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@dmjio
Forked from roelvandijk/Nub.hs
Created Feb 7, 2016
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Incredibly slow type-level Nub
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
module Nub where
import Data.Proxy
import Data.Type.Bool
import GHC.TypeLits
-- | Remove duplicates from a type-level list.
type family Nub xs where
Nub '[] = '[]
Nub (x ': xs) = x ': Nub (Remove x xs)
type family Remove x xs where
Remove x '[] = '[]
Remove x (x ': ys) = Remove x ys
Remove x (y ': ys) = y ': Remove x ys
class KnownNats ns where
natVals :: proxy ns -> [Integer]
instance KnownNats '[] where
natVals _ = []
instance (KnownNat n, KnownNats ns) => KnownNats (n ': ns) where
natVals _ = natVal (Proxy :: Proxy n) : natVals (Proxy :: Proxy ns)
type Example = '[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 1, 1, 1, 1, 1]
exampleVals :: [Integer]
exampleVals = natVals (Proxy :: Proxy (Nub Example))
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