根據上面的註釋,似乎可能沒有乾淨的方式來編寫我需要的函數,使用由Control.Newtype
提供的hofs。但是,似乎還有其他選擇:爲非 -newtypes創建實例。
與newtype包一個例子:
{-# LANGUAGE MultiParamTypeClasses, UndecidableInstances,
FlexibleInstances #-}
import Control.Newtype
instance (Newtype n o) => Newtype [n] [o] where
pack = map pack
unpack = map unpack
instance (Newtype n o) => Newtype (Maybe n) (Maybe o) where
pack = fmap pack
unpack = fmap unpack
instance (Newtype n o, Newtype n' o') => Newtype (n -> n') (o -> o') where
pack f = pack . f . unpack
unpack f = unpack . f . pack
-- a newtype wrapper for Nums
newtype NNum a = NNum {unNNum :: a}
instance Newtype (NNum a) a where
pack = NNum
unpack = unNNum
ntimes5 :: (Num a) => NNum a -> NNum a
ntimes5 = pack sum . replicate 5
foo :: a -> Maybe [a]
foo = undefined
bar :: NNum a -> Maybe [NNum a]
bar = pack foo
如前所述bheklilr,這需要UndecidableInstances
,但它似乎並不過分要求籤名。然而,我們可以做的更好使用newtype-generics包:(當然,你總是可以在此手動導出Newtype
情況下,也節省了延伸和一個進口)
{-# LANGUAGE TypeFamilies, DeriveGeneriC#-}
import Control.Newtype
import GHC.Generics
instance (Newtype a) => Newtype [a] where
type O [a] = [O a]
pack = map pack
unpack = map unpack
instance (Newtype a) => Newtype (Maybe a) where
type O (Maybe a) = Maybe (O a)
pack = fmap pack
unpack = fmap unpack
instance (Newtype a, Newtype b) => Newtype (a -> b) where
type O (a -> b) = (O a -> O b)
pack f = pack . f . unpack
unpack f = unpack . f . pack
newtype NNum a = NNum {unNNum :: a} deriving (Generic)
instance Newtype (NNum a)
ntimes5 :: (Num a) => NNum a -> NNum a
ntimes5 = pack sum . replicate 5
foo :: a -> Maybe [a]
foo = undefined
bar :: NNum a -> Maybe [NNum a]
bar = pack foo
因此這會由於出現的任何問題UndecidableInstances
或FlexibleInstances
現在沒有意義。類型家庭與fundeps的比較總結爲here和here。這個例子似乎是一個類型家庭提供明確勝利的例子。
我相信第一個最好的解決方法是使用'pack'和'unpack',使用'Data.Function'和'Data.Composition'可以簡單地用'f1 operator = pack。:on operator unpack'實現。另外兩個也很簡單:'f1 f = fmap pack。 F 。解壓縮「和」f3 f = pack「。 F 。 fmap解包「。事實上,'f2'和'f3'在'over'和'overF'兩個版本之間是中途的,這兩個版本都是在'newtype'包中用'pack'和'unpack'實現的。我會遵循作者的例子並保持簡單。 – bheklilr
對Newtype(也許n)(也許o)','Newtype [n] [o]'和Newtype(o - > o')(n - > n')''有實例嗎? – crockeea
你甚至可以定義這些實例嗎? – bheklilr