Author: larry
Date: Wed Mar 28 12:46:50 2007
New Revision: 14356
Modified:
doc/trunk/design/syn/S04.pod
doc/trunk/design/syn/S06.pod
Log:
Simplified -> binding not to autoalias $_ to first arg, per TheDamian++
Also clarified why "when" must always break from immediately surrounding block.
Juerd++'s double-pointy block now allowed as convenient way to make all
parameters default to "rw".
Implicit $_ arg now defined in terms of <->
Modified: doc/trunk/design/syn/S04.pod
==============================================================================
--- doc/trunk/design/syn/S04.pod (original)
+++ doc/trunk/design/syn/S04.pod Wed Mar 28 12:46:50 2007
@@ -12,9 +12,9 @@
Maintainer: Larry Wall <[EMAIL PROTECTED]>
Date: 19 Aug 2004
- Last Modified: 12 Mar 2007
+ Last Modified: 28 Mar 2007
Number: 4
- Version: 54
+ Version: 55
This document summarizes Apocalypse 4, which covers the block and
statement syntax of Perl.
@@ -390,8 +390,23 @@
for =$*ARGS {...}
-Parameters are by default readonly within the block. You can
-declare a parameter read/write by including the "C<is rw>" trait.
+Arguments bound to the formal parameters of a pointy block are by
+default readonly within the block. You can declare a parameter
+read/write by including the "C<is rw>" trait. The following treats
+every other value in C<@values> as modifiable:
+
+ for @values -> $even is rw, $odd { ... }
+
+In the case where you want all your parameters to default to C<rw>,
+you may use the visually suggestive double-ended arrow to indicate that
+values flow both ways:
+
+ for @values <-> $even, $odd { ... }
+
+This is equivalent to
+
+ for @values -> $even is rw, $odd is rw { ... }
+
If you rely on C<$_> as the implicit parameter to a block,
then C<$_> is considered read/write by default. That is,
the construct:
@@ -400,10 +415,9 @@
is actually short for:
- for @foo -> $_ is rw {...}
+ for @foo <-> $_ {...}
-so you can modify the current list element in that case. However,
-any time you specify the arguments, they default to read only.
+so you can modify the current list element in that case.
When used as statement modifiers, C<for> and C<given> use a private
instance of C<$_> for the left side of the statement. The outer C<$_>
@@ -551,16 +565,20 @@
}
The current topic is always aliased to the special variable C<$_>.
-The C<given> block is just one way to set the current topic, but a
-switch statement can be any block that sets C<$_>, including a C<for>
-loop (in which the first loop parameter is the topic) or the body
-of a method (if you have declared the invocant as C<$_>). So switching
-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<$_>) and the argument
-of the C<when>. If the smart match succeeds, C<when>'s associated block
-is executed, and the surrounding block is automatically broken out of.
-The value of the inner block is returned as the value of the outer block.
+The C<given> block is just one way to set the current topic, but
+a switch statement can be any block that sets C<$_>, including a
+C<for> loop (assuming one of its loop variables is bound to C<$_>)
+or the body of a method (if you have declared the invocant as C<$_>).
+So switching 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<$_>) and
+the argument of the C<when>. If the smart match succeeds, C<when>'s
+associated block is executed, and the immediatly surrounding block
+is automatically broken out of. (If that is not the block you wish
+to leave, you must use the C<LABEL.leave> method to be more specific,
+since the compiler may find it difficult to guess which surrounding
+construct last set C<$_> as the topic.) 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>
@@ -583,14 +601,12 @@
fall off the last C<when> into ordinary code. But use of a C<default>
block is good documentation.
-If you use a C<for> loop with a named parameter, the parameter is
-also aliased to C<$_> so that it can function as the topic of any
-C<when> statements within the loop. If you use a C<for> statement
-with multiple parameters, only the first parameter is aliased to C<$_>
-as the topic.
+If you use a C<for> loop with a parameter named C<$_> (either
+explicitly or implicitly), that parameter can function as the topic
+of any C<when> statements within the loop.
You can explicitly break out of a C<when> block (and its surrounding
-switch) early using the C<break> verb. You can explicitly break out
+block) early using the C<break> verb. You can explicitly break out
of a C<when> block and go to the next statement by using C<continue>.
(Note that, unlike C's idea of falling through, subsequent C<when>
conditions are evaluated. To jump into the next C<when> block you
Modified: doc/trunk/design/syn/S06.pod
==============================================================================
--- doc/trunk/design/syn/S06.pod (original)
+++ doc/trunk/design/syn/S06.pod Wed Mar 28 12:46:50 2007
@@ -13,9 +13,9 @@
Maintainer: Larry Wall <[EMAIL PROTECTED]>
Date: 21 Mar 2003
- Last Modified: 15 Mar 2007
+ Last Modified: 28 Mar 2007
Number: 6
- Version: 81
+ Version: 82
This document summarizes Apocalypse 6, which covers subroutines and the
@@ -166,6 +166,14 @@
enclosing C<sub> or C<method>, not the block itself. It is referenced
by C<&?BLOCK>, not C<&?ROUTINE>.
+A normal pointy block's parameters default to C<readonly>, just like
+parameters to a normal sub declaration. However, the double-pointy variant
+defaults parameters to C<rw>:
+
+ for @list <-> $elem {
+ $elem++;
+ }
+
=head2 Stub declarations
To predeclare a subroutine without actually defining it, use a "stub block":
