相关疑难解决方法(0)

我有一个函数,它在where子句中定义了一个值,我想给它一个显式的类型注释.注释需要使用顶级函数中的类型变量,因此我的理解是我需要使用ScopedTypeVariables.这是问题的最小减少:

{-# LANGUAGE ScopedTypeVariables #-}

import Control.Monad.Trans.Except
import Data.Functor.Identity

f :: ExceptT String Identity a -> Maybe a
f m = Nothing
  where x :: Identity (Either String a)
        x = runExceptT m

此代码不能进行类型检查.它失败并显示以下错误消息:

Couldn't match type ‘a’ with ‘a1’
  ‘a’ is a rigid type variable bound by
      the type signature for f :: ExceptT String Identity a -> Maybe a
      at src/Lib.hs:20:6
  ‘a1’ is a rigid type variable bound by
       the type signature …