Iterator definitions were described on See Iterator definitions. Iterators are used extensively in Sather to control loops. This section elaborates their semantics and describes the built-in iterators that correspond to statements such as 'while' and 'do' found in other languages.
Example 8-1. Example:
loop ... end
loop_statement ==> loop statement_list end
Iteration is done with loop statements, used in conjunction with iterator calls. An execution state is maintained for each textual iterator call. When a loop is entered, the execution state of all enclosed iterator calls is initialized. When an iterator is first called in a loop, the expressions for self and for each once argument are evaluated left to right. Then the expressions for arguments which are not once (in or inout before the call, out or inout after the call; see See Method call expressions) are evaluated left to right. On subsequent calls, only the expressions for arguments which are not once are re-evaluated. self and any once arguments retain their earlier values. The expressions for self and for once arguments may not themselves contain iterator calls (such iters would only execute their first iteration.) Method call semantics are detailed on See Method call expressions.
When an iterator is called, it executes the statements in its body in order. If it executes a yield statement, control is returned to the caller. Subsequent calls on the iterator resume execution with the statement following the yield statement. If an iterator executes quit or reaches the end of its body, control passes immediately to the end of the innermost enclosing loop statement in the caller and no value is returned.