functional-programming - 解决脆弱模式匹配的建议

Question

我经常需要匹配应该具有相同构造函数的值元组。大杂烩_,_总是在最后结束。这当然是脆弱的，任何添加到该类型的额外构造函数都可以很好地编译。我目前的想法是让匹配连接第一个但不连接第二个参数。但是，还有其他选择吗？

例如，

type data = | States of int array 
            | Chars  of (char list) array

let median a b = match a,b with
    | States xs, States ys ->
        assert( (Array.length xs) = (Array.length ys) );
        States (Array.init (Array.length xs) (fun i -> xs.(i) lor ys.(i)))
    | Chars xs, Chars ys -> 
        assert( (Array.length xs) = (Array.length ys) );
        let union c1 c2 = (List.filter (fun x -> not (List.mem x c2)) c1) @ c2 in
        Chars (Array.init (Array.length xs) (fun i -> union xs.(i) ys.(i)))
    (* inconsistent pairs of matching *)
    | Chars  _, _
    | States _, _ -> assert false

Answer 1

您可以使用下面稍短的模式:

| (Chars _| States _), _ -> assert false

其实你可以让编译器帮你生成，因为还是有点繁琐。
输入以下内容并编译:

let median a b = match a,b with
| States xs, States ys ->
    assert( (Array.length xs) = (Array.length ys) );
    States (Array.init (Array.length xs) (fun i -> xs.(i) lor ys.(i)))
| Chars xs, Chars ys -> 
    assert( (Array.length xs) = (Array.length ys) );
    let union c1 c2 = (List.filter (fun x -> not (List.mem x c2)) c1) @ c2 in
    Chars (Array.init (Array.length xs) (fun i -> union xs.(i) ys.(i)))

Warning 8: this pattern-matching is not exhaustive. Here is an example of a value that is not matched: (Chars _, States _)

您现在可以将建议的模式复制粘贴回您的代码中。这通常是我为具有数十个构造函数的类型生成非脆弱的全能模式的方式。您可能需要多次启动编译器，但它仍然比自己键入它们要快。