为什么Haskell中没有`Cofunctor`类型类? [英] Why there is no `Cofunctor` typeclass in Haskell?

问题描述

单子从Functor类型类获取fmap.为什么comonads不需要在Cofunctor类中定义的cofmap方法?

Monads get fmap from Functor typeclass. Why comonads don't need a cofmap method defined in a Cofunctor class?

推荐答案

Functor定义为:

class Functor f where
    fmap :: (a -> b) -> (f a -> f b)

Cofunctor可以定义如下:

class Cofunctor f where
    cofmap :: (b -> a) -> (f b -> f a)

因此，两者在技术上都是相同的，这就是Cofunctor不存在的原因. '一般的functor'的双重概念仍然是'一般的functor'".

So, both are technically the same, and that's why Cofunctor does not exist. "The dual concept of 'functor in general' is still 'functor in general'".

由于Functor和Cofunctor相同，所以通过使用Functor定义单子和共母.但是，不要让那让您认为单子和子母是同一回事，它们不是同一回事.

Since Functor and Cofunctor are the same, both monads and comonads are defined by using Functor. But don't let that make you think that monads and comonads are the same thing, they're not.

一个monad的定义(简化)为:

A monad is defined (simplifying) as:

class Functor m => Monad where
    return :: a -> m a
    (>>=) :: m a -> (a -> m b) -> m b

一个通俗的(再次简化)是:

whether a comonad (again, simplified) is:

class Functor w => Comonad where
    extract :: w a -> a
    extend :: (w a -> b) -> w a -> w b

注意对称性".

另一件事是逆函子，定义为:

Another thing is a contravariant functor, defined as:

import Data.Functor.Contravariant
class Contravariant f where
    contramap :: (b -> a) -> (f a -> f b)

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