Chapter 13: Add a Batch Async Boundary

Work in progress. This course has not yet received the author’s final audit. Chapters, exercises, and commands may still change before the course is marked complete.

Some engines need a uniform asynchronous interface even when a local expression is immediately ready. The useful boundary is one future per batch, not one future per row.

Prerequisites: Chapter 12, Future, Pin, and return-position impl Trait.

By the end of this chapter, you will:

• wrap static expression evaluation in a borrowed batch future;
• erase that future behind an object-safe asynchronous interface; and
• preserve every synchronous result and error without evaluating twice.

cargo x copy-test --chapter 13
cargo test -p type-exercise-starter chapter_13 --locked

The first run should fail on the missing static or erased batch future boundary.

Checkpoint 1: return one static future

• Target: type-exercise-starter/src/expression.rs::evaluate_static.
• Change: return impl Future<Output = anyhow::Result<ArrayImpl>> + Send + 'a that borrows the expression and input slice.
• Preserve: the body delegates to the existing synchronous batch evaluation exactly once.
• Run: the Chapter 13 focused test.
• Passing means: static sync and async paths return identical arrays and errors.

Checkpoint 2: erase the future

• Target: type-exercise-starter/src/expression.rs::{BatchFuture, AsyncExpression, AsyncExpressionAdapter}.
• Change: add AsyncExpressionAdapter::new, then box and pin the borrowed batch future so it can be returned from a trait object.
• Preserve: the future lifetime covers the expression, the view slice, and every borrowed backing array.
• Run: the focused test.
• Passing means: erased async evaluation matches static and synchronous evaluation.

Checkpoint 3: forward a bound plan

• Target: type-exercise-starter/src/binder.rs::BoundExpression::evaluate_async.
• Change: delegate to the already-selected expression without repeating logical binding.
• Preserve: arity, type, length, null, and operation errors keep the same contextual messages and precedence.
• Run: focused and cumulative tests.
• Passing means: the planning boundary remains one-time and the batch kernel remains synchronous.

Required and extension work

One ready future per batch is required. I/O, timers, retries, background threads, cancellation protocols, custom runtimes, and per-row futures are outside this course.

cargo test -p type-exercise-starter chapter_13 --locked
cargo test -p type-exercise-starter --doc --locked
cargo test -p type-exercise-starter --lib --locked

You have now moved type selection, representation dispatch, validation, promotion, and runtime selection out of the row loop while keeping each failure boundary explicit.