Question

我正在尝试编写一个show实例来显示格式良好的公式，但是在模拟整个语法之后，我仍然面临着与下面相同的错误。

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

以下是我的.hs文件的内容包括数据类型和相关的show instance。

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中，空白很重要。您需要缩进属于show实例的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，您不需要括号来传递要显示的参数。 show(x)应为show x。

如果您正在学习haskell，我推荐这些特殊资源：

Haskell：显示实例歧义

1 个答案: