如何将多态函数应用于 Either 的两边？

Question

我试过这个：

type TestT = Either Int Float

testM :: (a -> a) -> TestT -> TestT
testM f (Left x) = Left (f x)
testM f (Right x) = Right (f x)

但是还是不行，请问有什么办法吗？我环顾四周，类似的东西真的很复杂和有限。

错误消息，根据要求：

Main.hs:101:28: error:
    • Couldn't match expected type ‘a’ with actual type ‘Int’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          testM :: forall a. (a -> a) -> TestT -> TestT
        at Main.hs:100:1-35
    • In the first argument of ‘f’, namely ‘x’
      In the first argument of ‘Left’, namely ‘(f x)’
      In the expression: Left (f x)
    • Relevant bindings include
        f :: a -> a (bound at Main.hs:101:7)
        testM :: (a -> a) -> TestT -> TestT (bound at Main.hs:101:1)

Answer 1

我认为您无法用基础语言做到这一点。如评论中所述，您可能需要启用几个扩展，例如 RankNTypes。

由于所有涉及的类型都是数字类型，因此很容易使用增量函数，例如 (+1) 作为多态函数。

让我们试试 ghci:

$ ghci
GHCi, version 8.6.5: http://www.haskell.org/ghc/  :? for help
 λ> 
 λ> type TestT = Either Int Float 
 λ> 
 λ> :set +m
 λ> 
 λ> :set -XRankNTypes
 λ> :set -XScopedTypeVariables
 λ> 
 λ> {-
|λ> let { testM :: (forall a. Num a => a -> a) -> TestT -> TestT ;
|λ>       testM fn (Left x) = Left (fn x) ;
|λ>       testM fn (Right x) = Right (fn x) }
|λ> -}
 λ> 
 λ> :type testM
testM :: (forall a. Num a => a -> a) -> TestT -> TestT
 λ> 
 λ> testM (+3) (Left 42)
Left 45
 λ> 
 λ> testM (+3) (Right 3.14159)
Right 6.14159
 λ>

注意 1： 如果省略语言扩展，它会中断，并显示一条提示 RankNTypes 的消息。

注2：如果用forall a. Num a => (a -> a)代替(forall a. Num a => a -> a)，它也会中断。

注 3： 这里有一些现有技术： Alexis King 的有用评论。

Answer 2

一种方法是使用 Bifunctor：

Prelude> :m +Data.Bifunctor
Prelude Data.Bifunctor> bimap show show (Left 3)
Left "3"
Prelude Data.Bifunctor> bimap show show (Right 'x')
Right "'x'"
Prelude Data.Bifunctor>

如何将多态函数应用于 Either 的两边？

How to apply a polymorphic function to both sides of an Either?

polymorphism

haskell

rank-n-types