{-# LANGUAGE GADTs, DataKinds #-} | |
{-# LANGUAGE KindSignatures #-} | |
{-# LANGUAGE RankNTypes #-} | |
{-# LANGUAGE ScopedTypeVariables #-} | |
{-# LANGUAGE TypeApplications #-} | |
module Main where | |
import Data.Typeable (cast) | |
import Type.Reflection | |
-- See: https://new.reddit.com/r/haskell/comments/yebk9v/generalising_drying_functions_which_apply_to_gadts/ | |
data MyType = TypeOne | TypeTwo | |
data MyEntity (t :: MyType) where | |
MyEntityOne :: String -> MyEntity 'TypeOne | |
MyEntityTwo :: String -> String -> MyEntity 'TypeTwo | |
-- Every value of @MyEntity b@ contains enough information to determine the | |
-- type @b@ to be a known type. | |
-- | |
-- This fact can be represented by the following function: | |
hasTypeable :: MyEntity b -> (Typeable b => MyEntity b -> r) -> r | |
hasTypeable x@(MyEntityOne {}) f = f x | |
hasTypeable x@(MyEntityTwo {}) f = f x | |
filterFor :: forall a b. Typeable a => MyEntity b -> Maybe (MyEntity a) | |
filterFor x = hasTypeable x (cast :: Typeable b => MyEntity b -> Maybe (MyEntity a)) | |
filterForOne :: forall b. MyEntity b -> Maybe (MyEntity 'TypeOne) | |
filterForOne = filterFor @'TypeOne | |
main :: IO () | |
main = putStrLn "compiles" |
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