rhai/doc/src/engine/expressions.md

Evaluate Expressions Only

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Sometimes a use case does not require a full-blown scripting language, but only needs to evaluate expressions.

In these cases, use the Engine::compile_expression and Engine::eval_expression methods or their _with_scope variants.

let result = engine.eval_expression::<i64>("2 + (10 + 10) * 2")?;

When evaluating expressions, no full-blown statement (e.g. if, while, for) - not even variable assignment - is supported and will be considered parse errors when encountered.

// The following are all syntax errors because the script is not an expression.

engine.eval_expression::<()>("x = 42")?;

let ast = engine.compile_expression("let x = 42")?;

let result = engine.eval_expression_with_scope::<i64>(&mut scope, "if x { 42 } else { 123 }")?;