Evaluate Expressions Only

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.

#![allow(unused)]
fn main() {
let result = engine.eval_expression::<i64>("2 + (10 + 10) * 2")?;
}

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

Closures and anonymous functions are also not supported because in the background they compile to functions.

#![allow(unused)]
fn main() {
// 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 }")?;
}