如何修复"PartialOrd is not implemented for HashMap"错误？

Question

我正在用Rust写一个解释器，遇到了这个我不知道如何解决的错误。它是由包含HashMap的枚举变体之一引起的:

use std::collections::HashMap;
// stubbed type
#[derive(Debug, PartialEq, Clone, Eq, PartialOrd, Hash)]
struct Expression;
#[derive(Debug, PartialEq, Clone, Eq, PartialOrd, Hash)]
enum Literal {
Integer(i64),
Bool(bool),
String(String),
Array(Vec<Expression>),
Hash(HashMap<Expression, Expression>),
}

error[E0277]: can't compare `HashMap<Expression, Expression>` with `HashMap<Expression, Expression>`
--> src/lib.rs:13:10
|
7  | #[derive(Debug, PartialEq, Clone, Eq, PartialOrd, Hash)]
|                                       ---------- in this derive macro expansion
...
13 |     Hash(HashMap<Expression, Expression>),
|          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `HashMap<Expression, Expression> < HashMap<Expression, Expression>` and `HashMap<Expression, Expression> > HashMap<Expression, Expression>`
|
= help: the trait `PartialOrd` is not implemented for `HashMap<Expression, Expression>`
error[E0277]: the trait bound `HashMap<Expression, Expression>: Hash` is not satisfied
--> src/lib.rs:13:10
|
7  | #[derive(Debug, PartialEq, Clone, Eq, PartialOrd, Hash)]
|                                                   ---- in this derive macro expansion
...
13 |     Hash(HashMap<Expression, Expression>),
|          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Hash` is not implemented for `HashMap<Expression, Expression>`

错误说PartialOrd不是为HashMap<Expression, Expression>实现的，但据我所知，HashMap是无序的，所以我为什么需要它?

我需要做什么来修复它?

Answer 1

上面是#[derive(..., PartialOrd, ...)]。这意味着所有枚举变体都应该实现PartialOrd。这就是为什么编译器希望HashMap<Expression, Expression>实现PartialOrd。

将HashMap从变体中移除，将PartialOrd从#[derive(...)]中移除，或者使用像BTreeMap一样实现PartialOrd的映射类型

Hash性状也是如此。