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我正在为自己的语言编写解释器，并且我有一个具有这种类型的抽象语法树：

data Expression = 
  PInt Int
  | PFloat Double
  | PString String
  | PChar Char
  | PBool Bool
  | Var String
  | Unbound String String
  | Unary String Expression
  | Binary String Expression Expression
  | Call Expression [Expression]
  | Lambda Expression
  | Assign String Expression Expression
  | Conditional Expression Expression Expression
  deriving Eq

我正在尝试为我的班级编写一个 Num 实例，以便我可以使用现有的机器进行数值运算。这是我写的：

instance Num Expression where
   PInt a + PInt b = PInt $ a + b
   PInt a + PFloat b = PFloat $ a + b
   PFloat a + PInt b = PFloat $ a + b
   PFloat a + PFloat b = PFloat $ a + b
   _ + _ = undefined
   PInt a - PInt b = PInt $ a - b
   PInt a - PFloat b = PFloat $ a - b
   PFloat a - PInt b = PFloat $ a - b
   PFloat a - PFloat b = PFloat $ a - b
   _ - _ = undefined
   PInt a * PInt b = PInt $ a * b
   PInt a * PFloat b = PFloat $ a * b
   PFloat a * PInt b = PFloat $ a * b
   PFloat a * PFloat b = PFloat $ a * b
   _ * _ = undefined
   negate (PInt a) = PInt (-a)
   negate (PFloat a) = PFloat (-a)
   negate _ = undefined
   abs (PInt a) = PInt $ abs a
   abs (PFloat a) = PFloat $ abs a
   abs _ = undefined
   signum (PInt a) = PInt $ signum a
   signum (PFloat a) = PFloat $ signum a
   signum _ = undefined
   fromInteger i = (PInt $ fromInteger i)

这在我结合整数和浮点数的地方特别给了我错误。

Prelude> :load AST.hs
[1 of 1] Compiling AST          ( AST.hs, interpreted )

AST.hs:38:36:
    Couldn't match expected type `Double' with actual type `Int'
    In the first argument of `(+)', namely `a'
    In the first argument of `PFloat', namely `(a + b)'
    In the expression: PFloat (a + b)

AST.hs:39:37:
    Couldn't match expected type `Double' with actual type `Int'
    In the second argument of `(+)', namely `b'
    In the second argument of `($)', namely `a + b'
    In the expression: PFloat $ a + b

AST.hs:43:33:
    Couldn't match expected type `Double' with actual type `Int'
    In the first argument of `(-)', namely `a'
    In the second argument of `($)', namely `a - b'
    In the expression: PFloat $ a - b

AST.hs:44:37:
    Couldn't match expected type `Double' with actual type `Int'
    In the second argument of `(-)', namely `b'
    In the second argument of `($)', namely `a - b'
    In the expression: PFloat $ a - b

AST.hs:48:33:
    Couldn't match expected type `Double' with actual type `Int'
    In the first argument of `(*)', namely `a'
    In the second argument of `($)', namely `a * b'
    In the expression: PFloat $ a * b

AST.hs:49:37:
    Couldn't match expected type `Double' with actual type `Int'
    In the second argument of `(*)', namely `b'
    In the second argument of `($)', namely `a * b'
    In the expression: PFloat $ a * b
Failed, modules loaded: none.

这对我来说没有意义，因为 Haskell 中 Int + Double 的类型是 Double，所以 a + b 应该解析为 Double，并且由于 PFloat 的构造函数采用 Double，没问题......为什么不是这样吗？

已解决：fromIntegral在类型变量前面使用Int可修复它。

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1 回答 1

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Num 类型类中的数学运算符希望它们的两个参数具有相同的类型，因此您必须先使用将 Int 转换为 DoublefromIntegral才能将它们相加。

例如，替换这个

PInt a + PFloat b = PFloat $ a + b

有了这个

PInt a + PFloat b = PFloat $ fromIntegral a + b
于 2013-08-14T22:02:30.110 回答