Author: lwall
Date: 2009-11-24 21:31:37 +0100 (Tue, 24 Nov 2009)
New Revision: 29187
Modified:
docs/Perl6/Spec/S04-control.pod
Log:
[S04] clarifications suggested by TheDamian++
Modified: docs/Perl6/Spec/S04-control.pod
===================================================================
--- docs/Perl6/Spec/S04-control.pod 2009-11-24 17:50:52 UTC (rev 29186)
+++ docs/Perl6/Spec/S04-control.pod 2009-11-24 20:31:37 UTC (rev 29187)
@@ -13,8 +13,8 @@
Created: 19 Aug 2004
- Last Modified: 19 Nov 2009
- Version: 89
+ Last Modified: 24 Nov 2009
+ Version: 90
This document summarizes Apocalypse 4, which covers the block and
statement syntax of Perl.
@@ -196,11 +196,21 @@
=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
+In the absence of explicit control flow terminating the block early,
+the return value of a block is the value of its final statement.
+This is defined as the textually last statement of its top-level
+list of statements; any statements embedded within those top-level
+statements are in their own lower-level list of statements and,
+while they may be a final statement in their subscope, they're not
+considered the final statement of the outer block in question.
+
+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.) If there are no statements in the block,
-the result is C<Nil>.
+was just a conditional. Unlike in Perl 5, if a final statement in
+Perl 6 is a conditional that does not execute any of its branches, it
+doesn't matter what the value of the condional is, the value of that
+conditional statement is always C<Nil>. If there are no statements
+in the block at all, the result is also C<Nil>.
=head1 Statement-ending blocks
@@ -919,6 +929,10 @@
doit() if $_ ~~ 42;
+This is particularly useful for list comprehensions:
+
+ @lucky = ($_ when /7/ for 1..100);
+
=head1 Exception handlers
X<CATCH>
@@ -1243,9 +1257,13 @@
sets the variable to 0 at C<END> time, since that is when the "my"
declaration is actually executed. Only argumentless phasers may
-use the statement form. This means that C<CHECK> and C<CONTROL>
+use the statement form. This means that C<CATCH> and C<CONTROL>
always require a block, since they take an argument that sets C<$_>
-to the current topic.
+to the current topic, so that the innards are able to behave
+as a switch statement. (If bare statements were allowed, the
+temporary binding of C<$_> would leak out past the end of the C<CATCH>
+or C<CONTROL>, with unpredictable and quite possibly dire consequences.
+Exception handlers are supposed to reduce uncertainty, not increase it.)
Code that is generated at run time can still fire off C<CHECK>
and C<INIT> phasers, though of course those phasers can't do things that
