使用计算值作为匹配模式的惯用方法 [英] Idiomatic way to use a computed value as a pattern in match

问题描述

使用在运行时计算的值作为匹配模式之一的最佳方法是什么?

What is the best way to use a value computed at runtime as one of the patterns in a match?

我有一个值 (byte)，我需要与其他值匹配.一些值是固定的(b'0'..b'9').其他在运行时计算(c = some_function()).

I have a value (byte) that I need to match against other values. Some of the values are fixed (b'0'..b'9'). Others are computed at runtime (c = some_function()).

我目前的解决方案是使用一个假变量和一个 if 守卫(即 k if (k == c))，但对我来说它看起来不太好.我尝试只使用 c 但这被解释为一个包罗万象的变量，而不是在该上下文中替换 c 的值.

My current solution is to use a fake variable and a if guard (i.e. k if (k == c)), but it does not look very nice to me. I tried using just c but that gets interpreted as a catch-all variable, rather than substituted for the value of c in that context.

以下代码片段显示了问题:(也在游戏围栏上)

The following code snippet shows the problem: (also on playpen)

fn main() {
    fun(b'5', 0);
    fun(b'C', 0);
    fun(b'C', 2);
}

fn fun(byte: u8, i: uint) {
    let CHARS = b"ABCDEFGH";
    let c = CHARS[i];

    let msg = match byte {
        b'0'..b'9' => "numeric",
        // c => "same char", // <-- I would had preferred this
        k if (k == c) => "same char",
        _ => "different char",
    };

    println!("fun({}, {} [{}]) = {}", byte, i, c, msg);
}

这是 Rust 可以提供的最惯用的构造吗?

Is this the most idiomatic construct that Rust can offer?

推荐答案

简而言之，是的，您应该为此使用模式保护.

In short, yes, you should use pattern guards for this.

为了使其成为可能，应该有一种方法来区分普通绑定和相等性检查.例如，Scala 是基于可变大小写的:如果以大写字母开头，则是相等性检查；否则就是模式绑定.Rust 中没有这样的机制，所以不，现在不可能了.

To make it possible there should be a way to differentiate plain bindings and equality checks. Scala, for example, does this based on variable case: if it starts with capital letter, it is an equality check; otherwise it is a pattern binding. There is no such mechanism in Rust, so no, it is impossible now.

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