模糊的类型变量，但不在ghci中？ [英] Ambiguous type variable but not in ghci?

问题描述

任何人都可以解释为什么haskell在下面的例子中强制显式类型签名，以及如何修改它以避免需要显式声明？

 将限定的Data.List导入为L 
 
 main = do 
 print $ length $ L.nub [1,1,2,3]  -  3，通过（方程a，数字a）=> [a] 
 print $ length $ L.nub []  - 模糊类型错误，已通过（Eq a）=> [a] 
  - 可以帮助键入带有明确签名的系统，但如何避免？ 
 print $ length $ L.nub（[] :: [Int]）
  

令人惊讶的是在ghci中交互式编写的相同代码没有歧义，并且打印零长度：

 λ> ：m + Data.List 
λ>打印$ length $ nub [] 
 0  -  ??你可以解释吗 ？？ 
  

更新：
看起来与< c $ c> Data.List.nub 长度函数不会停止对不明确类型的投诉：

  length':: Eq a => [a]  - > Int 
 length'[] = 0 
 length'（x：xs）= 1 + length'xs 
 
 main = do 
 print $ length'$ nub [ ] 
  - 没有使用'length'引起的（Eq a0）实例
  - 类型变量'a0'不明确
  

解决方案

问题是 [] 具有多态类型（等式a）=> [α] 。由于长度不会添加任何特定约束。 $ b

特别是长度是：

 长度:: [a]  - > Int 
  

这比 nub ：

  nub :: Eq a => [a]  - > [a] 
  

编译器需要使用长度那里，并且不能推断出 a 的类型。

现在你有两个选项：

1. 使用来打开 ExtendedDefaultRules ＃LANGUAGE ExtendedDefaultRules＃ - }


2. 显式： ... L.nub（[]： ：[Int]）

默认情况下，我会推荐第二个，除非您完全理解第一个的后果。

Can anybody explain why haskell enforces explicit type signature in the following example and how to modify it to avoid need of the explicit declaration ?

import qualified Data.List as L

main = do
    print $ length $ L.nub [1,1,2,3]  -- 3, passed (Eq a, Num a) => [a]
    print $ length $ L.nub []         -- ambiguous type error, passed (Eq a) => [a]
    -- can help type system with explicit signature but how to avoid ?
    print $ length $ L.nub ([] :: [Int])

Surprisingly the same code written interactively in ghci has no issue with ambiguity and does prints zero length:

λ> :m +Data.List
λ> print $ length $ nub []
0  -- ?? can you explain ??

Update: It seems even same restricted as Data.List.nub length function won't stop complaints about ambiguous type:

length' :: Eq a => [a] -> Int
length' [] = 0
length' (x:xs) = 1 + length' xs

main = do
    print $ length' $ nub []
-- No instance for (Eq a0) arising from a use of ‘length'’
-- The type variable ‘a0’ is ambiguous

解决方案

The problem is that [] has the polymorphic type (Eq a) => [a]. Since length doesn't add any particular constraint.

Specifically the type of length is:

length :: [a] -> Int

which is even more permissive than nub:

nub :: Eq a => [a] -> [a]

The compiler needs to use a specific instance of length there and is not able to deduce a type for a.

Now you have two options:

1. Turn on the ExtendedDefaultRules extension with {-# LANGUAGE ExtendedDefaultRules #-} at the beginning of the file.
2. Be explicit: ... L.nub ([] :: [Int])

I'd recommend the 2nd one by default, unless you fully understand the consequences of the first one.

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