如何在使用GHCi时为函数提供显式类型声明？ [英] How to provide explicit type declarations for functions when using GHCi?

问题描述

如何在GHCi中定义此功能的等价物（取自 learnyouahaskell ）？

  import Data.List 
 
 numUniques ::（Eq a）=> [a]  - > Int 
 numUniques = length。 nub 
  

如果没有类型声明，GHCi会接受函数定义，但最终会出现无用的类型：

  Prelude Data.List> import Data.List 
 Prelude Data.List>让numUniques'=长度。 nub 
 Prelude Data.List> ：t numUniques'
 numUniques':: [（）]  - > Int 
  

结果函数只接受单位列表作为参数。

有没有办法在GHCi中提供类型声明？还是有另一种方式来定义这些不需要类型声明的函数？

我没有看到GHCi指南中的明显线索，并尝试使用类似以下的表达式（无效）：

 >让numUniques'=（（length.nub）::（等式a）=> [a]  - > Int）
> ：t numUniques'
 numUniques':: [（）]  - > Int 
  

解决方案

在GHCi中提供类型声明？

  let numUniques'::（Eq a）=> ; [a]  - >诠释; numUniques'=长度。或者是否有另一种方法来定义类似这样的函数，它不会' t要求类型声明？
 
如果关闭 -XNoMonomorphismRestriction ，它会推断出正确的类型。
 
How to I define the equivalent of this function (taken from learnyouahaskell) inside GHCi?
import Data.List  

numUniques :: (Eq a) => [a] -> Int  
numUniques = length . nub  
Without the type declaration, GHCi accepts the function definition, but it ends up with an unhelpful type:
Prelude Data.List> import Data.List 
Prelude Data.List> let numUniques' = length . nub
Prelude Data.List> :t numUniques'
numUniques' :: [()] -> Int
The resulting function only accepts a list of units as a parameter.

Is there a way provide type declarations in GHCi? Or is there another way to define functions like these which doesn't require type declarations?

I saw no obvious clues in the GHCi guide, and experimented with expressions like the following (to no avail):
> let numUniques' = ((length . nub) :: (Eq a) => [a] -> Int)
> :t numUniques'
numUniques' :: [()] -> Int

解决方案

  
Is there a way provide type declarations in GHCi?


let numUniques' :: (Eq a) => [a] -> Int; numUniques' = length . nub



  
Or is there another way to define functions like these which doesn't require type declarations?
If you turn off the monomorphism restriction with -XNoMonomorphismRestriction, it will infer the right type.

                        
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