问题描述
我试图将闭包存储为HashMap值。如果我通过值传递闭包arg,一切都很好:
使用std :: collections :: hash_map :: HashMap;
fn main(){
let mut cmds:HashMap< String,Box< FnMut(String) - >()>
= HashMap :: new();
cmds.insert(ping.to_string(),Box :: new(|& mut:s | {println!({},s);}));
match cmds.get_mut(ping){
Some(f)=> f(pong.to_string()),
_ => ()
}
}
()
但是如果我想要一个包含引用arg的闭包,
使用std :: collections :: hash_map :: HashMap;
fn main(){
let mut cmds:HashMap< String,Box< FnMut(& str) - >()>
= HashMap :: new();
cmds.insert(ping.to_string(),Box :: new(|& mut:s | {println!({},s);}));
match cmds.get_mut(ping){
Some(f)=> f(pong),
_ => ()
}
}
< anon>:8:37:8:78错误:类型不匹配:类型`closure [< anon> :8:46:8:77]`实现trait`core :: ops :: FnMut(_)`,但是`for` core:ops :: FnMut(&'r str)`是必需的(预期的具体生存期,找到的绑定生命周期参数)
< anon>:8 cmds.insert(ping.to_string :: new(|& mut:s | {println!({},s);}));
^ ~~~~~~~~~~~~~~~~~~~~~~
< ; anon>:8:37:8:78注意:需要转换为对象类型`for<'r> core :: ops :: FnMut(&'r str)`
< anon>:8 cmds.insert(ping.to_string(),Box :: new(|& mut:s | println!({},s);}));
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
错误:由于先前的错误而中止
()
我阅读了,并尝试打破地图构建到自己的功能,以有一个地方挂起其中
子句,但没有骰子:
use std :: collections: :hash_map :: HashMap;
fn mk_map< F>() - > HashMap< String,(String,Box< F>)>
其中F:for<'a> FnMut(& a str) - > ()
{
let mut cmds:HashMap< String,(String,Box< F>)> = HashMap :: new();
cmds.insert(ping.to_string(),(ping.to_string(),Box :: new(|& mut:s | {println!({},s);} )));
cmds
}
fn main(){
let cmds = mk_map();
match cmds.get_mut(ping){
一些(& mut(_,ref mut f))=> f(pong),
_ => println!(invalid command)
}
}
< anon>:8:58:8:99错误: ; F>`,found`Box< closure [< anon>:8:67:8:98]>`(expected type parameter,found closure)
< anon>:8 cmds.insert ping.to_string(),(ping.to_string(),Box :: new(|& mut:s | {println!({},s);})));
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(幼儿围栏
我的解决方案:
#![allow(unstable)]
use std :: collections :: hash_map :: HashMap ;
//#1返回一个trait对象
fn mk_map<'a>() - > HashMap< String,(String,Box< FnMut(& str)+'a>)> {
let mut cmds:HashMap< _,(_,Box< FnMut(& str)>)& = HashMap :: new();
cmds.insert(ping.to_string(),(ping.to_string(),
Box :: new(|& mut:s:& str | println!({},s);})));
//#2 ^ - 给编译器一点帮助
cmds
}
fn main(){
let mut cmds = mk_map();
//小改变:cmds需要是可变的
match cmds.get_mut(ping){
一些(& mut(_,ref mut f))=> f(pong),
_ => println!(invalid command)
}
}
/ p>
- 返回一个trait对象
- 根据closure参数的类型为编译器提供一些帮助:
Box :: new(& mut:s:& str |
老实说,我不是100%确定#2的原因(我的意思是,至少应该给出一个更清楚的错误消息)。可能是rustc的问题。
在#1,我几乎肯定它是必需的,因为你不能为从函数返回的闭包命名一个具体的返回类型(它是由编译器在运行时创建的匿名类型)因此现在的Trait对象应该是返回闭包的唯一方法。
附录
响应注释:
trait Foo {}
: trait Foo {}
impl Foo for u32 {}
impl Foo for Vec< f32& {}
如果你编写一个类似于mk_map的函数(我们称之为make_foo)它将很难实现它。让我们看看:
fn mk_foo< F>() - > Box< F&其中F:Foo {
未实现!()
}
mk_foo说,我应该能够调用该函数与任何类型实现Foo。所以这应该都是有效的:
let a:Box< Vec< f32> = mk_foo ::< Vec< f32>>();
let b:Box< u32> = mk_foo ::< u32>();
函数,如写,不返回trait对象。它有希望返回一个框与任何具体类型的调用者选择。这就是为什么它不容易实际实现的功能。它应该知道如何从一无所有创建几种类型。
I'm trying to store a closure as a HashMap value. If I pass the closure arg by value, everything works great:
use std::collections::hash_map::HashMap;
fn main() {
let mut cmds: HashMap<String, Box<FnMut(String)->()>>
= HashMap::new();
cmds.insert("ping".to_string(), Box::new(|&mut:s| { println!("{}", s); }));
match cmds.get_mut("ping") {
Some(f) => f("pong".to_string()),
_ => ()
}
}
(playpen)
But if I want a closure that takes a reference arg, things go south:
use std::collections::hash_map::HashMap;
fn main() {
let mut cmds: HashMap<String, Box<FnMut(&str)->()>>
= HashMap::new();
cmds.insert("ping".to_string(), Box::new(|&mut:s| { println!("{}", s); }));
match cmds.get_mut("ping") {
Some(f) => f("pong"),
_ => ()
}
}
<anon>:8:37: 8:78 error: type mismatch: the type `closure[<anon>:8:46: 8:77]` implements the trait `core::ops::FnMut(_)`, but the trait `for<'r> core::ops::FnMut(&'r str)` is required (expected concrete lifetime, found bound lifetime parameter )
<anon>:8 cmds.insert("ping".to_string(), Box::new(|&mut:s| { println!("{}", s); }));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:8:37: 8:78 note: required for the cast to the object type `for<'r> core::ops::FnMut(&'r str)`
<anon>:8 cmds.insert("ping".to_string(), Box::new(|&mut:s| { println!("{}", s); }));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
(playpen)
I read the answer to How to rewrite code to new unboxed closures, and tried breaking out the map building into its own function in order to have a place to hang the where
clause, but no dice:
use std::collections::hash_map::HashMap;
fn mk_map<F>() -> HashMap<String, (String, Box<F>)>
where F: for<'a> FnMut(&'a str) -> ()
{
let mut cmds: HashMap<String, (String, Box<F>)> = HashMap::new();
cmds.insert("ping".to_string(), ("ping".to_string(), Box::new(|&mut:s| { println!("{}", s); })));
cmds
}
fn main() {
let cmds = mk_map();
match cmds.get_mut("ping") {
Some(&mut (_, ref mut f)) => f("pong"),
_ => println!("invalid command")
}
}
<anon>:8:58: 8:99 error: mismatched types: expected `Box<F>`, found `Box<closure[<anon>:8:67: 8:98]>` (expected type parameter, found closure)
<anon>:8 cmds.insert("ping".to_string(), ("ping".to_string(), Box::new(|&mut:s| { println!("{}", s); })));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(playpen)
What's the right way to do this?
My solution:
#![allow(unstable)]
use std::collections::hash_map::HashMap;
// #1 returning a trait object
fn mk_map<'a>() -> HashMap<String, (String, Box<FnMut(&str) + 'a>)> {
let mut cmds : HashMap<_, (_, Box<FnMut(&str)>)> = HashMap::new();
cmds.insert("ping".to_string(), ("ping".to_string(),
Box::new(|&mut: s: &str| { println!("{}", s); })));
// #2 ^-- give a little help to the compiler here
cmds
}
fn main() {
let mut cmds = mk_map();
// minor change: cmds needs to be mutable
match cmds.get_mut("ping") {
Some(&mut (_, ref mut f)) => f("pong"),
_ => println!("invalid command")
}
}
Ingredients:
- return a trait object
- give some help to the compiler on the type of the closure's parameter:
Box::new(|&mut: s: &str|
To be honest, I'm not 100% sure about the reason for #2 (I mean, at least leaving it out should give a more intelligible error message). Probably an issue with rustc.
On #1, I'm almost sure it's required because you can't name a concrete return type for a closure returned from a function (it's an anonymous type created on the fly by the compiler), so Trait objects for now should be the only way to return a closure.
Appendixresponding to comment:
imagine you have a trait Foo {}
implemented by a few types:
trait Foo {}
impl Foo for u32 {}
impl Foo for Vec<f32> {}
if you write a function like you did with mk_map (let's call it make_foo) I commented that it's going to be hard to implement it. Let's see:
fn mk_foo<F>() -> Box<F> where F: Foo {
unimplemented!()
}
the signature of mk_foo says that I should be able to call the function with any type that implements Foo. So this should all be valid:
let a: Box<Vec<f32>> = mk_foo::<Vec<f32>>();
let b: Box<u32> = mk_foo::<u32>();
i.e. the function, as written, is not returning a trait object. It's promising to return a Box with any concrete type the caller chooses. That's why it's not easy to actually implement the function. It should know how to create several types from nothing.
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