Control Flow¶

Overview¶

Control flow statements alter the sequential execution of program statements.

Syntax¶

if (cond) {
    doSomething();
} else if (cond2) {
    doOtherThing();
} else {
    doFallback();
}

while (cond) {
    doWork();
}

break;

continue;

return 123;

Rules¶

Condition Types¶

The condition expression MUST be convertible to bool. All primitive types MUST be convertible to bool where zero, 0.0, or false evaluates to false and any non-zero value evaluates to true. Struct handles (&StructType) MUST be convertible to bool where null evaluates to false and non-null evaluates to true. Slice handles ([]ElementType) MUST NOT be convertible to bool; use the length operator # to check for empty slices.

If Statement¶

The if statement evaluates the condition and executes the appropriate branch. An if statement without an else clause MAY be followed by an else if clause. else if chains MUST be supported as a single syntactic form. An else clause MUST be associated with a directly preceding if without an else.

While Statement¶

The while statement MUST evaluate the condition, if true, the body MUST be executed, after the body is executed the sequence MUST repeat until the condition returns false. If false, execution MUST continue from directly after the loop.

Deprecated with Prefix Compatibility¶

The if (...) with names { ... } form and while (...) with names { ... } form MAY be accepted as compatibility syntax, where names is one identifier or a comma-separated list of identifiers. In these forms, with MUST have no semantic effect. Names listed after with MUST be ignored and MUST NOT require name resolution or type validation. No other with form in if or while syntax MUST be accepted.

Break Statement¶

break MUST only appear inside a while loop. break MUST terminate the nearest enclosing while loop and continue execution on the first statement after the loop.

Continue Statement¶

continue MUST only appear inside a while loop. continue MUST skip to the next condition evaluation of the nearest enclosing while loop.

Return Statement¶

A function that declares a return type MUST return a value of that type on all code paths that reach the end of the function's execution. A return statement without an expression MUST only be allowed in functions which don't have a return type.

Flow-Sensitive Promotion¶

When a struct handle (type &StructType) is used as the condition of an if statement, the compiler MUST track it as known-non-null in all code paths where non-nullness can be statically guaranteed. For example, when the consequent of if (!h) terminates via return or break, the handle MUST be tracked as known-non-null in all subsequent statements in the enclosing scope after the if statement as well as in the else block. The compiler MAY include additional static tracking of handle validity.

Block Semantics¶

The body of an if branch or while loop MUST be a non-empty braced block. An else if chain MUST be permitted as an else branch form.

An if statement MUST NOT require a trailing semicolon after its closing brace.

A while statement MUST NOT require a trailing semicolon after its closing brace.

Statements inside control-flow blocks MUST follow their own statement syntax rules for semicolon use.