使用 GHCi 时如何为函数提供显式的类型声明？

如何在 GHCi 中定义这个函数的等价物(取自学会了) ？

import Data.List


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

如果没有类型声明，GHCi 接受函数定义，但它最终得到的是一个无用的类型:

Prelude Data.List> import Data.List
Prelude Data.List> let numUniques' = length . nub
Prelude Data.List> :t numUniques'
numUniques' :: [()] -> Int

结果函数只接受一个单元列表作为参数。

在 GHCi 中有没有提供类型声明的方法？或者还有其他方法来定义这些不需要类型声明的函数吗？

我在 GHCi 指南中没有看到任何明显的线索，并且尝试了下面这些表达式(没有用) :

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

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最佳答案

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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Note that you can also avoid the monomorphism restriction simply by adding "points" (i.e. explicit variables) back to your expression. So this also gives the correct type:

let numUniques x = length . nub $ x

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The GHC User's Guide shows two additional ways to achieve this. This subsection introduces the :{ ... :} construct, which can be used as follows:

> :{
| numUniques :: (Eq a) => [a] -> Int
| numUniques = length . nub
| :}

Alternatively, you can enable multiline mode:

> :set +m
> let
| numUniques :: (Eq a) => [a] -> Int
| numUniques = length . nub
|