`::next_gen`

Safe generators on stable Rust.

Examples

Reimplementing a `range` iterator

```rust use ::next_gen::prelude::*;

[generator(yield(u8))]

fn range (start: u8, end: u8) { let mut current = start; while current < end { yield_!(current); current += 1; } }

mkgen!(let generator = range(3, 10)); asserteq!( generator.into_iter().collect::>(), (3 .. 10).collect::>(), ); ```

Implementing an iterator over prime numbers using the sieve of Eratosthenes

```rust use ::next_gen::prelude::*;

enum NeverSome {}

/// Generator over all the primes less or equal to up_to.

[generator(yield(usize))]

fn primesupto (upto: usize) -> Option { if upto < 2 { return None; } let mut sieve = vec![true; upto.checkedadd(1).expect("Overflow")]; let mut p: usize = 1; loop { p += 1 + sieve .get(p + 1 ..)? .iter() .position(|&isprime| isprime)? ; yield!(p); let p2 = if let Some(p2) = p.checkedmul(p) { p2 } else { continue }; if p2 >= upto { continue; } sieve[p2 ..] .itermut() .stepby(p) .foreach(|isprime| *isprime = false) ; } }

mkgen!(let primes = primesupto(10000)); for prime in primes { assert!( (2usize ..) .takewhile(|&n| n.saturating_mul(n) <= prime) .all(|n| prime % n != 0) ); } ```

Defining an iterator with self-borrowed state

This is surely the most useful feature of a generator.

Consider, for instance, the following problem:

rust fn iter_locked (elems: &'_ Mutex<Set<i32>>) -> impl '_ + Iterator<Item = i32>

Miserable attempts without generators

No matter how hard you try, without using unsafe, or some other unsafe-using self-referential library/tool, you won't be able to feature such a signature!

The following fails:

rust fn iter_locked (mutexed_elems: &'_ Mutex<Set<i32>>) -> impl '_ + Iterator<Item = i32> { ::std::iter::from_fn({ let locked_elems = mutexed_elems.lock().unwrap(); let mut elems = locked_elems.iter().copied(); move || { // let _ = locked_elems; elems.next() } // Error, borrowed `locked_elems` is not captured and is thus dropped! }) }

rust error[E0515]: cannot return value referencing local variable `locked_elems` --> src/lib.rs:122:5 | 11 | / ::std::iter::from_fn({ 12 | | let locked_elems = mutexed_elems.lock().unwrap(); 13 | | let mut elems = locked_elems.iter().copied(); | | ------------------- `locked_elems` is borrowed here 14 | | move || { ... | 17 | | } // Error, borrowed `locked_elems` is not captured and is thus dropped! 18 | | }) | |______^ returns a value referencing data owned by the current function | = help: use `.collect()` to allocate the iterator
as well as this:

rust fn iter_locked (mutexed_elems: &'_ Mutex<Set<i32>>) -> impl '_ + Iterator<Item = i32> { ::std::iter::from_fn({ let locked_elems = mutexed_elems.lock().unwrap(); let mut elems = locked_elems.iter().copied(); move || { let _ = &locked_elems; // ensure `locked_elems` is captured (and thus moved) elems.next() // Error, can't use borrow of moved value! } }) }

``rust error[E0515]: cannot return value referencing local variablelockedelems--> src/lib.rs:144:5 | 11 | / ::std::iter::from_fn({ 12 | | let locked_elems = mutexed_elems.lock().unwrap(); 13 | | let mut elems = locked_elems.iter().copied(); | | -------------------lockedelemsis borrowed here 14 | | move || { ... | 17 | | } 18 | | }) | |______^ returns a value referencing data owned by the current function | = help: use.collect()` to allocate the iterator

error[E0505]: cannot move out of locked_elems because it is borrowed --> src/lib.rs:147:9 | 8 | fn iterlocked (mutexedelems: &'_ Mutex>) | - let's call the lifetime of this reference '1 ... 11 | / ::std::iter::fromfn({ 12 | | let lockedelems = mutexedelems.lock().unwrap(); 13 | | let mut elems = lockedelems.iter().copied(); | | ------------------- borrow of locked_elems occurs here 14 | | move || { | | ^^^^^^^ move out of locked_elems occurs here 15 | | let _ = &lockedelems; // ensure locked_elems is captured (and thus moved) | | ------------ move occurs due to use in closure 16 | | elems.next() // Error, can't use borrow of moved value! 17 | | } 18 | | }) | |_- returning this value requires that locked_elems is borrowed for '1

error: aborting due to 2 previous errors ```
In other cases sub-efficient workarounds may be available

Such as when that Set would be a Vec instead. In that case, we can use indices as a poorman's self-reference, with no "official" lifetimes and thus Rust not complaining:

rust fn iter_locked (mutexed_vec: &'_ Mutex<Vec<i32>>) -> impl '_ + Iterator<Item = i32> { ::std::iter::from_fn({ let locked_vec = mutexed_vec.lock().unwrap(); let mut indices = 0.. locked_vec.len(); move /* locked_vec, indices */ || { let i = indices.next()?; Some(locked_vec[i]) // copies, so OK. } }) } let mutexed_elems = Mutex::new(vec![27, 42]); let mut iter = iter_locked(&mutexed_elems); assert_eq!(iter.next(), Some(27)); assert_eq!(iter.next(), Some(42)); assert_eq!(iter.next(), None);

But with generators this is easy:

```rust use ::next_gen::prelude::*;

[generator(yield(i32))]

fn geniterlocked (mutexedelems: &' Mutex>) { let lockedelems = mutexedelems.lock().unwrap(); for elem in lockedelems.iter().copied() { yield!(elem); } } ```

and voilà!

That #[generator] fn is the key constructor for our safe self-referential iterator!

Now, instantiating an iterator off a self-referential generator has a subtle aspect, muck alike that of polling a self-referential Future (that's what a missing Unpin bound means): we need to get it pinned before it can be polled!

About pinning "before use", and the two forms of pinning

Getting a Future:

rust let future = async { ... }; // or let future = some_async_fn(...);
- Pinning an instantiated Future in the heap (Boxed):
rust // Pinning it in the heap (boxed): let mut pinned_future = Box::pin(future) // or, through an extension trait (`::futures::future::FutureExt`): let mut pinned_future = future.boxed() // this also incidentally `dyn`-erases the future.
- Now we can return it, or poll it:
  
  rust if true { pinned_future.as_mut().poll(...); } // and/or return it: return pinned_future;
  - Pinning an instantiated Future in the stack (pinned to the local scope):
``rust use ::some_lib::some_pinning_macro as stack_pinned; // Pinning it in the "stack" stack_pinned!(mut future); /* the above shadowsfuture`, thus acting as: let mut future = magic::Stack::pin(future); // */

// Let's rename it for clarity: let mut pinned_future = future; ```
- Now we can poll it / use it within the current stack frame, but we cannot return it.
  
  rust pinned_future.as_mut().poll(...)

Well, it turns out that for generators it's similar:

Once you have a #[generator] fn "generator constructor"

```rust use ::next_gen::prelude::*;

[generator(yield(u8))]

fn foo () { yield_!(42); } ```
Instantiation requires pinning, and thus:
- Stack-pinning: cheap, no_std compatible, usable within the same scope. But it cannot be returned.
  
```rust mk_gen!(let mut generator = foo());

// can be used within the same scope asserteq!(generator.next(), Some(42)); asserteq!(generator.next(), None);

// but it can't be returned // return generator; /* Error, can't return borrow to local value */ ```
- Heap-pinning: a bit more expensive, requires an ::allocator or not being no_std, but the so-pinned generator can be returned.
  
```rust mk_gen!(let mut generator = box foo());

// can be used within the same scope if somecondition { asserteq!(generator.next(), Some(42)); assert_eq!(generator.next(), None); }

// and/or it can be returned return generator; // OK ```

So, back to our example, this is what we need to do:

```rust use ::next_gen::prelude::*;

/// We already have:

[generator(yield(i32))]

fn geniterlocked (mutexedelems: &' Mutex>) ...

/// Now let's wrap-it so that it yields a nice iterator: fn iterlocked (mutexedelems: &'_ Mutex>) -> impl '_ + Iterator { if true { // One possible syntax to instantiate the generator mkgen!(let generator = box geniterlocked(mutexedelems)); generator } else { // or, since we are box-ing, we can directly do: geniterlocked.callboxed((mutexedelems, )) } // : Pin>> // : impl '_ + Iterator }

let mutexedelems = Mutex::new([27, 42].iter().copied().collect::>()); let mut iter = iterlocked(&mutexedelems); asserteq!(iter.next(), Some(27)); asserteq!(iter.next(), Some(42)); asserteq!(iter.next(), None); ```

If the iter_locked() function you are trying to implement is part of a trait definition and thus need to name the type, at which point the impl '_ + Iterator… existential syntax can be problematic, you can then use dyn instead of impl, at the cost of having to mention the Pin<Box<>> layer:

rust // instead of -> impl '_ + Iterator<Item = i32> // write: -> Pin<Box<dyn '_ + Generator<Yield = i32, Return = ()>>>
An example

```rust use ::next_gen::prelude::*;

struct Once(T); impl IntoIterator for Once { type Item = T; type IntoIter = Pin>;
```
fn into_iter (self: Once<T>)
  -> Self::IntoIter
{
    #[generator(yield(T))]
    fn once_generator<T> (value: T)
    {
        yield_!(value);
    }

    once_generator.call_boxed((self.0, ))
}
```
} asserteq!(Once(42).intoiter().next(), Some(42)); ```

See Generator for more info.

Features

Performance

The crate enables no-allocation generators, thanks the usage of stack pinning. When used in that fashion, it should thus be close to zero-cost.

Ergonomics / sugar

A lot of effort has been put into macros and an attribute macro providing the most ergonomic experience when dealing with these generators, despite the complex / subtle internals involved, such as stack pinning.

Safe

Almost no unsafe is used, the exception being:

Stack pinning, where it uses the official ::pin_utils::pin_mut implementation;
Using the pinning guarantee to extend a lifetime;

`no_std` support

This crates supports #![no_std]. For it, just disable the default "std" feature:

toml [dependencies] next-gen.version = "..." next-gen.default-features = false # <- ADD THIS to disable `std`&`alloc` for `no_std` compat next-gen.features = [ "alloc", # If your no_std platform has access to allocators. # "std", # `default-features` bundles this. ]

Idea

Since generators and coroutines rely on the same internals, one can derive a safe implementation of generators using the async / await machinery, which is only already in stable Rust.

A similar idea has also been implemented in https://docs.rs/genawaiter.

::next_gen

Examples

Reimplementing a range iterator

[generator(yield(u8))]

Implementing an iterator over prime numbers using the sieve of Eratosthenes

[generator(yield(usize))]

Defining an iterator with self-borrowed state

Miserable attempts without generators

But with generators this is easy:

[generator(yield(i32))]

[generator(yield(u8))]

[generator(yield(i32))]

Features

Performance

Ergonomics / sugar

Safe

no_std support

Idea

`::next_gen`

Reimplementing a `range` iterator

`no_std` support