为什么我会收到 "match redudant" 错误？

Question

我对运行以下代码时遇到的 "match redundant" 错误感到有点困惑：

    datatype expression =  Constant of int |
            Variable of string |
            Operator of string * expression |
            Pair of expression list |
            List of expression list

fun add2 (ul1: expression, ul2: expression) =
    let
        fun gcd (a, b) =
            if a >= b
            then 
                if (a mod b) = 0
                then b
                else gcd (b,(a mod b))
            else
            if (b mod a) = 0
            then a
            else gcd (a, (b mod a))
        fun lcm (a,b) =
            a*b div (gcd(a,b))      
    in

        case ul1 of
            Operator("/",Pair [Constant a, Constant b]) =>
                case ul2 of
                    Operator("/",Pair [Constant c, Constant d]) => 
                    a*d + c*b//(b*d)
                    |Operator("/",Pair [Variable c, Constant d])=>
                    Operator("/",Pair [(Operator("+", Pair [Constant a, Variable c])),Constant (lcm(b,d))])

            |Operator("/",Pair [Variable a, Constant b]) =>
                case ul2 of
                    Operator("/",Pair [Constant c, Constant d]) => 
                    Operator("/",Pair [(Operator("+", Pair [Variable a, Constant c])),Constant (lcm(b,d))])
                    |Operator("/",Pair [Variable c, Constant d])=>
                    Operator("/",Pair [(Operator("+", Pair [Variable a, Variable c])),Constant (lcm(b,d))])


    end

确切的错误消息是：

C:\Users\erikj\Dropbox\Fakulteta Laptop\Programiranje\domacanaloga 6.sml:91.5-102.93 Error: match redundant and nonexhaustive
          Operator ("/",Pair (Constant c :: Constant d :: nil)) => ...
          Operator ("/",Pair (Variable c :: Constant d :: nil)) => ...
    -->   Operator ("/",Pair (Variable a :: Constant b :: nil)) => ...

我不介意非详尽的匹配，因为它只是一个练习。提前感谢您的帮助！

Answer 1

考虑以下代码：

case ul1 of
    Operator("/",Pair [Constant a, Constant b]) =>
        case ul2 of
            Operator("/",Pair [Constant c, Constant d]) => 
            ...
            |Operator("/",Pair [Variable c, Constant d])=> (* Case A *)
            ...
            |Operator("/",Pair [Variable a, Constant b]) => (* Case B *)
            ...

应该很清楚案例B是多余的，因为它与案例A相同（除了变量名）。

此代码与您的代码之间的唯一变化是我更改了案例 B 的缩进。现在缩进不会影响 SML 程序的语义，因此案例 B 在您的代码中与此处一样冗余。

对于人类 reader 从缩进可以清楚地看出您希望案例 B 属于外部模式匹配，但是，正如我所说，编译器不关心缩进，所以您'您将需要使用括号或开始结束块来告诉编译器内部匹配结束的位置。

为什么我会收到 "match redudant" 错误？

Why am i getting the "match redudant" error?

sml