Created
October 21, 2017 09:50
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Closed kinds aren't as closed as you'd think
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{-# language PolyKinds, DataKinds, RankNTypes, TypeApplications, ScopedTypeVariables, TypeFamilies, FlexibleInstances #-} | |
import Data.Proxy | |
class KnownUnit (k :: ()) where | |
reflect :: Proxy k -> Int | |
instance KnownUnit '() where | |
reflect _ = 0 | |
instance KnownUnit x => KnownUnit (f x) where | |
reflect _ = 1 + reflect (Proxy :: Proxy x) | |
type family Succ :: k -> k | |
reify :: Int -> (forall k. KnownUnit k => Proxy k -> r) -> r | |
reify 0 f = f (Proxy :: Proxy '()) | |
reify n f = reify (n - 1) $ \(Proxy :: Proxy k) -> f (Proxy :: Proxy (Succ k)) | |
-- ghci> reify 23 reflect | |
-- 23 |
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I mentioned it in the tweet that originally linked here, but the technique that I used here is originally due to aarvar.