去除临时多态性

Question

删除 haskell 中的临时多态性的最佳方法是什么？

80% 的时间，我不需要 fmap 在 Functor f 中是多态的，我实际上知道我将它应用于哪个实例。用特定实例替换它给我：

读代码时少动脑，多动脑检查
类型检查器验证时发现更多类型错误

像在范畴论中一样，将函子 F 应用于 haskell 中的态射的最佳方法是什么？

-- F is a functor : it maps objects of * to objects of *
data F r = Z | Suc r

-- F is a functor : it maps arrows of *  to arrows of *
-- generic fmap will be found for this type, I inherit much code for free, great
instance Functor F where
  fmap f Z       = Z
  fmap f (Suc n) = Suc (f n)

-- But I care writing code specific for this functor only
-- Applies F for arrows of *
fmapF = fmap :: (a -> b) -> (F a -> F b)

-- an arrow in *, aka a function -- (and also a value as * is CCC)
f = id :: a -> a

-- works for values F a, not any functor f
r = fmapF f Z :: F a
r' = fmap f "hi" -- as opposed to

Answer 1

我想你真的想要

{-# LANGUAGE TypeApplications #-}
r = fmap @ F f Z

@ F 部分指定我们想要函子 F.

的 fmap

去除临时多态性

removing ad-hoc polymorphism

haskell

functor

category-theory