任何方式在do/while/let塊中打印出一個變量類型？

Question

有沒有什麼辦法在ghci中打印出嵌套變量的推斷類型？考慮代碼，

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

然後，很高興查詢g的類型，例如， :t f.g將打印出Int -> Int。

Answer 1

您可以通過給出錯誤的類型註釋並檢查錯誤消息來哄騙這些信息。

*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... 

Answer 2

ghci的調試器可以與正確放置斷點打印你（但你需要加載一個模塊中的定義）：

{-# LANGUAGE ScopedTypeVariables #-} 

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

然後在ghci的：

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>