expr

The pure expression language of Mix Scenes bindings. A scene binds a port to an expression such as = $model.rows, = not $model.volume.has_icon or = $model.prefix .. $item.cells[0]; this crate compiles the expression, reports which globals it reads, and evaluates it over serde_json::Value globals under size and time limits.

The language is the expression subset of Mix with every builtin denied: literals, $name globals with field and index access, arithmetic, .. concatenation, comparisons, and/or/??, the ternary and the if … then … else … end expression. Calls, assignments, statements and shell or Bus forms are refused at compile time with ErrorKind::NotAllowed. The crate does not depend on the Mix interpreter.

let model = serde_json::json!({"prefix": "live ", "rows": [{"cells": ["one"]}]});
let binding = expr::compile("$model.prefix .. $model.rows[0].cells[0]")?;
assert_eq!(binding.roots().iter().collect::<Vec<_>>(), ["model"]);
let text = binding.eval(&[("model", &model)], &expr::Limits::default())?;
assert_eq!(text, "live one");

Test: cargo test -p expr. The semantics are pinned in tests/semantics.rs; the crate documentation in src/lib.rs lists what is accepted, refused and how errors are classified.

The source is libs/expr.