59 lines
1.8 KiB
Markdown
59 lines
1.8 KiB
Markdown
Statement Expression
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====================
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{{#include ../links.md}}
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```admonish warning.side "Differs from Rust"
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This is different from Rust where, if the last statement is terminated by a semicolon, the block's
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return value defaults to `()`.
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```
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Like Rust, a statement can be used anywhere where an _expression_ is expected.
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These are called, for lack of a more creative name, "statement expressions."
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The _last_ statement of a statements block is _always_ the block's return value when used as a statement,
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_regardless_ of whether it is terminated by a semicolon or not.
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If the last statement has no return value (e.g. variable definitions, assignments) then it is
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assumed to be [`()`].
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```rust
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let x = {
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let foo = calc_something();
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let bar = foo + baz;
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bar.further_processing(); // <- this is the return value
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}; // <- semicolon is needed here...
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// The above is equivalent to:
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let result;
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{
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let foo = calc_something();
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let bar = foo + baz;
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result = bar.further_processing();
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}
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let x = result;
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// Statement expressions can be inserted inside normal expressions
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// to avoid duplicated calculations
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let x = foo(bar) + { let v = calc(); process(v, v.len, v.abs) } + baz;
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// The above is equivalent to:
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let foo_result = foo(bar);
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let calc_result;
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{
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let v = calc();
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result = process(v, v.len, v.abs); // <- avoid calculating 'v'
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}
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let x = foo_result + calc_result + baz;
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// Statement expressions are also useful as function call arguments
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// when side effects are desired
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do_work(x, y, { let z = foo(x, y); print(z); z });
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// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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// statement expression
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```
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Statement expressions can be disabled via [`Engine::set_allow_statement_expression`][options].
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