Performance

The engine is built to run on every keystroke, which sets the performance bar.

Bytecode, not tree walking

Expressions compile to a compact bytecode and run on a stack machine. Compiling once and executing many times is much faster than re-walking a syntax tree, and it makes the execution step easy to bound.

Caching, in layers

Compiled bytecode is cached by expression text, so an unchanged line skips lexing, normalising, parsing and compiling entirely. Line results are cached separately, so an unchanged line skips execution too.

1. 1ColdA line nobody has seen.
2. 2Bytecode cacheSame text, seen before.
3. 3Line cacheSame line, nothing it depends on changed.
4. 4TogetherWhy a keystroke is cheap.

1. Lex
2. Normalise
3. Parse
4. Compile
5. Execute

Every stage runs. This is the full cost, and it is paid once per distinct expression text.

Three layers

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Incremental re-evaluation

A dependency graph records which lines read which variables, so editing a line recomputes only what transitively depends on it.

What to avoid

Constructing a new engine per evaluation throws away every cache and re-registers every package. Create one engine and call clear() between documents.

Enabling diagnostics collects a large amount of per-stage detail. It is intended for a devtool and should be off in production.