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enums - 比较两个枚举的变体及其值时如何避免代码重复？

问题描述

我需要比较两个serde_json::Values：

enum Value {
    Null,
    Bool(bool),
    Number(Number),
    String(String),
    Array(Vec<Value>),
    Object(Map<String, Value>),
}

比较将返回另一个枚举：

enum Diff {
    Equal,
    Different,
    NotFound,
}

我开始编写结构如下的代码：

fn compare(val1: &Value, val2: &Value) {
    let cmp = match val1 {
        Value::Null => {
            if let Value::Null = val2 {
                Diff::Equal
            } else {
                Diff::NotFound
            }
        }
        Value::Bool(b1) => {
            if let Value::Bool(b2) = val2 {
                if b1 == b2 {
                    Diff::Equal
                } else {
                    Diff::Different
                }
            } else {
                Diff::NotFound
            }
        }
        Value::Number(ref n1) => {
            if let Value::Number(ref n2) = val2 {
                if n1 == n2 {
                    Diff::Equal
                } else {
                    Diff::Different
                }
            } else {
                Diff::NotFound
            }
        }
        Value::String(ref s1) => {
            if let Value::String(ref s2) = val2 {
                if s1 == s2 {
                    Diff::Equal
                } else {
                    Diff::Different
                }
            } else {
                Diff::NotFound
            }
        }
        _ => {
            // etc...
            Diff::NotFound
        }
    };

}

我想避免很多代码重复。

  1. 一般来说，有没有更好的方法来比较枚举？
  2. 我怎样才能避免这种类型的重复（例如在 C++ 中你可以使用宏）

标签： enumsrustcomparison

解决方案

您可以同时匹配 2 个值：

let cmp = match (val1, val2) {
    (Value::Null, Value::Null) => Diff::Equal,
    _ => {
        // etc...
        Diff::NotFound
    }
};

您还可以实现转换函数或特征并使用它：

impl Diff {
    fn new<T: PartialEq>(lhs: T, rhs: T) -> Diff {
        if lhs == rhs { Diff::Equal } else { Diff::Different }
    }
}

fn compare(val1: &Value, val2: &Value) {
    let cmp = match (val1, val2) {
        (Value::Null, Value::Null) => Diff::Equal,
        (Value::Bool(b1), Value::Bool(b2)) => Diff::new(b1, b2),
        _ => Diff::NotFound,
    };
}

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