That is, there's no way to say a reasonable lifetime for 'a. I also think that it's the same root problem as Can I write an Iterator that yields a reference into itself? Can I write an Iterator that yields a reference into itself?.
Instead, I might suggest that you use an associated type instead of the generic parameter use an associated type instead of the generic parameter:
That is, there's no way to say a reasonable lifetime for 'a. I also think that it's the same root problem as Can I write an Iterator that yields a reference into itself?.
Instead, I might suggest that you use an associated type instead of the generic parameter:
That is, there's no way to say a reasonable lifetime for 'a. I also think that it's the same root problem as Can I write an Iterator that yields a reference into itself?.
Instead, I might suggest that you use an associated type instead of the generic parameter:
such as files or in-memory strings
I'm going to give the advice that people dread: I don't think your abstraction is going to work here. When I first read "in-memory strings", I expected a String, not a &str. Since you mentioned a file, I think it's still a valid comparison. I don't believe you can implement this trait for such a type:
struct OwnedStringSource {
pub buff: String,
}
impl<'a> Source<'a> for OwnedStringSource {
fn at(&self, offset: usize) -> Option<(char, usize)> { None }
fn slice(&self, start: usize, end: usize) -> &'a str {
// Hmm.... what to put here?
}
}
That is, there's no way to say a reasonable lifetime for 'a. I also think that it's the same root problem as Can I write an Iterator that yields a reference into itself?.
Aside from that, your original error was probably something like:
error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
--> src/main.rs:117:6
|
117 | impl<'a, S> Iterator for TokenIter<S>
| ^^ unconstrained lifetime parameter
The best advice I've gotten about that specific error was helpful... after thinking about it for a while. Paraphrased and elided for this situation:
what [the error is] trying to tell you is that it cannot get [the generic type] back from either the implemented trait [...] or the type implemented on [...]. [The where clause] is not enough to extract [the generic] from [the type] because one [...] type can have multiple [...] impls with various arguments
Instead, I might suggest that you use an associated type instead of the generic parameter:
trait Source {
type Slice;
fn at(&self, offset: usize) -> Option<(char, usize)>;
fn slice(&self, start: usize, end: usize) -> Self::Slice;
}
This separates the lifetimes from the trait. Specific implementations can still participate in it:
impl<'a> Source for StringSource<'a> {
type Slice = &'a str;
// ...
}
And you can just bubble up the inner type out of the iterator:
impl<'a, S> Iterator for TokenIter<S>
where S: Source,
{
type Item = S::Slice;
// ...
}
You potentially might need to add extra bounds on that generic there (S::Slice: AsRef<str>) depending on what you need to be able to do with the slice in the iterator implementation.
trait Source {
type Slice;
fn at(&self, offset: usize) -> Option<(char, usize)>;
fn slice(&self, start: usize, end: usize) -> Self::Slice;
}
struct StringSource<'a> {
pub buff: &'a str
}
impl<'a> Source for StringSource<'a> {
type Slice = &'a str;
fn at(&self, offset: usize) -> Option<(char, usize)> {
self.buff[offset..].chars().nth(0).map(|ch| { (ch, offset + ch.len_utf8()) })
}
fn slice(&self, start: usize, end: usize) -> &'a str {
&self.buff[start..end]
}
}
struct TokenIter<S> {
source: S,
idx: usize,
}
impl<S> TokenIter<S> {
fn new(source: S) -> Self {
TokenIter {
source: source,
idx: 0,
}
}
}
impl<S> Iterator for TokenIter<S>
where S: Source,
{
type Item = S::Slice;
fn next(&mut self) -> Option<Self::Item> {
let ts = self.idx;
self.source.at(ts).map(|(_ch, next)| {
self.idx = next;
self.source.slice(ts, next)
})
}
}
fn main() {
let source = StringSource{ buff: "hello world" };
let iter = TokenIter::new(source);
println!("{:?}", iter.collect::<Vec<_>>());
}