type-exercise-in-rust

Build a Typed Database Expression Engine in Rust

A hand-written loop for one integer function is easy. A database expression engine also has to borrow strings without copying, preserve nulls, read several column representations, coerce types, and select functions at runtime.

This course builds the type families and checked boundaries that move those decisions out of each row loop. The result is a small auto-vectorized expression engine: authors write one scalar operation, while generic unary, binary, and ternary adapters reuse the checked batch path.

Read the course

The complete published course contains five modules and ten cumulative, independently testable checkpoints. Work through them in order; each extends the same starter and keeps the earlier supplied tests green. Plan roughly half a day for each checkpoint. An experienced Rust learner can finish in about five working days, while newer learners should expect to take longer.

What you will build

The five modules follow the engine from physical values to its outer async boundary:

• Type families and nullable views (Checkpoints 1–2): connect owned and borrowed scalars, arrays, builders, logical types, and checked Array, Constant, Indexed, and typed-null views.
• Shared evaluation and transactional strings (Checkpoints 3–4): lift scalar operations over batches and publish variable-width rows without exposing partial writes.
• Shape specialization and binary semantics (Checkpoints 5–6): specialize common column shapes while preserving fallback behavior, then separate total, fallible, and nullable-aware binary evaluation.
• Runtime erasure and the physical catalog (Checkpoints 7–8): erase whole typed expressions behind a checked batch boundary and build a discoverable catalog of concrete operations.
• Logical binding, one-level Lists, and batch async (Checkpoints 9–10): bind logical calls to physical expressions, add checked nullable List storage, and defer one complete batch in a borrowing future.

The course deliberately stops short of Decimal arithmetic, casts and rounding, lossy coercions, nested or list-producing functions, exhaustive fast paths, an aggregate engine, and per-row futures.

Start the exercises

You should know ordinary Cargo use, enums, traits, references, and Option. Follow the environment setup, then work only in type-exercise-starter:

git fetch origin
git switch --create course-work --track origin/main
cargo check -p type-exercise-starter-expr --lib --locked

Choose another branch name if course-work already exists. The starter baseline should compile. Do not inspect type-exercise/ or archived/ while solving an exercise.

How chapter tests work

Copy the cumulative supplied contract when you are ready to start a chapter:

cargo x copy-test --chapter 1
cargo test -p type-exercise-starter-supplied-tests chapter_1 --locked

The first focused run should fail because the new behavior is missing. Read the copied destination under type-exercise-starter/supplied-tests/src/, implement the named API in other starter files, and rerun the same command until it passes. Never edit copied tests or supplied-tests/src/lib.rs; keep all earlier copied chapters green.

Checkpoint 10 is the terminal unit. After completing it, cargo x copy-test --chapter 10 copies the complete cumulative supplied contract.

Questions and feedback

Join skyzh's Discord server for discussion. For a concrete bug or improvement, open an issue or pull request.

License

The source code is licensed under Apache 2.0. See LICENSE. The mdBook text is © 2022-2026 Alex Chi Z and licensed under CC BY-NC-SA 4.0.