speice.io/_posts/2020-07-08-on-implementing-futures.md

---
layout: post
title: "Tips for Implementing `Future`"
description: ""
category:
tags: [python]
---

When support for async/await launched in Rust, it came with a couple of technical caveats; it was
deemed more beneficial to release a minimum viable product than wait for a feature-complete release.
So far, this model seems to have worked out well. Asynchronous code originally required thread-local
storage for context tracking which mean that it could only be used in projects that included the
Rust standard library. It wasn't a hard requirement; nothing about the async design mandated context
tracking in this way. But given that most users of asynchronous code relied on the standard library
anyway, supporting asynchronous `no_std` projects was left as something to be addressed later. After
some fantastic work, thread-local storage is no longer used and there's some incredibly cool work
being done to enable Rust on `no_std` and embedded systems. While asynchronous programming is very
frequently used to model disk or network I/O, the same principles can be applied to monitoring
signals received from GPIO pins.

NOTE: Should I mention something about how cool it is that we can have `async` without needing heap
allocations or type erasure like in every other `async` implementation?

One other missing feature in the initial async support was being able to write traits that contained
`async fn` methods Normally, when an `async fn` function is declared, the compiler does some magic
to the function signature:

```rust
struct R;

// When you write a function like this:
async fn read_bytes(s: TcpStream) -> R { /* ... */ }

// ...the compiler effectively transforms it into this:
fn read_bytes(s: TcpStream) -> impl Future<Output = R> { /* ... */ }
```

This special return type (the `impl Future` thing) tells the compiler "I have no idea what the
_exact_ return type will be, but it will be something that implements `Future`, just figure it out
for me." If you're writing static or `struct` functions, that's no issue, the compiler can figure
everything out for you.

However, this "figure it out for me" mentality doesn't work when used with traits. The reasons are
varied and complex and out of scope for this discussion. But if we want to mix traits and
asynchronous code, we simply need to make sure the trait method returns a type that implements the
`Future` trait:

```rust
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

pub struct Byte(u8);

// Implementing this section of the code is what we'll be talking about.
// vvvvvvvvvvvvvvvvvvv
impl Future for Byte {
    type Output = u8;

    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
        return Poll::Ready(self.0);
    }
}
// ^^^^^^^^^^^^^^^^^^^

pub trait ByteReader {
    fn get_byte(&self) -> Byte;
}

pub async fn my_function(b: impl ByteReader) -> u8 {
    b.get_byte().await
}
```

Because of some Rust-specific issues (the `Pin`/`Unpin` system, unhelpful compiler messages),
implementing `Future` directly can be rather difficult. It's possible to use crates like
`async_trait` to work around the limitation, but if you're interested in building your own futures,
these techniques should make the process at least a bit easier.

# Implement functionality before structure

Principle: if possible, implement the desired behavior in a separate function where all state is
provided as arguments.

It's helpful to de-couple "what you need in order to function" from "how you get those things"; are
you supposed to use `#[pin_project]` or `let Self { } = &mut *self` or maybe just `&mut self.value`?
Instead, just pass everything that needs polled as `Pin<&mut Thing>` and deal with it later.

## Caveat 1: Don't reference this method until ready

Errors elsewhere in the code can mask issues in the implementation, or make it difficult to
understand if there are issues in specification (the `struct`) or implementation (the function).

## Caveat 2: Don't re-use type names

Can reconcile the names afterward, but it's helpful to separate issues of implementation from
specification:

```rust
use futures_io::AsyncBufRead;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

fn poll_once<R1: AsyncBufRead + ?Sized>(mut reader: Pin<&mut R1>, cx: &mut Context<'_>) -> Poll<()> {
    reader.as_mut().poll_fill_buf(cx);
    return Poll::Ready(());
}

struct MyStruct<'a, R2: ?Sized> {
    reader: &'a R2,
}

impl<R3: AsyncBufRead + ?Sized + Unpin> Future for MyStruct<'_, R3> {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        poll_once(Pin::new(&mut self.reader), cx)
    }
}
```

```text
error[E0277]: the trait bound `&R3: futures_io::if_std::AsyncBufRead` is not satisfied
  --> src/lib.rs:19:9
   |
6  | fn poll_once<R1: AsyncBufRead + ?Sized>(mut reader: Pin<&mut R1>, cx: &mut Context<'_>) -> Poll<()> {
   |                  ------------ required by this bound in `poll_once`
...
19 |         poll_once(Pin::new(&mut self.reader), cx)
   |         ^^^^^^^^^ the trait `futures_io::if_std::AsyncBufRead` is not implemented for `&R3`
```

I need to reduce this example though.

NOTE: Should also add something about how `AsyncBufRead` isn't implemented for `&R3`, but _is_ after
deref. The errors become a lot more obvious if you try to deref `self.reader`:

```rust
use futures_io::AsyncBufRead;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

fn poll_once<R1: AsyncBufRead + ?Sized>(mut reader: Pin<&mut R1>, cx: &mut Context<'_>) -> Poll<()> {
    reader.as_mut().poll_fill_buf(cx);
    return Poll::Ready(());
}

struct MyStruct<'a, R2: ?Sized> {
    reader: &'a R2,
}

impl<R3: AsyncBufRead + ?Sized + Unpin> Future for MyStruct<'_, R3> {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        poll_once(Pin::new(&mut *self.reader), cx)
    }
}
```

```text
error[E0596]: cannot borrow data in a dereference of `std::pin::Pin<&mut MyStruct<'_, R3>>` as mutable
  --> src/lib.rs:19:28
   |
12 |     reader: &'a R2,
   |             ------ help: consider changing this to be mutable: `&'a mut R2`
...
19 |         poll_once(Pin::new(&mut *self.reader), cx)
   |                            ^^^^^^^^^^^^^^^^^ cannot borrow as mutable

error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
  --> src/lib.rs:19:34
   |
18 |     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
   |             ---- help: consider changing this to be mutable: `mut self`
19 |         poll_once(Pin::new(&mut *self.reader), cx)
   |                                  ^^^^ cannot borrow as mutable
```

Now, we can see that `self` can't be borrowed as mutable (it needs to be marked
`mut self: Pin<&mut Self>`) and that the reader can't be borrowed as mutable (the struct definition
needs `&'a mut R2`).

# Don't feel bad about requiring `Unpin`

Principle: don't require it unless you need to, but don't hesitate to add it if the compiler thinks
you should.

```rust
use futures_io::AsyncBufRead;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

fn poll_once<R1: AsyncBufRead + ?Sized>(mut reader: Pin<&mut R1>, cx: &mut Context<'_>) -> Poll<()> {
    reader.as_mut().poll_fill_buf(cx);
    return Poll::Ready(());
}

struct MyStruct<'a, R2: ?Sized> {
    reader: &'a R2,
}

impl<R3: AsyncBufRead + ?Sized> Future for MyStruct<'_, R3> {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        poll_once(Pin::new(&mut self.reader), cx)
    }
}
```

The type bounds for `R1` and `R3` seem to be identical, but are actually slightly different:

```text
error[E0277]: `R3` cannot be unpinned
  --> src/lib.rs:19:9
   |
6  | fn poll_once<R1: AsyncBufRead + ?Sized>(mut reader: Pin<&mut R1>, cx: &mut Context<'_>) -> Poll<()> {
   |                  ------------ required by this bound in `poll_once`
...
19 |         poll_once(Pin::new(&mut self.reader), cx)
   |         ^^^^^^^^^ the trait `std::marker::Unpin` is not implemented for `R3`
   |
   = note: required because of the requirements on the impl of `futures_io::if_std::AsyncBufRead` for `&mut R3`
help: consider further restricting this bound
   |
15 | impl<R3: AsyncBufRead + ?Sized + std::marker::Unpin> Future for MyStruct<'_, R3> {
   |                                ^^^^^^^^^^^^^^^^^^^^
```

# Know what the escape hatches are

When used sparingly, either `#[async_trait]` or `Box::pin(async move {})` can enable async
functionality in code that will later not need the allocations. Use the escape hatch when you need
to such that you can continue making incremental improvements later.

Specific trick: use `BoxFuture` for opaque type erasure:

```rust
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};
use futures::future::BoxFuture;

async fn my_function() {}

struct MyStruct<F: Future<Output = ()>> {
    f: F
}

fn another_function() -> MyStruct<BoxFuture<'static, ()>> {
    MyStruct { f: Box::pin(async { my_function().await }) }
}
```

NOTE: Should also add something about owned data structures need to implemented `Unpin`:

```rust

struct First<T> {
    value: T,
}

// To get access to `T` through `self`, `T` must implement `Unpin`

struct Second<T> {
    values: Vec<T>
}

// Same thing - `T` must implement `Unpin` to get access to `values`
```