Evaluate Expressions Only
Use Dynamic if you’re uncertain of the return type.
Very often, 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: i64 = engine.eval_expression("2 + (10 + 10) * 2")?;
let result: Dynamic = engine.eval_expression("get_value(42)")?;
// Usually this is done together with a custom scope with variables...
let mut scope = Scope::new();
scope.push("x", 42_i64);
scope.push_constant("SCALE", 10_i64);
let result: i64 = engine.eval_expression_with_scope(&mut scope,
"(x + 1) * SCALE"
)?;When evaluating expressions, no full-blown statement (e.g. while, for, fn) –
not even variable assignment – is supported and will be considered syntax errors.
This is true also for statement expressions and closures.
// The following are all syntax errors because the script
// is not a strict expression.
engine.eval_expression::<()>("x = 42")?;
let ast = engine.compile_expression("let x = 42")?;
let result = engine.eval_expression_with_scope::<i64>(&mut scope,
"{ let y = calc(x); x + y }"
)?;
let fp: FnPtr = engine.eval_expression("|x| x + 1")?;if expressions are allowed if both statement blocks
contain only a single expression each.
switch expressions are allowed if all match
actions are expressions and not statements.
loop expressions are not allowed.
// The following are allowed.
let result = engine.eval_expression_with_scope::<i64>(&mut scope,
"if x { 42 } else { 123 }"
)?;
let result = engine.eval_expression_with_scope::<i64>(&mut scope, "
switch x {
0 => x * 42,
1..=9 => foo(123) + bar(1),
10 => 0,
}
")?;