我正在尝试使用Rust构建一个非常简单的基于栈的求值器。我希望用户能够在后面定义函数,因此我将所有运算符存储在一个HashMap中,其中闭包作为值。
use std::collections::HashMap;
pub type Value = i32;
pub struct Evaluator<'a> {
stack: Vec<Value>,
ops: HashMap<String, &'a dyn FnMut(&'a mut Vec<Value>)>,
}
impl<'a> Evaluator<'a> {
pub fn new() -> Evaluator<'a> {
let stack: Vec<Value> = vec![];
let mut ops: HashMap<String, &'a dyn FnMut(&'a mut Vec<Value>)> = HashMap::new();
ops.insert("+".to_string(), &|stack: &'a mut Vec<Value>| {
if let (Some(x), Some(y)) = (stack.pop(), stack.pop()) {
stack.push(y + x);
}
});
Evaluator { stack, ops }
}
pub fn stack(&self) -> &[Value] {
&self.stack
}
pub fn eval(&'a mut self, input: &str) {
let symbols = input
.split_ascii_whitespace()
.collect::<Vec<_>>();
for sym in symbols {
if let Ok(n) = sym.parse::<i32>() {
self.stack.push(n);
} else {
let s = sym.to_ascii_lowercase();
if let Some(f) = self.ops.get(&s) {
f(&mut self.stack);
} else {
println!("error");
}
}
}
}
}
fn main() {
let mut e = Evaluator::new();
e.eval("1 2 +")
}
我目前遇到两个错误:
error[E0499]: cannot borrow `self.stack` as mutable more than once at a time
--> src/sample.rs:34:17
|
10 | impl<'a> Evaluator<'a> {
| -- lifetime `'a` defined here
...
34 | self.stack.push(n);
| ^^^^^^^^^^ second mutable borrow occurs here
...
38 | f(&mut self.stack);
| ------------------
| | |
| | first mutable borrow occurs here
| argument requires that `self.stack` is borrowed for `'a`
error[E0596]: cannot borrow `**f` as mutable, as it is behind a `&` reference
--> src/sample.rs:38:21
|
38 | f(&mut self.stack);
| ^ cannot borrow as mutable
error[E0499]: cannot borrow `self.stack` as mutable more than once at a time
--> src/sample.rs:38:23
|
10 | impl<'a> Evaluator<'a> {
| -- lifetime `'a` defined here
...
38 | f(&mut self.stack);
| --^^^^^^^^^^^^^^^-
| | |
| | `self.stack` was mutably borrowed here in the previous iteration of the loop
| argument requires that `self.stack` is borrowed for `'a`
error: aborting due to 3 previous errors
Some errors have detailed explanations: E0499, E0596.
For more information about an error, try `rustc --explain E0499`.
我关心的是第一个问题。 我不确定我做错了什么,因为我没有同时借用它们。 我能告诉Rust之前的借用 (self.stack.pop()
) 已经完成了吗? 非常感谢任何帮助。