Expression conformance

Expressions are the part of MapLibre compatibility that is exact rather than approximate. They are evaluated by maplibre-expr — a pure-Rust parser and evaluator in its own repository, published as maplibre-expr — which passes 100 % of MapLibre’s official expression spec fixtures (563/563).

What the fixtures cover

The suite is MapLibre’s own, vendored into the crate and run through libtest-mimic so each fixture is an individual test. It exercises:

• every operator in the spec — arithmetic, comparison, match, case, step, interpolate (linear / exponential / cubic-bezier), string and array functions, let/var, coalesce;
• the type system — to-number, to-string, to-boolean, to-color, array with type and length assertions, and the errors that are supposed to be errors;
• collator and locale-aware comparison;
• the geo functions (within, distance);
• legacy filter conversion, using MapLibre’s own pre-compile conversion so an old bare-field filter means what it always meant, strict-type semantics included.

Passing all of them means a *-expr field evaluates to the value MapLibre would compute for the same feature — not “close enough”, the same.

What it does not tell you

Conformance is about expression evaluation. It says nothing about what the renderer does with the resulting value. A fill-expr returns exactly MapLibre’s colour, and then fill-solid rasterizes it with ezu’s rasterizer, which is not GL-JS’s. Antialiasing, stroke geometry, and label placement are separate concerns — see gaps and differences.

Two evaluation-context differences are worth naming:

• Zoom is integer. A tile is rendered at its own zoom level, so ["zoom"] takes integer values. Curves evaluate at integer steps, not continuously.
• expr-op scalars see no feature. The expr op evaluates once per tile with zoom in the context; feature properties are only available on per-feature *-expr fields. See expression fields.

Using the engine on its own

maplibre-expr has no dependency on ezu, so it is usable in any Rust project that needs to evaluate MapLibre expressions or convert legacy filters:

[dependencies]
maplibre-expr = "0"

It lives at reearth/maplibre-expr-rs, and that is where expression bugs and fixture failures belong.