=head1 NAME
-perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $)
+perlfaq7 - Perl Language Issues ($Revision: 1.1 $, $Date: 2001/09/20 03:03:00 $)
=head1 DESCRIPTION
* for all types of that symbol name. In version 4 you used them like
pointers, but in modern perls you can just use references.
-A couple of others that you're likely to encounter that aren't
-really type specifiers are:
+There are couple of other symbols that you're likely to encounter that aren't
+really type specifiers:
<> are used for inputting a record from a filehandle.
\ takes a reference to something.
Note that <FILE> is I<neither> the type specifier for files
nor the name of the handle. It is the C<< <> >> operator applied
-to the handle FILE. It reads one line (well, record - see
+to the handle FILE. It reads one line (well, record--see
L<perlvar/$/>) from the handle FILE in scalar context, or I<all> lines
in list context. When performing open, close, or any other operation
-besides C<< <> >> on files, or even talking about the handle, do
+besides C<< <> >> on files, or even when talking about the handle, do
I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0,
2)> and "copying from STDIN to FILE".
=head2 What's an extension?
-A way of calling compiled C code from Perl. Reading L<perlxstut>
-is a good place to learn more about extensions.
+An extension is a way of calling compiled C code from Perl. Reading
+L<perlxstut> is a good place to learn more about extensions.
=head2 Why do Perl operators have different precedence than C operators?
Actually, they don't. All C operators that Perl copies have the same
precedence in Perl as they do in C. The problem is with operators that C
doesn't have, especially functions that give a list context to everything
-on their right, eg print, chmod, exec, and so on. Such functions are
+on their right, eg. print, chmod, exec, and so on. Such functions are
called "list operators" and appear as such in the precedence table in
L<perlop>.
# if using RCS/CVS, this next line may be preferred,
# but beware two-digit versions.
- $VERSION = do{my@r=q$Revision: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r};
+ $VERSION = do{my@r=q$Revision: 1.1 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r};
@ISA = qw(Exporter);
@EXPORT = qw(&func1 &func2 &func3);
}
our @EXPORT_OK;
+ # exported package globals go here
+ our $Var1;
+ our %Hashit;
+
# non-exported package globals go here
our @more;
our $stuff;
}
This is not C<-w> clean, however. There is no C<-w> clean way to
-detect taintedness - take this as a hint that you should untaint
+detect taintedness--take this as a hint that you should untaint
all possibly-tainted data.
=head2 What's a closure?
Closures make sense in any programming language where you can have the
return value of a function be itself a function, as you can in Perl.
Note that some languages provide anonymous functions but are not
-capable of providing proper closures; the Python language, for
+capable of providing proper closures: the Python language, for
example. For more information on closures, check out any textbook on
functional programming. Scheme is a language that not only supports
but encourages closures.
objects. See L<perlsub/"Pass by Reference"> for this particular
question, and L<perlref> for information on references.
+See ``Passing Regexes'', below, for information on passing regular
+expressions.
+
=over 4
=item Passing Variables and Functions
-Regular variables and functions are quite easy: just pass in a
+Regular variables and functions are quite easy to pass: just pass in a
reference to an existing or anonymous variable or function:
func( \$some_scalar );
=item Passing Filehandles
To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations.
-These are "typeglobs" - see L<perldata/"Typeglobs and Filehandles">
+These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles">
and especially L<perlsub/"Pass by Reference"> for more information.
Here's an excerpt:
If you're planning on generating new filehandles, you could do this:
sub openit {
- my $name = shift;
+ my $path = shift;
local *FH;
return open (FH, $path) ? *FH : undef;
}
}
}
-Or you can use a closure to bundle up the object and its method call
-and arguments:
+Or, you can use a closure to bundle up the object, its
+method call, and arguments:
my $whatnot = sub { $some_obj->obfuscate(@args) };
func($whatnot);
that was initialized at compile time.
To declare a file-private variable, you'll still use a my(), putting
-it at the outer scope level at the top of the file. Assume this is in
-file Pax.pm:
+the declaration at the outer scope level at the top of the file.
+Assume this is in file Pax.pm:
package Pax;
my $started = scalar(localtime(time()));
=head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
-C<local($x)> saves away the old value of the global variable C<$x>,
-and assigns a new value for the duration of the subroutine, I<which is
+C<local($x)> saves away the old value of the global variable C<$x>
+and assigns a new value for the duration of the subroutine I<which is
visible in other functions called from that subroutine>. This is done
at run-time, so is called dynamic scoping. local() always affects global
variables, also called package variables or dynamic variables.
C<my($x)> creates a new variable that is only visible in the current
-subroutine. This is done at compile-time, so is called lexical or
+subroutine. This is done at compile-time, so it is called lexical or
static scoping. my() always affects private variables, also called
lexical variables or (improperly) static(ly scoped) variables.
variables. It gives a global variable a temporary value. my() is
what you're looking for if you want private variables.
-See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary
-Values via local()"> for excruciating details.
+See L<perlsub/"Private Variables via my()"> and
+L<perlsub/"Temporary Values via local()"> for excruciating details.
=head2 How can I access a dynamic variable while a similarly named lexical is in scope?
If you want to override a predefined function, such as open(),
then you'll have to import the new definition from a different
-module. See L<perlsub/"Overriding Builtin Functions">. There's
+module. See L<perlsub/"Overriding Built-in Functions">. There's
also an example in L<perltoot/"Class::Template">.
If you want to overload a Perl operator, such as C<+> or C<**>,
=head2 What's the difference between calling a function as &foo and foo()?
When you call a function as C<&foo>, you allow that function access to
-your current @_ values, and you by-pass prototypes. That means that
-the function doesn't get an empty @_, it gets yours! While not
+your current @_ values, and you bypass prototypes.
+The function doesn't get an empty @_--it gets yours! While not
strictly speaking a bug (it's documented that way in L<perlsub>), it
would be hard to consider this a feature in most cases.
This is explained in more depth in the L<perlsyn>. Briefly, there's
no official case statement, because of the variety of tests possible
in Perl (numeric comparison, string comparison, glob comparison,
-regex matching, overloaded comparisons, ...). Larry couldn't decide
-how best to do this, so he left it out, even though it's been on the
-wish list since perl1.
+regex matching, overloaded comparisons, ...).
+Larry couldn't decide how best to do this, so he left it out, even
+though it's been on the wish list since perl1.
+
+Starting from Perl 5.8 to get switch and case one can use the
+Switch extension and say:
-The general answer is to write a construct like this:
+ use Switch;
+
+after which one has switch and case. It is not as fast as it could be
+because it's not really part of the language (it's done using source
+filters) but it is available, and it's very flexible.
+
+But if one wants to use pure Perl, the general answer is to write a
+construct like this:
for ($variable_to_test) {
if (/pat1/) { } # do something
For example, let's say you wanted to test which of many answers you were
given, but in a case-insensitive way that also allows abbreviations.
You can use the following technique if the strings all start with
-different characters, or if you want to arrange the matches so that
+different characters or if you want to arrange the matches so that
one takes precedence over another, as C<"SEND"> has precedence over
C<"STOP"> here:
Some possible reasons: your inheritance is getting confused, you've
misspelled the method name, or the object is of the wrong type. Check
-out L<perltoot> for details on these. You may also use C<print
-ref($object)> to find out the class C<$object> was blessed into.
+out L<perltoot> for details about any of the above cases. You may
+also use C<print ref($object)> to find out the class C<$object> was
+blessed into.
Another possible reason for problems is because you've used the
indirect object syntax (eg, C<find Guru "Samy">) on a class name
before Perl has seen that such a package exists. It's wisest to make
sure your packages are all defined before you start using them, which
will be taken care of if you use the C<use> statement instead of
-C<require>. If not, make sure to use arrow notation (eg,
+C<require>. If not, make sure to use arrow notation (eg.,
C<< Guru->find("Samy") >>) instead. Object notation is explained in
L<perlobj>.
Make sure to read about creating modules in L<perlmod> and
-the perils of indirect objects in L<perlobj/"WARNING">.
+the perils of indirect objects in L<perlobj/"Method Invocation">.
=head2 How can I find out my current package?
my $packname = __PACKAGE__;
-But if you're a method and you want to print an error message
+But, if you're a method and you want to print an error message
that includes the kind of object you were called on (which is
not necessarily the same as the one in which you were compiled):
This works I<sometimes>, but it is a very bad idea for two reasons.
-The first reason is that they I<only work on global variables>.
-That means above that if $fred is a lexical variable created with my(),
-that the code won't work at all: you'll accidentally access the global
-and skip right over the private lexical altogether. Global variables
-are bad because they can easily collide accidentally and in general make
-for non-scalable and confusing code.
+The first reason is that this technique I<only works on global
+variables>. That means that if $fred is a lexical variable created
+with my() in the above example, the code wouldn't work at all: you'd
+accidentally access the global and skip right over the private lexical
+altogether. Global variables are bad because they can easily collide
+accidentally and in general make for non-scalable and confusing code.
Symbolic references are forbidden under the C<use strict> pragma.
They are not true references and consequently are not reference counted
or garbage collected.
The other reason why using a variable to hold the name of another
-variable a bad idea is that the question often stems from a lack of
+variable is a bad idea is that the question often stems from a lack of
understanding of Perl data structures, particularly hashes. By using
symbolic references, you are just using the package's symbol-table hash
(like C<%main::>) instead of a user-defined hash. The solution is to
$str = 'this has a $fred and $barney in it';
$str =~ s/(\$\w+)/$1/eeg; # need double eval
-Instead, it would be better to keep a hash around like %USER_VARS and have
+it would be better to keep a hash around like %USER_VARS and have
variable references actually refer to entries in that hash:
$str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
$str = 'this has a %fred% and %barney% in it';
$str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
-Another reason that folks sometimes think they want a variable to contain
-the name of a variable is because they don't know how to build proper
-data structures using hashes. For example, let's say they wanted two
-hashes in their program: %fred and %barney, and to use another scalar
-variable to refer to those by name.
+Another reason that folks sometimes think they want a variable to
+contain the name of a variable is because they don't know how to build
+proper data structures using hashes. For example, let's say they
+wanted two hashes in their program: %fred and %barney, and that they
+wanted to use another scalar variable to refer to those by name.
$name = "fred";
$$name{WIFE} = "wilma"; # set %fred
So, sometimes you might want to use symbolic references to directly
manipulate the symbol table. This doesn't matter for formats, handles, and
-subroutines, because they are always global -- you can't use my() on them.
-But for scalars, arrays, and hashes -- and usually for subroutines --
-you probably want to use hard references only.
+subroutines, because they are always global--you can't use my() on them.
+For scalars, arrays, and hashes, though--and usually for subroutines--
+you probably only want to use hard references.
=head1 AUTHOR AND COPYRIGHT
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
All rights reserved.
-When included as part of the Standard Version of Perl, or as part of
-its complete documentation whether printed or otherwise, this work
-may be distributed only under the terms of Perl's Artistic License.
-Any distribution of this file or derivatives thereof I<outside>
-of that package require that special arrangements be made with
-copyright holder.
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit. A simple comment in the code giving
credit would be courteous but is not required.
+