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.
+expression or defined-or operator, so don't do that.
=head2 Declarations
statements as if it were an ordinary statement. That means it actually
has both compile-time and run-time effects.
-=head2 Simple statements
+=head2 Simple Statements
The only kind of simple statement is an expression evaluated for its
side effects. Every simple statement must be terminated with a
} while $x++ <= $z;
}
-=head2 Compound statements
+B<NOTE:> The behaviour of a C<my> statement modified with a statement
+modifier conditional or loop construct (e.g. C<my $x if ...>) is
+B<undefined>. The value of the C<my> variable may be C<undef>, any
+previously assigned value, or possibly anything else. Don't rely on
+it. Future versions of perl might do something different from the
+version of perl you try it out on. Here be dragons.
+
+=head2 Compound Statements
In Perl, a sequence of statements that defines a scope is called a block.
Sometimes a block is delimited by the file containing it (in the case
looking back your call-stack at run time to find the LABEL. Such
desperate behavior triggers a warning if you use the C<use warnings>
pragma or the B<-w> flag.
-Unlike a C<foreach> statement, a C<while> statement never implicitly
-localises any variables.
If there is a C<continue> BLOCK, it is always executed just before the
-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).
+conditional is about to be evaluated again. Thus it can be used to
+increment a loop variable, even when the loop has been continued via
+the C<next> statement.
=head2 Loop Control
-The C<next> command is like the C<continue> statement in C; it starts
-the next iteration of the loop:
+The C<next> command starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
-The C<last> command is like the C<break> statement in C (as used in
-loops); it immediately exits the loop in question. The
+The C<last> command immediately exits the loop in question. The
C<continue> block, if any, is not executed:
LINE: while (<STDIN>) {
# do something
}
+Using C<readline> (or the operator form, C<< <EXPR> >>) as the
+conditional of a C<for> loop is shorthand for the following. This
+behaviour is the same as a C<while> loop conditional.
+
+ for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
+ # do something
+ }
+
=head2 Foreach Loops
The C<foreach> loop iterates over a normal list value and sets the
implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with C<my>, it uses
that variable instead of the global one, but it's still localized to
-the loop.
+the loop. This implicit localisation occurs I<only> in a C<foreach>
+loop.
The C<foreach> keyword is actually a synonym for the C<for> keyword, so
you can use C<foreach> for readability or C<for> for brevity. (Or because
}
(That's actually not as strange as it looks once you realize that you can
-use loop control "operators" within an expression, That's just the normal
-C comma operator.)
+use loop control "operators" within an expression. That's just the binary
+comma operator in scalar context. See L<perlop/"Comma Operator">.)
or
=head2 Plain Old Comments (Not!)
-Much like the C preprocessor, Perl can process line directives. Using
+Perl can process line directives, much like the C preprocessor. Using
this, one can control Perl's idea of filenames and line numbers in
error or warning messages (especially for strings that are processed
with C<eval()>). The syntax for this mechanism is the same as for most
C preprocessors: it matches the regular expression
-C</^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/> with C<$1> being the line
-number for the next line, and C<$2> being the optional filename
-(specified within quotes).
+
+ # example: '# line 42 "new_filename.plx"'
+ /^# \s*
+ line \s+ (\d+) \s*
+ (?:\s"([^"]+)")? \s*
+ $/x
+
+with C<$1> being the line number for the next line, and C<$2> being
+the optional filename (specified within quotes).
There is a fairly obvious gotcha included with the line directive:
Debuggers and profilers will only show the last source line to appear
projects / p5sagit/p5-mst-13.2.git / blobdiff