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Not quite sure how to implement
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| {-# LANGUAGE DataKinds | |
| , GADTs | |
| , KindSignatures | |
| , ScopedTypeVariables | |
| , TypeOperators | |
| #-} | |
| module LengthList | |
| ( LengthList (..) | |
| -- examples | |
| , list2 | |
| , list3 | |
| -- list functions | |
| , zipWith | |
| , length | |
| ) where | |
| import Prelude hiding (length, zipWith) | |
| import Data.Proxy (Proxy (..)) | |
| import GHC.TypeLits | |
| data LengthList :: Nat -> * -> * where | |
| Cons :: a -> LengthList (n-1) a -> LengthList n a | |
| Nil :: LengthList (0 :: Nat) a | |
| instance Show a => Show (LengthList n a) where | |
| show = show . toList | |
| -- length-list examples | |
| list2 :: LengthList 2 Char | |
| list2 = Cons 'a' (Cons 'b' Nil) | |
| list3 :: LengthList 3 Char | |
| list3 = Cons 'a' (Cons 'b' (Cons 'c' Nil)) | |
| -- list functions duplicated for length-lists | |
| zipWith :: (a -> b -> c) | |
| -> LengthList (n :: Nat) a | |
| -> LengthList (n :: Nat) b | |
| -> LengthList (n :: Nat) c | |
| zipWith f (Cons x xs) (Cons y ys) = Cons (f x y) (zipWith f xs ys) | |
| zipWith _ Nil Nil = Nil | |
| length :: forall n a. (KnownNat n) => LengthList n a -> Integer | |
| length _ = natVal (Proxy :: Proxy n) | |
| -- conversions | |
| toList :: LengthList n a -> [a] | |
| toList (Cons x xs) = x : toList xs | |
| toList Nil = [] |
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