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[英]Functor instance for (newtype Mu f = InF {outF :: f (Mu f)})
[英]Why can I not make a (Functor f) => ConcreteType -> f String into an instance Functor ((->) ConcreteType)?
我构造类型的方式,我相信这将遵循Functor法则,该法则规定应该有一个恒等函数,fmap为其返回原始函子。
码:
-- apply a style function to a shell prompt functor
-- e.g.
-- bold & fgColor red `style` gitCurrentBranch
style :: (String -> ShellPromptType -> String) -> ShellPromptSegment String
-> ShellPromptType -> ShellPromptSegment String
style f segment = \shType -> (flip f) shType <$> segment
-- this is fine
style' :: (String -> ShellPromptType -> String)
-> (ShellPromptType -> ShellPromptSegment String)
-> ShellPromptType -> ShellPromptSegment String
style' f makeSegment = flip f >>= \g shellType -> fmap g $ makeSegment shellType
-- this apparently is not. Compiler complains that it wants the type (String -> String) -> ShellPromptType -> b
-- for my lambda function there, but it gets (String -> String) -> ShellPromptType -> ShellPromptSegment String
-- instead. I guess 'b' is not allowed to be a functor?
instance Functor ((->) ShellPromptType) where
fmap f makeSegment = ((flip f) :: ShellPromptType -> String -> String)
>>= ((\g shellType -> fmap g $ makeSegment shellType)
:: (String -> String) -> ShellPromptType -> (ShellPromptSegment String))
错误信息:
LambdaLine/Shells/ShellPromptSegment.hs|81 col 30 error| Couldn't match type `ShellPromptType -> String'
|| with `ShellPromptSegment String'
|| Expected type: (String -> String) -> ShellPromptType -> b
|| Actual type: (String -> String)
|| -> ShellPromptType -> ShellPromptSegment String
|| In the second argument of `(>>=)', namely
|| `((\ g shellType -> fmap g $ makeSegment shellType) ::
|| (String -> String)
|| -> ShellPromptType -> (ShellPromptSegment String))'
|| In the expression:
|| ((flip f) :: ShellPromptType -> String -> String)
|| >>=
|| ((\ g shellType -> fmap g $ makeSegment shellType) ::
|| (String -> String)
|| -> ShellPromptType -> (ShellPromptSegment String))
|| In an equation for `fmap':
|| fmap f makeSegment
|| = ((flip f) :: ShellPromptType -> String -> String)
|| >>=
|| ((\ g shellType -> fmap g $ makeSegment shellType) ::
|| (String -> String)
|| -> ShellPromptType -> (ShellPromptSegment String))
LambdaLine/Shells/ShellPromptSegment.hs|81 col 56 error| Couldn't match type `[Char]' with `ShellPromptSegment String'
|| Expected type: ShellPromptSegment String
|| Actual type: a
|| In the return type of a call of `makeSegment'
|| In the second argument of `($)', namely `makeSegment shellType'
|| In the expression: fmap g $ makeSegment shellType
您太专业了。
函子的定义如下:
class Functor f where
fmap :: (a -> b) -> f a -> f b
这个想法是,它需要一个正常的功能,并将其提升到某种上下文中。 但这还不止于此:想法是它可以接受任何正常功能并将其提升到上下文中。 对于列表函子, fmap
可以执行任何功能并在适当类型的列表上执行它。
您所做的总是总是从函子中返回相同的类型,这违反了将其用作函子的目的,因此Haskell不允许这样做。
考虑一下Functor
的定义:
class Functor f where
fmap :: (a -> b) -> f a -> f b
该类或方法不限制类型a
和b
,因此您定义的任何fmap
必须适用于任何类型a
和b
。 您可以定义自己的类型类,例如:
class MyFunctor f where
myfmap :: (String -> String) -> f String -> f String
但是那不是Functor
。
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