混淆特征边界而不指定具体的关联类型 [英] Aliasing trait bounds without specifying concrete associated types
问题描述
pub fn foo< T>>我发现自己在同一界限下写了不同的函数, (mut self,path:T) - > Self其中
T:IntoIterator,
T :: Item:AsRef< str> ;,
{
// ...
}
pub fn bar< T>(mut self,path:T) - > Self其中
T:IntoIterator,
T :: Item:AsRef< str> ;,
{
// ...
}
感觉这有点麻烦,我尝试过别名这些边界。但我没有找到办法。我仔细检查了几个地方 [1],[2] ,结果如下:
trait Path {
type I:IntoIterator< Item = Self :: S>;
type S:AsRef< str>;
}
impl< T,U> Path for T其中
T:IntoIterator< Item = U> ;,
U:AsRef< str>,
{
type I = T;
type S = U;
}
现在,作为一个例子,这样编译得很好:
fn into_vec< T:Path>(it:T :: I) - > VEC<字符串> {
it.into_iter()
.map ::< String,_>(| x | x.as_ref()。into())
.collect()
}
但是当我尝试使用它时:
< T>(& [one,two]);<
}
我收到以下错误:
src / lib.rs:104:19:104:34错误:不匹配的类型:
预计`< T as Path> :: I`,
找到了& [& str; 2]`
(期望的关联类型,
找到& -ptr)[b]
src / lib.rs:104 into_vec ::< T>(& [one ,two]);
^ ~~~~~~~~~~~~~~
所以,没有运气。如何前进?
1 https://github.com/rust-lang/rust/issues/8634
2 https://stackoverflow.com/a/30424219/3957040
如何前进?
你不能直接。让我们来看看你的函数:
fn消耗< T:Path>(){
应该是,因为它不被用作参数或返回值。
into_vec ::< T> ;(& [one,two]);
这表示对于任何类型的
Path
,用一段字符串调用into_vec
。但是,您无法保证无论T
是什么,它都会接受一段字符串。
从另一方面来看,很多类型可能接受一段字符串,所以它会使
T
含糊不清。
从第三个方向看,类型推断无法决定
T
你可以通过明确指出哪些
T
$ p $fn consume(){
into_vec ::<& [&'静态str]>(& [one,two]);
}
为了澄清,这与将特征组合到另一个特征中无关。这将是任何特质的问题。
I found myself writing different functions with an argument under the same bounds, like this:
pub fn foo<T>(mut self, path: T) -> Self where
T: IntoIterator,
T::Item: AsRef<str>,
{
// ...
}
pub fn bar<T>(mut self, path: T) -> Self where
T: IntoIterator,
T::Item: AsRef<str>,
{
// ...
}
Feeling this to be a bit of a chore, I tried to alias these bounds. But I didn’t find a way to. The closest I got, after checking a couple places[1],[2], is this:
trait Path {
type I: IntoIterator<Item = Self::S>;
type S: AsRef<str>;
}
impl<T, U> Path for T where
T: IntoIterator<Item = U>,
U: AsRef<str>,
{
type I = T;
type S = U;
}
Now, as an example, this compiles fine:
fn into_vec<T: Path>(it: T::I) -> Vec<String> {
it.into_iter()
.map::<String, _>(|x| x.as_ref().into())
.collect()
}
But when I try to use it:
fn consume<T: Path>() {
into_vec::<T>(&["one", "two"]);
}
I get the following error:
src/lib.rs:104:19: 104:34 error: mismatched types:
expected `<T as Path>::I`,
found `&[&str; 2]`
(expected associated type,
found &-ptr) [E0308]
src/lib.rs:104 into_vec::<T>(&["one", "two"]);
^~~~~~~~~~~~~~~
So, no luck. How can I go forward?
1 https://github.com/rust-lang/rust/issues/8634
2 https://stackoverflow.com/a/30424219/3957040
How can I go forward?
You can't directly. Let's look at your function:
fn consume<T: Path>() {
into_vec::<T>(&["one", "two"]);
}
This says "for any type that implements Path
, call into_vec
with a slice of strings". However, there's no way that you can guarantee that whatever the T
is, it will accept a slice of strings.
Looking in another direction, many types might accept a slice of strings, so it would make T
ambiguous.
Looking in a third direction, there's no way for type inference to decide what T
should be, as it's not used as an argument or return value.
You can get it to work by explicitly stating which T
you want:
fn consume() {
into_vec::<&[&'static str]>(&["one", "two"]);
}
To clarify, this has nothing to do with combining traits into another trait. This would be a problem with any trait.
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