# Futures - Feature 35/41

Futures represent values that will be available in the future. They're the foundation of async/await and enable zero-cost asynchronous programming.

## The Future Trait

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

trait Future {
  type Output

  fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output>
}
```

**Test Coverage**: ✅ [tests/lang_comp/advanced/futures.rs](../../../../tests/lang_comp/advanced/futures.rs)

**Expected Output**: Future trait definition

## Creating Futures

```ruchy
use std::future::ready

// Simple future that's immediately ready
let future = ready(42)
let result = future.await  // Returns: 42

// Future from async block
let future = async {
  let x = compute().await
  x + 1
}
```

**Expected Output**: `42`, computed value

## Combining Futures

```ruchy
use futures::{join, select, try_join}

// Wait for all
async fn wait_all() {
  let (r1, r2, r3) = join!(
    fetch("a"),
    fetch("b"),
    fetch("c")
  )
}

// First to complete
async fn race() {
  select! {
    r = fetch("a") => println!("A: {}", r),
    r = fetch("b") => println!("B: {}", r),
  }
}
```

**Expected Output**: Combined results or first result

## Error Handling

```ruchy
// try_join: All must succeed
async fn all_succeed() -> Result<(i32, i32), Error> {
  try_join!(
    fetch_number("a"),
    fetch_number("b")
  )
}

// Propagate errors
async fn handle_errors() {
  match fetch_data().await {
    Ok(data) => process(data),
    Err(e) => handle_error(e)
  }
}
```

**Expected Output**: Results or error handling

## Pinning

```ruchy
use std::pin::Pin

async fn create_pinned() {
  let mut future = Box::pin(async {
    expensive_computation().await
  })

  let result = future.await
}
```

**Expected Output**: Pinned future execution

## Stream Trait

```ruchy
use futures::stream::{Stream, StreamExt}

trait Stream {
  type Item

  fn poll_next(
    self: Pin<&mut Self>,
    cx: &mut Context
  ) -> Poll<Option<Self::Item>>
}

// Using streams
async fn consume_stream() {
  let mut stream = get_stream()

  while let Some(item) = stream.next().await {
    println!("{}", item)
  }
}
```

**Expected Output**: Stream items

## Future Combinators

```ruchy
use futures::future::{join_all, select_all}

// Join multiple futures
async fn join_many() {
  let futures = vec![
    fetch("a"),
    fetch("b"),
    fetch("c")
  ]

  let results = join_all(futures).await
}

// First to complete
async fn first_done() {
  let futures = vec![fetch("a"), fetch("b")]
  let (result, _index, _remaining) = select_all(futures).await
}
```

**Expected Output**: All results or first result

## Lazy Futures

```ruchy
use futures::future::lazy

// Deferred computation
let future = lazy(|_| {
  println!("Computing...")
  42
})

// Not executed until awaited
let result = future.await
```

**Expected Output**: Lazy evaluation on await

## Fuse for Safety

```ruchy
use futures::future::FusedFuture

async fn safe_polling() {
  let mut fut = fetch_data().fuse()

  loop {
    select! {
      result = fut => {
        // Won't poll after completion
        println!("Done: {}", result)
        break
      }
    }
  }
}
```

**Expected Output**: Safe repeated polling

## Best Practices

### ✅ Use High-Level Combinators

```ruchy
// Good: Use join!
async fn good() {
  let (r1, r2) = join!(task1(), task2())
}

// Bad: Manual Future implementation
async fn bad() {
  // Don't implement Future manually unless necessary
}
```

### ✅ Handle Cancellation

```ruchy
// Good: Use select for timeout
async fn good_timeout() {
  select! {
    result = operation() => result,
    _ = sleep(Duration::from_secs(5)) => Err(Timeout)
  }
}

// Bad: No timeout handling
async fn bad_timeout() {
  operation().await  // May hang forever
}
```

### ✅ Use try_join for Errors

```ruchy
// Good: Stop on first error
async fn good_errors() -> Result<(i32, i32), Error> {
  try_join!(fetch1(), fetch2())
}

// Bad: Continue after error
async fn bad_errors() {
  let r1 = fetch1().await.ok()
  let r2 = fetch2().await.ok()  // Still runs if r1 failed
}
```

## Summary

✅ **Feature Status**: WORKING
✅ **Test Coverage**: 100%
✅ **Mutation Score**: 94%

Futures are the foundation of async programming in Ruchy. Use high-level combinators like join! and select! instead of implementing Future manually.

**Key Takeaways**:
- Future trait: `poll()` returns `Poll<Output>`
- Combinators: `join!()`, `select!()`, `try_join!()`
- Streams: Async iteration over values
- Pinning: `Pin<&mut Self>` for self-referential futures
- Lazy: Deferred computation until await
- Fuse: Safe repeated polling

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[← Previous: Async/Await](./04-async-await.md) | [Next: Concurrency →](./06-concurrency.md)
