不同情况下使用的多态变量haskell [英] Polymorphic variable used in different contexts haskell

问题描述

我有一段Haskell代码：

I have a following piece of Haskell code:

foo :: Num a => (a -> a) -> Either Integer Double -> Either Integer Double
foo f x = case x of
  Left i -> Left $ f i
  Right d -> Right $ f d

它不会编译时出现错误无法匹配整型与双精度。我知道 f 类型的计算方式为整数 - >整数和双精度 - > Double 在左和右表达式中分别创建冲突。

It doesn't compile with error "Couldn't match type Integer with Double". I understand that type of f is computed as Integer -> Integer and Double -> Double in Left and Right expressions respectively, creating a collision.

如何更改此函数，以便每次使用正确版本的 f ？

How can I change this function, so that correct version of f would be used each time?

推荐答案

您需要 RankNTypes 。

{-# LANGUAGE RankNTypes #-}

foo :: (forall a. Num a => a -> a) -> Either Integer Double -> Either Integer Double
foo f x = case x of
    Left i -> Left $ f i
    Right d -> Right $ f d

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