類型A不等於Haskell（ghci解釋器）中的類型A？

Question

GHCi告訴我，類型A不是類型A。爲什麼？與作用域打交道時，下面是清楚地表明瞭這縮短會議

>>> data A = A 
>>> let x = A 
>>> let id A = A 
>>> 
>>> data A = A 
>>> let x' = A 
>>> let id' A = A 
>>> 
>>> data A = A 
>>> 
>>> let y = id' x 

<interactive>:18:13: 
    Couldn't match expected type `main::Interactive.A' 
       with actual type `main::Interactive.A' 
    In the first argument of id', namely `x' 
    In the expression: id' x 
    In an equation for `y': y = id' x 

Answer 1

GHCI有一些奇怪的行爲：

Prelude> data A = A 
Prelude> let meh A = A 
Prelude> data A = A 
Prelude> meh A 

<interactive>:5:5: 
    Couldn't match expected type `main::Interactive.A' 
      with actual type `A' 
    In the first argument of `meh', namely `A' 
    In the expression: meh A 
    In an equation for `it': it = meh A 

至於GHCI的關注，你不妨這樣做：

Prelude> data A = A 
Prelude> let meh A = A 
Prelude> data A' = A' 
Prelude> meh A' 

<interactive>:5:5: 
    Couldn't match expected type `A' with actual type A' 
    In the first argument of `meh', namely A' 
    In the expression: meh A' 
    In an equation for `it': it = meh A' 

它認爲它是完全不同的數據類型，只是它們具有相同的名稱。

你可以閱讀所有關於它here。相關部分是2.4.4。