Haskell：顯示實例歧義

Question

我想寫一個顯示實例來顯示格式良好的公式，但在模仿完整的語法後，我仍然面臨如下的錯誤。

Hugs> :load "C:\\Users\\Devil\\Desktop\\CASESTUDY1.hs" 
ERROR file:.\CASESTUDY1.hs:15 - Ambiguous variable occurrence "show" 
*** Could refer to: CASESTUDY1.show Hugs.Prelude.show 

下面是我的.hs文件包含數據類型和相關show實例的內容。

module CASESTUDY1 

where 

data Wff = VAR String 
      | NEG Wff 
      | AND Wff Wff 
      | OR Wff Wff 
      | IMPL Wff Wff 

instance Show Wff where 
show (VAR x) = x 
show (NEG x) = "~" ++ show(x) 
show (AND x y) = "(" ++ show(x) ++ "^" ++ show(y) ++ ")" 
show (OR x y) = "(" ++ show(x) ++ "v" ++ show(y) ++ ")" 
show (IMPL x y) = "(" ++ show(x) ++ "-->" ++ show(y) ++ ")" 

Answer 1

在Haskell，空白是很重要的。您需要縮進屬於您的Show實例的show。

instance Show Wff where 
    show (VAR x)  = show x 
    show (NEG x)  = "~" ++ show x 
    show (AND x y) = "(" ++ show x ++ "^" ++ show y ++ ")" 
    show (OR x y) = "(" ++ show x ++ "v" ++ show y ++ ")" 
    show (IMPL x y) = "(" ++ show x ++ "-->" ++ show y ++ ")" 

此外，您不需要括號來傳遞參數來顯示。 show(x)應該是show x。

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如果你正在學習哈斯克爾我建議這些特殊資源：

• Learn You a Haskell For Great Good
• Real World Haskell