Question

大家好我正在学习haskell并且我使用这段代码来实现分数但是当我编译它时我收到错误parse error on input ‘|’ 我的代码如下：

module Fraccion(Frac,Div,mcd,fraccion) where


data Frac = Div Integer Integer deriving (Eq, Show)


mcd :: Integer -> Integer -> Integer
mcd a b | a > b = mcd (a-b) b
        | a < b = mcd a (b-a)
        | a == b = a

fraccion :: Integer -> Integer -> Frac 
fraccion a b | b == 0 = error "divisor can't be 0"
             | b <  0 = fraccion (-a) (-b)
             | b >  0 = Div (a `div` mcdab) (b `div` mcdab)
                        where mcdab = mcd (abs a) b

instance (Num Frac) where
    negate (Div a b)      = (Div (-a) b)
    abs    (Div a b)      = (Div (abs a) b)
    (Div a b) * (Div c d) = fraccion (a*c) (b*d)
    (Div a b) + (Div c d) = fraccion (a*d+b*c) (b*d)
    (Div a b) - (Div c d) = fraccion (a*d-b*c) (b*d)
    fromInteger n = (Div n 1)
    signum (Div a b) | a > 0 = 1
                     | a < 0 + -1
                     | a == b = 0

instance show Frac where
    show (Div a b) = (show a) ++ "/" ++ (show b)

你可以帮我解决这个错误吗？

Answer 1

第27行：

| a < 0 + -1

一个简单的错字。您将+放在了=

的位置

| a < 0 = -1

您还希望在倒数第二行上大写show。您的类型的deriving子句也不应包含Show，因为您明确定义了一个实例而不是派生它。最后，您的模块导出列表应该如下所示。

module Fraccion(Frac(Div),mcd,fraccion) where

尝试编译时，输入“|”上的Haskell解析错误

1 个答案: