有没有办法像这样定义一个函数:
f :: (C a, C b) => a -> Maybe b
这样:
f = Just
当输入a
= b
时,并且
f _ = Nothing
何时输入a
/= b
?
请注意:
- 我不想使用重叠的实例。
- 我不希望有类约束
C
,但我非常确定它是必要的(特别是如果我想避免重叠实例)。 - 我想在此方案中为要比较的每种类型仅定义一个实例,定义一个类
Equal a b
使这个问题变得微不足道,但它需要n^2
code> 我想避免的实例。
最佳答案
就这样
http://hackage.haskell.org/package/base-4.9.1.0/docs/Data-Typeable.html#v:cast
cast :: (Typeable a, Typeable b) => a -> Maybe b
使用示例
函数转换可以是“像面向对象语言这样的类型转换系统”的一部分。细菌和真菌是生命。他们可以被提升到生命。生命也许能够转化为细菌或真菌。
{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-}
import Data.Typeable
import Data.Maybe
data SomeLife = forall l . Life l => SomeLife l
deriving Typeable
instance Show SomeLife where
show (SomeLife l) = "SomeLife " ++ show l
class (Typeable l, Show l) => Life l where
toLife :: l -> SomeLife
fromLife :: SomeLife -> Maybe l
toLife = SomeLife
fromLife (SomeLife l) = cast l
instance Life SomeLife where
toLife = id
fromLife = Just
data Bacteria = Bacteria deriving (Typeable, Show)
instance Life Bacteria
data Fungus = Fungus deriving (Typeable, Show)
instance Life Fungus
castLife :: (Life l1, Life l2) => l1 -> Maybe l2
castLife = fromLife . toLife
filterLife :: (Life l1, Life l2) => [l1] -> [l2]
filterLife = catMaybes . map castLife
withLifeIO :: (Life l1, Life l2) => l1 -> (l2 -> IO ()) -> IO ()
withLifeIO l f = maybe (return ()) f $ castLife l
withYourFavoriteLife :: Life l => (l -> IO ()) -> IO ()
withYourFavoriteLife act = do
withLifeIO Bacteria act
withLifeIO Fungus act
main :: IO ()
main = do
let sls :: Life l => [l]
sls = filterLife [
SomeLife Bacteria,
SomeLife Fungus,
SomeLife Fungus ]
print (sls :: [SomeLife])
print (sls :: [Bacteria])
print (sls :: [Fungus])
withYourFavoriteLife (print :: SomeLife -> IO ())
withYourFavoriteLife (print :: Bacteria -> IO ())
withYourFavoriteLife (print :: Fungus -> IO ())
该技术扩展到类型层次结构系统。一生模块就在这里。 https://github.com/YoshikuniJujo/test_haskell/blob/master/features/existential_quantifications/type_hierarchy/Life.hs
异常系统使用这种技术。 https://hackage.haskell.org/package/base-4.9.1.0/docs/Control-Exception.html
关于haskell - 类型相等函数,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/44647096/