Related
I'm trying to implement a simple interpreter in Rust, for which I have created a Tokens struct, which takes source characters and produces either a Token or a ScanError inside a Result:
pub struct Tokens<'src> {
chars: Chars<'src>,
}
impl<'src> Iterator for Tokens<'src> {
type Item = Result<Token, ScanError>;
fn next(&mut self) -> Option<Result<Token, ScanError>> {
// ...
}
}
Since Result implements FromIterator, it is simple to collect the result to either the first ScanError or a vector of Tokens:
fn scan_tokens(source: &str) -> Result<Vec<Token>, ScanError> {
let iter = Tokens {
chars: source.chars(),
};
iter.collect()
}
In the case of multiple errors I really want to return every error:
fn scan_tokens(source: &str) -> Result<Vec<Token>, Vec<ScanError>> {
// what goes here?
}
It isn't possible as far as I know to implement my own version of FromIterator because neither that trait or Result are local to my crate. Can anyone suggest a clean way of doing this?
I have written an implementation using partition on the iterator, then unwrapping each Result, below, but it's not fun to read and doesn't feel like good use of iterators:
type T = Vec<Result<Token, ScanError>>;
fn scan_tokens(source: &str) -> Result<Vec<Token>, Vec<ScanError>> {
let iter = Tokens {
chars: source.chars(),
};
let (tokens_results, error_results): (T, T) = iter.partition(|result| result.is_ok());
let errors: Vec<ScanError> = error_results
.into_iter()
.map(|result| result.unwrap_err())
.collect();
if errors.len() > 0 {
return Err(errors);
}
Ok(tokens_results
.into_iter()
.map(|result| result.unwrap())
.collect())
}
unwrapping each Result
I would use itertools' partition_map to avoid the need to unwrap:
use itertools::{Either, Itertools}; // 0.8.0
fn iterator() -> impl Iterator<Item = Result<i32, bool>> {
vec![Ok(1), Err(false), Ok(2), Err(true), Ok(3)].into_iter()
}
fn example() -> Result<Vec<i32>, Vec<bool>> {
let (values, errors): (Vec<_>, Vec<_>) = iterator().partition_map(|v| match v {
Ok(v) => Either::Left(v),
Err(e) => Either::Right(e),
});
if errors.is_empty() {
Ok(values)
} else {
Err(errors)
}
}
See also:
What's the most idiomatic way of working with an Iterator of Results?
How do I stop iteration and return an error when Iterator::map returns a Result::Err?
How do I perform iterator computations over iterators of Results without collecting to a temporary vector?
You could also use the fact that Option and Result implement IntoIterator to avoid the exact unwrap, although this still processes one collection twice:
fn example2() -> Result<Vec<i32>, Vec<bool>> {
let (values, errors): (Vec<_>, Vec<_>) = iterator().partition(|result| result.is_ok());
if errors.is_empty() {
Ok(values.into_iter().flat_map(Result::ok).collect())
} else {
Err(errors.into_iter().flat_map(Result::err).collect())
}
}
See also:
Why does `Option` support `IntoIterator`?
An imperative solution is often the most expressive and efficient way to implement some algorithm. It's Rust, not Haskell; not everything needs to be functional.
fn scan_tokens(source: &str) -> Result<Vec<Token>, Vec<ScanError>> {
let iter = Tokens {
chars: source.chars(),
};
let mut tokens = Vec::new();
let mut errors = Vec::new();
for result in iter {
match result {
Ok(token) => {
tokens.push(token);
}
Err(e) => {
errors.push(e);
}
}
}
if errors.is_empty() {
Ok(tokens)
} else {
Err(errors)
}
}
Here is as far as I could get, using rental, partly based on How can I store a Chars iterator in the same struct as the String it is iterating on?. The difference here is that the get_iter method of the locked member has to take a mutable self reference.
I'm not tied to using rental: I'd be just as happy with a solution using reffers or owning_ref.
The PhantomData is present here just so that MyIter bears the normal lifetime relationship to MyIterable, the thing being iterated over.
I also tried changing #[rental] to #[rental(deref_mut_suffix)] and changing the return type of MyIterable.get_iter to Box<Iterator<Item=i32> + 'a> but that gave me other lifetime errors originating in the macro that I was unable to decipher.
#[macro_use]
extern crate rental;
use std::marker::PhantomData;
pub struct MyIterable {}
impl MyIterable {
// In the real use-case I can't remove the 'mut'.
pub fn get_iter<'a>(&'a mut self) -> MyIter<'a> {
MyIter {
marker: PhantomData,
}
}
}
pub struct MyIter<'a> {
marker: PhantomData<&'a MyIterable>,
}
impl<'a> Iterator for MyIter<'a> {
type Item = i32;
fn next(&mut self) -> Option<i32> {
Some(42)
}
}
use std::sync::Mutex;
rental! {
mod locking_iter {
pub use super::{MyIterable, MyIter};
use std::sync::MutexGuard;
#[rental]
pub struct LockingIter<'a> {
guard: MutexGuard<'a, MyIterable>,
iter: MyIter<'guard>,
}
}
}
use locking_iter::LockingIter;
impl<'a> Iterator for LockingIter<'a> {
type Item = i32;
#[inline]
fn next(&mut self) -> Option<Self::Item> {
self.rent_mut(|iter| iter.next())
}
}
struct Access {
shared: Mutex<MyIterable>,
}
impl Access {
pub fn get_iter<'a>(&'a self) -> Box<Iterator<Item = i32> + 'a> {
Box::new(LockingIter::new(self.shared.lock().unwrap(), |mi| {
mi.get_iter()
}))
}
}
fn main() {
let access = Access {
shared: Mutex::new(MyIterable {}),
};
let iter = access.get_iter();
let contents: Vec<i32> = iter.take(2).collect();
println!("contents: {:?}", contents);
}
As user rodrigo has pointed out in a comment, the solution is simply to change #[rental] to #[rental_mut].
I want to implement the IntoIterator trait for a struct containing a String. The iterator is based on the chars() iterator, is supposed to count the '1' chars and accumulate the result. This is a simplified version of what I got so far:
use std::iter::Map;
use std::str::Chars;
fn main() {
let str_struct = StringStruct { system_string: String::from("1101") };
for a in str_struct {
println!("{}", a);
}
}
struct StringStruct {
system_string: String
}
impl IntoIterator for StringStruct {
type Item = u32;
type IntoIter = Map<Chars, Fn(char) -> u32>;
fn into_iter(self) -> Self::IntoIter {
let count = 0;
return self.system_string.chars().map(|c| match c {
Some('1') => {
count += 1;
return Some(count);
},
Some(chr) => return Some(count),
None => return None
});
}
}
Expected output: 1, 2, 2, 3
This fails with:
error[E0107]: wrong number of lifetime parameters: expected 1, found 0
--> src/main.rs:17:25
|
17 | type IntoIter = Map<Chars, Fn(char) -> u32>;
| ^^^^^ expected 1 lifetime parameter
The chars iterator should have the same lifetime as the StringStruct::system_string, but I have no idea how to express this or if this approach is viable at all.
To answer the question you asked, I'd recommend to impl IntoIterator for &StringStruct (a reference to a StringStruct instead of the struct directly). The code would look like this:
impl<'a> IntoIterator for &'a StringStruct {
type Item = u32;
type IntoIter = Map<Chars<'a>, Fn(char) -> u32>;
// ...
}
However, you will notice many more errors that have a different origin afterwards. The next error that pops up is that Fn(char) -> u32 does not have a constant size at compile time.
The problem is that you try to name the type of your closure by writing Fn(char) -> u32. But this is not the type of your closure, but merely a trait which is implemented by the closure. The type of a closure can't be named (sometimes called "Voldemort type").
This means that, right now, you can't specify the type of a Map<_, _> object. This is a known issue; the recently accepted impl Trait-RFC might offer a workaround for cases like this. But right now, it's not possible, sorry.
So how to solve it then? You need to create your own type that implements Iterator and use it instead of Map<_, _>. Note that you can still use the Chars iterator. Here is the full solution:
struct StringStructIter<'a> {
chars: Chars<'a>,
count: u32,
}
impl<'a> Iterator for StringStructIter<'a> {
type Item = u32;
fn next(&mut self) -> Option<Self::Item> {
self.chars.next().map(|c| {
if c == '1' {
self.count += 1;
}
self.count
})
}
}
impl<'a> IntoIterator for &'a StringStruct {
type Item = u32;
type IntoIter = StringStructIter<'a>;
fn into_iter(self) -> Self::IntoIter {
StringStructIter {
chars: self.system_string.chars(),
count: 0,
}
}
}
fn main() {
let str_struct = StringStruct { system_string: String::from("1101") };
for a in &str_struct {
println!("{}", a);
}
}
And just a small note: an explicit return when not necessary is considered bad style in Rust. Better stick to rule and write idiomatic code by removing return whenever possible ;-)
I am having trouble expressing the lifetime of the return value of an Iterator implementation. How can I compile this code without changing the return value of the iterator? I'd like it to return a vector of references.
It is obvious that I am not using the lifetime parameter correctly but after trying various ways I just gave up, I have no idea what to do with it.
use std::iter::Iterator;
struct PermutationIterator<T> {
vs: Vec<Vec<T>>,
is: Vec<usize>,
}
impl<T> PermutationIterator<T> {
fn new() -> PermutationIterator<T> {
PermutationIterator {
vs: vec![],
is: vec![],
}
}
fn add(&mut self, v: Vec<T>) {
self.vs.push(v);
self.is.push(0);
}
}
impl<T> Iterator for PermutationIterator<T> {
type Item = Vec<&'a T>;
fn next(&mut self) -> Option<Vec<&T>> {
'outer: loop {
for i in 0..self.vs.len() {
if self.is[i] >= self.vs[i].len() {
if i == 0 {
return None; // we are done
}
self.is[i] = 0;
self.is[i - 1] += 1;
continue 'outer;
}
}
let mut result = vec![];
for i in 0..self.vs.len() {
let index = self.is[i];
result.push(self.vs[i].get(index).unwrap());
}
*self.is.last_mut().unwrap() += 1;
return Some(result);
}
}
}
fn main() {
let v1: Vec<_> = (1..3).collect();
let v2: Vec<_> = (3..5).collect();
let v3: Vec<_> = (1..6).collect();
let mut i = PermutationIterator::new();
i.add(v1);
i.add(v2);
i.add(v3);
loop {
match i.next() {
Some(v) => {
println!("{:?}", v);
}
None => {
break;
}
}
}
}
(Playground link)
error[E0261]: use of undeclared lifetime name `'a`
--> src/main.rs:23:22
|
23 | type Item = Vec<&'a T>;
| ^^ undeclared lifetime
As far as I understand, you want want the iterator to return a vector of references into itself, right? Unfortunately, it is not possible in Rust.
This is the trimmed down Iterator trait:
trait Iterator {
type Item;
fn next(&mut self) -> Option<Item>;
}
Note that there is no lifetime connection between &mut self and Option<Item>. This means that next() method can't return references into the iterator itself. You just can't express a lifetime of the returned references. This is basically the reason that you couldn't find a way to specify the correct lifetime - it would've looked like this:
fn next<'a>(&'a mut self) -> Option<Vec<&'a T>>
except that this is not a valid next() method for Iterator trait.
Such iterators (the ones which can return references into themselves) are called streaming iterators. You can find more here, here and here, if you want.
Update. However, you can return a reference to some other structure from your iterator - that's how most of collection iterators work. It could look like this:
pub struct PermutationIterator<'a, T> {
vs: &'a [Vec<T>],
is: Vec<usize>
}
impl<'a, T> Iterator for PermutationIterator<'a, T> {
type Item = Vec<&'a T>;
fn next(&mut self) -> Option<Vec<&'a T>> {
...
}
}
Note how lifetime 'a is now declared on impl block. It is OK to do so (required, in fact) because you need to specify the lifetime parameter on the structure. Then you can use the same 'a both in Item and in next() return type. Again, that's how most of collection iterators work.
#VladimirMatveev's answer is correct in how it explains why your code cannot compile. In a nutshell, it says that an Iterator cannot yield borrowed values from within itself.
However, it can yield borrowed values from something else. This is what is achieved with Vec and Iter: the Vec owns the values, and the the Iter is just a wrapper able to yield references within the Vec.
Here is a design which achieves what you want. The iterator is, like with Vec and Iter, just a wrapper over other containers who actually own the values.
use std::iter::Iterator;
struct PermutationIterator<'a, T: 'a> {
vs : Vec<&'a [T]>,
is : Vec<usize>
}
impl<'a, T> PermutationIterator<'a, T> {
fn new() -> PermutationIterator<'a, T> { ... }
fn add(&mut self, v : &'a [T]) { ... }
}
impl<'a, T> Iterator for PermutationIterator<'a, T> {
type Item = Vec<&'a T>;
fn next(&mut self) -> Option<Vec<&'a T>> { ... }
}
fn main() {
let v1 : Vec<i32> = (1..3).collect();
let v2 : Vec<i32> = (3..5).collect();
let v3 : Vec<i32> = (1..6).collect();
let mut i = PermutationIterator::new();
i.add(&v1);
i.add(&v2);
i.add(&v3);
loop {
match i.next() {
Some(v) => { println!("{:?}", v); }
None => {break;}
}
}
}
(Playground)
Unrelated to your initial problem. If this were just me, I would ensure that all borrowed vectors are taken at once. The idea is to remove the repeated calls to add and to pass directly all borrowed vectors at construction:
use std::iter::{Iterator, repeat};
struct PermutationIterator<'a, T: 'a> {
...
}
impl<'a, T> PermutationIterator<'a, T> {
fn new(vs: Vec<&'a [T]>) -> PermutationIterator<'a, T> {
let n = vs.len();
PermutationIterator {
vs: vs,
is: repeat(0).take(n).collect(),
}
}
}
impl<'a, T> Iterator for PermutationIterator<'a, T> {
...
}
fn main() {
let v1 : Vec<i32> = (1..3).collect();
let v2 : Vec<i32> = (3..5).collect();
let v3 : Vec<i32> = (1..6).collect();
let vall: Vec<&[i32]> = vec![&v1, &v2, &v3];
let mut i = PermutationIterator::new(vall);
}
(Playground)
(EDIT: Changed the iterator design to take a Vec<&'a [T]> rather than a Vec<Vec<&'a T>>. It's easier to take a ref to container than to build a container of refs.)
As mentioned in other answers, this is called a streaming iterator and it requires different guarantees from Rust's Iterator. One crate that provides such functionality is aptly called streaming-iterator and it provides the StreamingIterator trait.
Here is one example of implementing the trait:
extern crate streaming_iterator;
use streaming_iterator::StreamingIterator;
struct Demonstration {
scores: Vec<i32>,
position: usize,
}
// Since `StreamingIterator` requires that we be able to call
// `advance` before `get`, we have to start "before" the first
// element. We assume that there will never be the maximum number of
// entries in the `Vec`, so we use `usize::MAX` as our sentinel value.
impl Demonstration {
fn new() -> Self {
Demonstration {
scores: vec![1, 2, 3],
position: std::usize::MAX,
}
}
fn reset(&mut self) {
self.position = std::usize::MAX;
}
}
impl StreamingIterator for Demonstration {
type Item = i32;
fn advance(&mut self) {
self.position = self.position.wrapping_add(1);
}
fn get(&self) -> Option<&Self::Item> {
self.scores.get(self.position)
}
}
fn main() {
let mut example = Demonstration::new();
loop {
example.advance();
match example.get() {
Some(v) => {
println!("v: {}", v);
}
None => break,
}
}
example.reset();
loop {
example.advance();
match example.get() {
Some(v) => {
println!("v: {}", v);
}
None => break,
}
}
}
Unfortunately, streaming iterators will be limited until generic associated types (GATs) from RFC 1598 are implemented.
I wrote this code not long ago and somehow stumbled on this question here. It does exactly what the question asks: it shows how to implement an iterator that passes its callbacks a reference to itself.
It adds an .iter_map() method to IntoIterator instances. Initially I thought it should be implemented for Iterator itself, but that was a less flexible design decision.
I created a small crate for it and posted my code to GitHub in case you want to experiment with it, you can find it here.
WRT the OP's trouble with defining lifetimes for the items, I didn't run into any such trouble implementing this while relying on the default elided lifetimes.
Here's an example of usage. Note the parameter the callback receives is the iterator itself, the callback is expected to pull the data from it and either pass it along as is or do whatever other operations.
use iter_map::IntoIterMap;
let mut b = true;
let s = "hello world!".chars().peekable().iter_map(|iter| {
if let Some(&ch) = iter.peek() {
if ch == 'o' && b {
b = false;
Some('0')
} else {
b = true;
iter.next()
}
} else { None }
}).collect::<String>();
assert_eq!(&s, "hell0o w0orld!");
Because the IntoIterMap generic trait is implemented for IntoIterator, you can get an "iter map" off anything that supports that interface. For instance, one can be created directly off an array, like so:
use iter_map::*;
fn main()
{
let mut i = 0;
let v = [1, 2, 3, 4, 5, 6].iter_map(move |iter| {
i += 1;
if i % 3 == 0 {
Some(0)
} else {
iter.next().copied()
}
}).collect::<Vec<_>>();
assert_eq!(v, vec![1, 2, 0, 3, 4, 0, 5, 6, 0]);
}
Here's the full code - it was amazing it took such little code to implement, and everything just seemed to work smoothly while putting it together. It gave me a new appreciation for the flexibility of Rust itself and its design decisions.
/// Adds `.iter_map()` method to all IntoIterator classes.
///
impl<F, I, J, R, T> IntoIterMap<F, I, R, T> for J
//
where F: FnMut(&mut I) -> Option<R>,
I: Iterator<Item = T>,
J: IntoIterator<Item = T, IntoIter = I>,
{
/// Returns an iterator that invokes the callback in `.next()`, passing it
/// the original iterator as an argument. The callback can return any
/// arbitrary type within an `Option`.
///
fn iter_map(self, callback: F) -> ParamFromFnIter<F, I>
{
ParamFromFnIter::new(self.into_iter(), callback)
}
}
/// A trait to add the `.iter_map()` method to any existing class.
///
pub trait IntoIterMap<F, I, R, T>
//
where F: FnMut(&mut I) -> Option<R>,
I: Iterator<Item = T>,
{
/// Returns a `ParamFromFnIter` iterator which wraps the iterator it's
/// invoked on.
///
/// # Arguments
/// * `callback` - The callback that gets invoked by `.next()`.
/// This callback is passed the original iterator as its
/// parameter.
///
fn iter_map(self, callback: F) -> ParamFromFnIter<F, I>;
}
/// Implements an iterator that can be created from a callback.
/// does pretty much the same thing as `std::iter::from_fn()` except the
/// callback signature of this class takes a data argument.
pub struct ParamFromFnIter<F, D>
{
callback: F,
data: D,
}
impl<F, D, R> ParamFromFnIter<F, D>
//
where F: FnMut(&mut D) -> Option<R>,
{
/// Creates a new `ParamFromFnIter` iterator instance.
///
/// This provides a flexible and simple way to create new iterators by
/// defining a callback.
/// # Arguments
/// * `data` - Data that will be passed to the callback on each
/// invocation.
/// * `callback` - The callback that gets invoked when `.next()` is invoked
/// on the returned iterator.
///
pub fn new(data: D, callback: F) -> Self
{
ParamFromFnIter { callback, data }
}
}
/// Implements Iterator for ParamFromFnIter.
///
impl<F, D, R> Iterator for ParamFromFnIter<F, D>
//
where F: FnMut(&mut D) -> Option<R>,
{
type Item = R;
/// Iterator method that returns the next item.
/// Invokes the client code provided iterator, passing it `&mut self.data`.
///
fn next(&mut self) -> Option<Self::Item>
{
(self.callback)(&mut self.data)
}
}
I am having trouble expressing the lifetime of the return value of an Iterator implementation. How can I compile this code without changing the return value of the iterator? I'd like it to return a vector of references.
It is obvious that I am not using the lifetime parameter correctly but after trying various ways I just gave up, I have no idea what to do with it.
use std::iter::Iterator;
struct PermutationIterator<T> {
vs: Vec<Vec<T>>,
is: Vec<usize>,
}
impl<T> PermutationIterator<T> {
fn new() -> PermutationIterator<T> {
PermutationIterator {
vs: vec![],
is: vec![],
}
}
fn add(&mut self, v: Vec<T>) {
self.vs.push(v);
self.is.push(0);
}
}
impl<T> Iterator for PermutationIterator<T> {
type Item = Vec<&'a T>;
fn next(&mut self) -> Option<Vec<&T>> {
'outer: loop {
for i in 0..self.vs.len() {
if self.is[i] >= self.vs[i].len() {
if i == 0 {
return None; // we are done
}
self.is[i] = 0;
self.is[i - 1] += 1;
continue 'outer;
}
}
let mut result = vec![];
for i in 0..self.vs.len() {
let index = self.is[i];
result.push(self.vs[i].get(index).unwrap());
}
*self.is.last_mut().unwrap() += 1;
return Some(result);
}
}
}
fn main() {
let v1: Vec<_> = (1..3).collect();
let v2: Vec<_> = (3..5).collect();
let v3: Vec<_> = (1..6).collect();
let mut i = PermutationIterator::new();
i.add(v1);
i.add(v2);
i.add(v3);
loop {
match i.next() {
Some(v) => {
println!("{:?}", v);
}
None => {
break;
}
}
}
}
(Playground link)
error[E0261]: use of undeclared lifetime name `'a`
--> src/main.rs:23:22
|
23 | type Item = Vec<&'a T>;
| ^^ undeclared lifetime
As far as I understand, you want want the iterator to return a vector of references into itself, right? Unfortunately, it is not possible in Rust.
This is the trimmed down Iterator trait:
trait Iterator {
type Item;
fn next(&mut self) -> Option<Item>;
}
Note that there is no lifetime connection between &mut self and Option<Item>. This means that next() method can't return references into the iterator itself. You just can't express a lifetime of the returned references. This is basically the reason that you couldn't find a way to specify the correct lifetime - it would've looked like this:
fn next<'a>(&'a mut self) -> Option<Vec<&'a T>>
except that this is not a valid next() method for Iterator trait.
Such iterators (the ones which can return references into themselves) are called streaming iterators. You can find more here, here and here, if you want.
Update. However, you can return a reference to some other structure from your iterator - that's how most of collection iterators work. It could look like this:
pub struct PermutationIterator<'a, T> {
vs: &'a [Vec<T>],
is: Vec<usize>
}
impl<'a, T> Iterator for PermutationIterator<'a, T> {
type Item = Vec<&'a T>;
fn next(&mut self) -> Option<Vec<&'a T>> {
...
}
}
Note how lifetime 'a is now declared on impl block. It is OK to do so (required, in fact) because you need to specify the lifetime parameter on the structure. Then you can use the same 'a both in Item and in next() return type. Again, that's how most of collection iterators work.
#VladimirMatveev's answer is correct in how it explains why your code cannot compile. In a nutshell, it says that an Iterator cannot yield borrowed values from within itself.
However, it can yield borrowed values from something else. This is what is achieved with Vec and Iter: the Vec owns the values, and the the Iter is just a wrapper able to yield references within the Vec.
Here is a design which achieves what you want. The iterator is, like with Vec and Iter, just a wrapper over other containers who actually own the values.
use std::iter::Iterator;
struct PermutationIterator<'a, T: 'a> {
vs : Vec<&'a [T]>,
is : Vec<usize>
}
impl<'a, T> PermutationIterator<'a, T> {
fn new() -> PermutationIterator<'a, T> { ... }
fn add(&mut self, v : &'a [T]) { ... }
}
impl<'a, T> Iterator for PermutationIterator<'a, T> {
type Item = Vec<&'a T>;
fn next(&mut self) -> Option<Vec<&'a T>> { ... }
}
fn main() {
let v1 : Vec<i32> = (1..3).collect();
let v2 : Vec<i32> = (3..5).collect();
let v3 : Vec<i32> = (1..6).collect();
let mut i = PermutationIterator::new();
i.add(&v1);
i.add(&v2);
i.add(&v3);
loop {
match i.next() {
Some(v) => { println!("{:?}", v); }
None => {break;}
}
}
}
(Playground)
Unrelated to your initial problem. If this were just me, I would ensure that all borrowed vectors are taken at once. The idea is to remove the repeated calls to add and to pass directly all borrowed vectors at construction:
use std::iter::{Iterator, repeat};
struct PermutationIterator<'a, T: 'a> {
...
}
impl<'a, T> PermutationIterator<'a, T> {
fn new(vs: Vec<&'a [T]>) -> PermutationIterator<'a, T> {
let n = vs.len();
PermutationIterator {
vs: vs,
is: repeat(0).take(n).collect(),
}
}
}
impl<'a, T> Iterator for PermutationIterator<'a, T> {
...
}
fn main() {
let v1 : Vec<i32> = (1..3).collect();
let v2 : Vec<i32> = (3..5).collect();
let v3 : Vec<i32> = (1..6).collect();
let vall: Vec<&[i32]> = vec![&v1, &v2, &v3];
let mut i = PermutationIterator::new(vall);
}
(Playground)
(EDIT: Changed the iterator design to take a Vec<&'a [T]> rather than a Vec<Vec<&'a T>>. It's easier to take a ref to container than to build a container of refs.)
As mentioned in other answers, this is called a streaming iterator and it requires different guarantees from Rust's Iterator. One crate that provides such functionality is aptly called streaming-iterator and it provides the StreamingIterator trait.
Here is one example of implementing the trait:
extern crate streaming_iterator;
use streaming_iterator::StreamingIterator;
struct Demonstration {
scores: Vec<i32>,
position: usize,
}
// Since `StreamingIterator` requires that we be able to call
// `advance` before `get`, we have to start "before" the first
// element. We assume that there will never be the maximum number of
// entries in the `Vec`, so we use `usize::MAX` as our sentinel value.
impl Demonstration {
fn new() -> Self {
Demonstration {
scores: vec![1, 2, 3],
position: std::usize::MAX,
}
}
fn reset(&mut self) {
self.position = std::usize::MAX;
}
}
impl StreamingIterator for Demonstration {
type Item = i32;
fn advance(&mut self) {
self.position = self.position.wrapping_add(1);
}
fn get(&self) -> Option<&Self::Item> {
self.scores.get(self.position)
}
}
fn main() {
let mut example = Demonstration::new();
loop {
example.advance();
match example.get() {
Some(v) => {
println!("v: {}", v);
}
None => break,
}
}
example.reset();
loop {
example.advance();
match example.get() {
Some(v) => {
println!("v: {}", v);
}
None => break,
}
}
}
Unfortunately, streaming iterators will be limited until generic associated types (GATs) from RFC 1598 are implemented.
I wrote this code not long ago and somehow stumbled on this question here. It does exactly what the question asks: it shows how to implement an iterator that passes its callbacks a reference to itself.
It adds an .iter_map() method to IntoIterator instances. Initially I thought it should be implemented for Iterator itself, but that was a less flexible design decision.
I created a small crate for it and posted my code to GitHub in case you want to experiment with it, you can find it here.
WRT the OP's trouble with defining lifetimes for the items, I didn't run into any such trouble implementing this while relying on the default elided lifetimes.
Here's an example of usage. Note the parameter the callback receives is the iterator itself, the callback is expected to pull the data from it and either pass it along as is or do whatever other operations.
use iter_map::IntoIterMap;
let mut b = true;
let s = "hello world!".chars().peekable().iter_map(|iter| {
if let Some(&ch) = iter.peek() {
if ch == 'o' && b {
b = false;
Some('0')
} else {
b = true;
iter.next()
}
} else { None }
}).collect::<String>();
assert_eq!(&s, "hell0o w0orld!");
Because the IntoIterMap generic trait is implemented for IntoIterator, you can get an "iter map" off anything that supports that interface. For instance, one can be created directly off an array, like so:
use iter_map::*;
fn main()
{
let mut i = 0;
let v = [1, 2, 3, 4, 5, 6].iter_map(move |iter| {
i += 1;
if i % 3 == 0 {
Some(0)
} else {
iter.next().copied()
}
}).collect::<Vec<_>>();
assert_eq!(v, vec![1, 2, 0, 3, 4, 0, 5, 6, 0]);
}
Here's the full code - it was amazing it took such little code to implement, and everything just seemed to work smoothly while putting it together. It gave me a new appreciation for the flexibility of Rust itself and its design decisions.
/// Adds `.iter_map()` method to all IntoIterator classes.
///
impl<F, I, J, R, T> IntoIterMap<F, I, R, T> for J
//
where F: FnMut(&mut I) -> Option<R>,
I: Iterator<Item = T>,
J: IntoIterator<Item = T, IntoIter = I>,
{
/// Returns an iterator that invokes the callback in `.next()`, passing it
/// the original iterator as an argument. The callback can return any
/// arbitrary type within an `Option`.
///
fn iter_map(self, callback: F) -> ParamFromFnIter<F, I>
{
ParamFromFnIter::new(self.into_iter(), callback)
}
}
/// A trait to add the `.iter_map()` method to any existing class.
///
pub trait IntoIterMap<F, I, R, T>
//
where F: FnMut(&mut I) -> Option<R>,
I: Iterator<Item = T>,
{
/// Returns a `ParamFromFnIter` iterator which wraps the iterator it's
/// invoked on.
///
/// # Arguments
/// * `callback` - The callback that gets invoked by `.next()`.
/// This callback is passed the original iterator as its
/// parameter.
///
fn iter_map(self, callback: F) -> ParamFromFnIter<F, I>;
}
/// Implements an iterator that can be created from a callback.
/// does pretty much the same thing as `std::iter::from_fn()` except the
/// callback signature of this class takes a data argument.
pub struct ParamFromFnIter<F, D>
{
callback: F,
data: D,
}
impl<F, D, R> ParamFromFnIter<F, D>
//
where F: FnMut(&mut D) -> Option<R>,
{
/// Creates a new `ParamFromFnIter` iterator instance.
///
/// This provides a flexible and simple way to create new iterators by
/// defining a callback.
/// # Arguments
/// * `data` - Data that will be passed to the callback on each
/// invocation.
/// * `callback` - The callback that gets invoked when `.next()` is invoked
/// on the returned iterator.
///
pub fn new(data: D, callback: F) -> Self
{
ParamFromFnIter { callback, data }
}
}
/// Implements Iterator for ParamFromFnIter.
///
impl<F, D, R> Iterator for ParamFromFnIter<F, D>
//
where F: FnMut(&mut D) -> Option<R>,
{
type Item = R;
/// Iterator method that returns the next item.
/// Invokes the client code provided iterator, passing it `&mut self.data`.
///
fn next(&mut self) -> Option<Self::Item>
{
(self.callback)(&mut self.data)
}
}