Statement Expression

Differs from Rust

This is different from Rust where, if the last statement is terminated by a semicolon, the block’s return value defaults to ().

Like Rust, a statement can be used anywhere where an expression is expected.

These are called, for lack of a more creative name, “statement expressions.”

The last statement of a statement block is always the block’s return value when used as a statement, regardless of whether it is terminated by a semicolon or not.

If the last statement has no return value (e.g. variable definitions, assignments) then it is assumed to be ().

let x = {
    let foo = calc_something();
    let bar = foo + baz;
    bar.further_processing();       // <- this is the return value
};                                  // <- semicolon is needed here...

// The above is equivalent to:
let result;
{
    let foo = calc_something();
    let bar = foo + baz;
    result = bar.further_processing();
}
let x = result;

// Statement expressions can be inserted inside normal expressions
// to avoid duplicated calculations
let x = foo(bar) + { let v = calc(); process(v, v.len, v.abs) } + baz;

// The above is equivalent to:
let foo_result = foo(bar);
let calc_result;
{
    let v = calc();
    result = process(v, v.len, v.abs);  // <- avoid calculating 'v'
}
let x = foo_result + calc_result + baz;

// Statement expressions are also useful as function call arguments
// when side effects are desired
do_work(x, y, { let z = foo(x, y); print(z); z });
           // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           //       statement expression

Statement expressions can be disabled via Engine::set_allow_statement_expression.