Question

有没有办法在ghci中打印出嵌套变量的推断类型？考虑一下代码，

let f = g where
    g (x :: Int) = x

然后，查询g的类型会很好，例如:t f.g会打印Int -> Int。

Answer 1

ghci调试器可以使用正确放置的断点为您打印（但您需要在模块中加载定义）：

{-# LANGUAGE ScopedTypeVariables #-} 

f a = g a where
    g (x :: Int) = x

然后在ghci：

Prelude> :l tmp2.hs
[1 of 1] Compiling Main             ( tmp2.hs, interpreted )
Ok, modules loaded: Main.
*Main> :b 3 9
Breakpoint 0 activated at tmp2.hs:3:7-9
*Main> f undefined
Stopped at tmp2.hs:3:7-9
_result :: Int = _
a :: Int = _
g :: Int -> Int = _
[tmp2.hs:3:7-9] *Main>

Answer 2

您可以通过提供适当错误的类型注释并检查错误消息来哄骗此信息。

*Main> let f = g where g::a; g (x::Int) = x

<interactive>:1:23:
    Couldn't match type `a1' with `Int -> Int'
      `a1' is a rigid type variable bound by...

有什么办法在do / while / let块中打印出一个变量类型？

2 个答案: