Author: larry Date: Fri Dec 7 16:58:49 2007 New Revision: 14473 Modified: doc/trunk/design/syn/S04.pod
Log: More loop control tweakage. Modified: doc/trunk/design/syn/S04.pod ============================================================================== --- doc/trunk/design/syn/S04.pod (original) +++ doc/trunk/design/syn/S04.pod Fri Dec 7 16:58:49 2007 @@ -14,7 +14,7 @@ Date: 19 Aug 2004 Last Modified: 7 Dec 2007 Number: 4 - Version: 61 + Version: 62 This document summarizes Apocalypse 4, which covers the block and statement syntax of Perl. @@ -264,17 +264,34 @@ The C<.next> and C<.last> methods take an optional argument giving the final value of that loop iteration. So the old C<next LINE> syntax is still allowed but is really short for C<next LINE:> using -indirect object syntax. +indirect object syntax. Any block object can be used, not just labels, +so to return a value from this iteration of the current block you can say: + + &?BLOCK.next($retval); + +[Conjecuture: a bare C<next($retval)> function could be taught to do +the same, as long as C<$retval> isn't a loop label. Presumably multiple +dispatch could sort this out.] There is no longer a C<continue> block. Instead, use a C<NEXT> block within the body of the loop. See below. The value of a loop statement is the list of values from each -iteration that successfully completed without a C<next> or C<last>. -(This list is actually a two-dimensional list of Captures (a "slice") -with dimensional boundaries at each iteration. List context -ignores these boundaries and flattens the list. Slice context turns -the captures into subarrays) If you do not wish +iteration. Iterations that return a null list (such as by calling +C<next> with no extra return arguments) interpolate no values in the +resulting list. (This list is actually a two-dimensional list of +Captures (a "slice") with dimensional boundaries at each iteration. +Normal list context ignores these boundaries and flattens the list. +Slice context turns the captures into subarrays, so an iteration +returning a null list does show up as a null subarray when viewed as +a slice.) + +For finer-grained control of which iterations return values, use +C<gather> and C<take>. + +Since the final expression in a subroutine returns its value, it's +possible to accidentally return a loop's return value when you were +only evaluating the loop for its side effects. If you do not wish to accidentally return a list from the final loop statement in a subroutine, place an explicit return statement after it, or declare a return type of C<Void>. @@ -677,7 +694,7 @@ topicalizer block) early using the C<break> verb. More precisely, it leaves the innermost block outside the C<when> that uses C<$_> as one of its formal parameters, either explicitly or implicitly. -It does the essentially by simply going to the end of the block and +It does this essentially by going to the end of the block and returning normally from that block. In other words, a break (either implicit or explicit) is assumed to indicate success, not failure.
