Statement Expression
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 statements 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
.