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[PATCH] perlsyn.pod Revamp (take 2)
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From:
Shlomi Fish
Date:
May 10, 2003 02:42
Subject:
[PATCH] perlsyn.pod Revamp (take 2)
Message ID:
Pine.LNX.4.33L2.0305101241070.32193-200000@vipe.technion.ac.il
--- bleadperl/pod/perlsyn.pod 2003-04-27 03:07:07.000000000 +0300
+++ modified/pod/perlsyn.pod 2003-05-10 09:43:20.000000000 +0300
@@ -5,28 +5,26 @@
=head1 DESCRIPTION
A Perl script consists of a sequence of declarations and statements.
-The sequence of statements is executed just once, unlike in B<sed>
-and B<awk> scripts, where the sequence of statements is executed
-for each input line. While this means that you must explicitly
-loop over the lines of your input file (or files), it also means
-you have much more control over which files and which lines you look at.
-(Actually, I'm lying--it is possible to do an implicit loop with
-either the B<-n> or B<-p> switch. It's just not the mandatory
-default like it is in B<sed> and B<awk>.)
+The statements are executed one by one. This is unlike B<awk> or
+B<sed> (in case you are familiar with these tools) where the sequence
+of statements is executed for each input line. While it is possible
+to emulate this behavior, using the B<-n> or B<-p> switches, the
+default is to follow the standard of traditional imperative
+languages.
Perl is, for the most part, a free-form language. (The only exception
to this is format declarations, for obvious reasons.) Text from a
C<"#"> character until the end of the line is a comment, and is
-ignored. If you attempt to use C</* */> C-style comments, it will be
-interpreted either as division or pattern matching, depending on the
-context, and C++ C<//> comments just look like a null regular
-expression, so don't do that.
+ignored. C-style C</* */> or C++ C<//> comments will do something else
+entirely, so don't use them (and I mean it).
=head2 Declarations
The only things you need to declare in Perl are report formats
-and subroutines--and even undefined subroutines can be handled
-through AUTOLOAD. A variable holds the undefined value (C<undef>)
+and subroutines. (Undefined subroutines can be handled
+through the AUTOLOAD feature as documented in L<perlsub>)
+
+A variable holds the undefined value (C<undef>)
until it has been assigned a defined value, which is anything
other than C<undef>. When used as a number, C<undef> is treated
as C<0>; when used as a string, it is treated the empty string,
@@ -100,6 +98,23 @@
until EXPR
foreach EXPR
+C<if> executes a block of statements (or a single statement) once
+if C<EXPR> evaluated to a true value. C<unless> executes it once if
+it did not. A false value is C<undef>, 0, the empty string, and the
+floating number 0.0. A true value is everything else. Normally,
+you'll wish to use comparison operators (C<==>, C<eq>, C<<<=>>, C<gt>
+or friends), regular expression matches or other mechanisms that
+explicitly return a true or false value as a boolean expression
+there.
+
+C<while> and C<until> repeat the same block of statements while or
+until it is true. (I.e.: C<until> iterates while it is false)
+The C<while> and C<until> modifiers have the usual "C<while> loop"
+semantics (conditional evaluated first), except when applied to a
+C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
+which case the block executes once before the
+conditional is evaluated. This is so that you can write loops like:
+
The C<if> and C<unless> modifiers have the expected semantics,
presuming you're a speaker of English. The C<foreach> modifier is an
iterator: For each value in EXPR, it aliases C<$_> to the value and
@@ -141,6 +156,10 @@
it. Future versions of perl might do something different from the
version of perl you try it out on. Here be dragons.
+Finally, the C<foreach> modifier is an iterator: For each value
+in EXPR, it aliases the special "default" variable C<$_> to the
+value and executes the statement.
+
=head2 Compound statements
In Perl, a sequence of statements that defines a scope is called a block.
@@ -195,7 +214,10 @@
conditional is about to be evaluated again, just like the third part of a
C<for> loop in C. Thus it can be used to increment a loop variable, even
when the loop has been continued via the C<next> statement (which is
-similar to the C C<continue> statement).
+similar to the C C<continue> statement). Note that C<continue> in C
+and C<continue> in Perl do entirely different things. Fortunately, if
+one is used in place of the other by mistake it is likely to be a syntax
+error.
=head2 Loop Control
@@ -558,7 +580,8 @@
The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a C<foreach> loop. It
-also can't be used to go into a construct that is optimized away. It
+also can't be used to go into a construct that is optimized away at
+the compilation stage, like an C<if (0) { ... }> block. It
can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as C<last> or C<die>. The author of Perl has never felt the
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