SpecList

SpecList.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Main where

import Data.Proxy

class SpecList a where
    type List a :: *

    slNil       :: Proxy a -> List a
    slCons      :: a -> List a -> List a
    slLength    :: List a -> Int
    slFoldr     :: (a -> b -> b) -> b -> List a -> b
    slMap       :: SpecList b
                => (a -> b) -> List a -> List b

data IntList
  = ILNil
  | ILCons {-# UNPACK #-} !Int IntList

intListLength :: IntList -> Int
intListLength ILNil = 0
intListLength (ILCons _ l) = 1 + intListLength l

intListFoldr :: (Int -> b -> b) -> b -> IntList -> b
intListFoldr _ acc ILNil = acc
intListFoldr f acc (ILCons i rest) = f i (intListFoldr f acc rest)

intListMap :: forall b . SpecList b
           => (Int -> b) -> IntList -> List b
intListMap _ ILNil = slNil (Proxy::Proxy b)
intListMap f (ILCons i rest) = slCons (f i) (intListMap f rest)

instance SpecList Int where
    type List Int = IntList

    slNil (_ :: Proxy Int) = ILNil :: List Int
    slCons   = ILCons
    slLength = intListLength
    slFoldr  = intListFoldr
    slMap    = intListMap

main :: IO ()
main = return ()