Question

我无法在几个模糊的类型变量错误问题中找到我的问题的答案。

基本上我想将类型信息带到值级别。此示例中的最后一行失败。

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE TypeOperators #-}

module Test where

data Typ = TInteger | TString deriving Show

data Empty = Empty
data a ## b = Cons a b

class Typical a b | a -> b where
    typical :: a -> b

instance Typical Empty [Typ] where
    typical _ = []

instance Typical Integer Typ where
    typical _ = TInteger

instance Typical String Typ where
    typical _ = TString

instance (Typical a Typ, Typical b [Typ]) => Typical (a ## b) [Typ] where
    typical _ = typical (undefined :: a) : typical (undefined :: b)

以下是第一条错误消息：

Test.hs:27:17:
Could not deduce (Typical a0 Typ) arising from a use of `typical'
from the context (Typical a Typ, Typical b [Typ])
  bound by the instance declaration at Test.hs:26:10-67
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
  instance Typical String Typ -- Defined at Test.hs:23:10
  instance Typical Integer Typ -- Defined at Test.hs:20:10
Possible fix: add an instance declaration for (Typical a0 Typ)
In the first argument of `(:)', namely `typical (undefined :: a)'
In the expression:
  typical (undefined :: a) : typical (undefined :: b)
In an equation for `typical':
    typical _ = typical (undefined :: a) : typical (undefined :: b)

我只是不明白。

这里a0是什么？可能是我最后一行的a没有用最后一行的那个标识吗？

我应该在哪里放置类型签名？为什么？

请赐教！

Answer 1

好的，我有一个解决方案，但我不知道这是否是最干净的解决方法。

添加{-# LANGUAGE ScopedTypeVariables #-}会使代码编译。这样就可以从错误消息中识别a0 a（对应于代码最后两行的a）。

请评论！

Haskell - 模糊类型变量

1 个答案: