Author: larry Date: Mon Jan 22 10:46:25 2007 New Revision: 13531 Modified: doc/trunk/design/syn/S04.pod
Log: Implicit return policies clarified. Loop and conditional returns are now more conducive to list comprehension constructs (including accidental ones). Modified: doc/trunk/design/syn/S04.pod ============================================================================== --- doc/trunk/design/syn/S04.pod (original) +++ doc/trunk/design/syn/S04.pod Mon Jan 22 10:46:25 2007 @@ -12,9 +12,9 @@ Maintainer: Larry Wall <[EMAIL PROTECTED]> Date: 19 Aug 2004 - Last Modified: 13 Dec 2006 + Last Modified: 22 Jan 2007 Number: 4 - Version: 46 + Version: 47 This document summarizes Apocalypse 4, which covers the block and statement syntax of Perl. @@ -121,6 +121,13 @@ Note that temporizations that are undone upon scope exit must be prepared to be redone if a continuation within that scope is taken. +=head1 The Relationship of Blocks and Statements + +The return value of a block is the value of its final statement. +(This is subtly different from Perl 5's behavior, which was to return +the value of the last expression evaluated, even if that expression +was just a conditional.) + =head1 Statement-ending blocks A line ending with a closing brace "C<}>", followed by nothing but @@ -176,6 +183,10 @@ Conditional statement modifiers work as in Perl 5. So do the implicit conditionals implied by short-circuit operators. +If the final statement is a conditional which does not execute any +branch, the return value is C<undef> in item context and C<()> in +list context. + =head1 Loop statements Looping statement modifiers are the same as in Perl 5. @@ -184,6 +195,14 @@ 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. (This list actually a multidimensional list with +dimensional boundaries at each iteration. Most list contexts +ignore these boundaries and flatten the list.) 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>. + =head2 The C<while> and C<until> statements The C<while> and C<until> statements work as in Perl 5, except that you @@ -443,9 +462,11 @@ behavior is actually caused by the C<when> statements in the block, not by the nature of the block itself. A C<when> statement implicitly does a "smart match" between the current topic (C<$_>) and the argument -of the C<when>. If the smart match succeeds, the associated closure -is executed, and the surrounding block is automatically broken out -of. If the smart match fails, control passes to the next statement +of the C<when>. If the smart match succeeds, C<when>'s associated block +is executed, and the surrounding block is automatically broken out of. +The value of the inner block is returned as the value of the outer block. + +If the smart match fails, control passes to the next statement normally, which may or may not be a C<when> statement. Since C<when> statements are presumed to be executed in order like normal statements, it's not required that all the statements in a switch block be C<when>
