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July 23, 2022 20:10
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Heterogeneous difference lists
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{-# LANGUAGE DataKinds #-} | |
{-# LANGUAGE FlexibleInstances #-} | |
{-# LANGUAGE GADTs #-} | |
{-# LANGUAGE StandaloneKindSignatures #-} | |
{-# LANGUAGE TypeOperators #-} | |
{-# LANGUAGE UndecidableInstances #-} | |
module HDList where | |
import qualified Control.Category as Cat (Category(..)) | |
import Data.Function (on) | |
import Data.Kind (Type) | |
import qualified HList as HList | |
import HList (HList(..), AllC, type (++)) | |
import Prelude hiding (head) | |
type HDList :: [Type] -> [Type] -> Type | |
newtype HDList xs ys = UnsafeHDList { unsafeApplyHDList :: HList xs -> HList ys } | |
instance AllC Eq ys => Eq (HDList '[] ys) where | |
(==) = (==) `on` toList | |
instance AllC Show ys => Show (HDList '[] ys) where | |
showsPrec p hdl = | |
showParen (p > 10) $ | |
showString "fromList " . shows (toList hdl) | |
instance Cat.Category HDList where | |
id = empty | |
(.) = append | |
fromList :: HList xs -> HDList ys (xs ++ ys) | |
fromList = UnsafeHDList . (HList.++) | |
toList :: HDList '[] ys -> HList ys | |
toList = ($ Nil) . unsafeApplyHDList | |
empty :: HDList xs xs | |
empty = UnsafeHDList id | |
singleton :: x -> HDList xs (x:xs) | |
singleton = UnsafeHDList . (:>) | |
cons :: y -> HDList xs ys -> HDList xs (y:ys) | |
cons x xs = UnsafeHDList $ (x :>) . unsafeApplyHDList xs | |
snoc :: HDList (x:xs) ys -> x -> HDList xs ys | |
snoc xs x = UnsafeHDList $ unsafeApplyHDList xs . (x :>) | |
append :: HDList ys zs -> HDList xs ys -> HDList xs zs | |
append xs ys = UnsafeHDList $ unsafeApplyHDList xs . unsafeApplyHDList ys | |
head :: HDList '[] (y:ys) -> y | |
head ys = case toList ys of | |
y :> _ -> y | |
tail :: HDList '[] (y:ys) -> HList ys | |
tail ys = case toList ys of | |
_ :> ys -> ys |
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{-# LANGUAGE DataKinds #-} | |
{-# LANGUAGE GADTs #-} | |
{-# LANGUAGE PolyKinds #-} | |
{-# LANGUAGE StandaloneDeriving #-} | |
{-# LANGUAGE StandaloneKindSignatures #-} | |
{-# LANGUAGE TypeFamilies #-} | |
{-# LANGUAGE TypeOperators #-} | |
{-# LANGUAGE UndecidableInstances #-} | |
module HList where | |
import Data.Kind (Constraint, Type) | |
import Prelude hiding ((++)) | |
type HList :: [Type] -> Type | |
data HList xs where | |
Nil :: HList '[] | |
(:>) :: x -> HList xs -> HList (x:xs) | |
infixr 5 :> | |
deriving instance AllC Eq xs => Eq (HList xs) | |
deriving instance AllC Show xs => Show (HList xs) | |
type AllC :: (k -> Constraint) -> [k] -> Constraint | |
type family AllC f xs where | |
AllC _ '[] = () | |
AllC f (x:xs) = (f x, AllC f xs) | |
type (++) :: [a] -> [a] -> [a] | |
type family xs ++ ys where | |
'[] ++ ys = ys | |
(x:xs) ++ ys = x:(xs ++ ys) | |
infixr 5 ++ | |
(++) :: HList xs -> HList ys -> HList (xs ++ ys) | |
Nil ++ ys = ys | |
(x :> xs) ++ ys = x :> (xs ++ ys) |
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