X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq7.pod;h=a735edc7c880dbfb1e36234675090ae53c9f6fe5;hb=d44161bfbb2e964e9675634d6bf5e566d1d1d4f7;hp=0299c2d8934a92a29de5ed21e4e2f9ac49c1acac;hpb=0e06870bf080a38cda51c06c6612359afc2334e1;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq7.pod b/pod/perlfaq7.pod index 0299c2d..a735edc 100644 --- a/pod/perlfaq7.pod +++ b/pod/perlfaq7.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $) +perlfaq7 - General Perl Language Issues ($Revision: 1.15 $, $Date: 2003/07/24 02:17:21 $) =head1 DESCRIPTION @@ -38,7 +38,7 @@ really type specifiers: Note that is I the type specifier for files nor the name of the handle. It is the C<< <> >> operator applied to the handle FILE. It reads one line (well, record--see -L) from the handle FILE in scalar context, or I lines +L>) from the handle FILE in scalar context, or I lines in list context. When performing open, close, or any other operation besides C<< <> >> on files, or even when talking about the handle, do I use the brackets. These are correct: C, C pragma @@ -167,81 +172,15 @@ details, read L. You'll also find L helpful. If you're writing a C or mixed-language module with both C and Perl, then you should study L. -Here's a convenient template you might wish you use when starting your -own module. Make sure to change the names appropriately. - - package Some::Module; # assumes Some/Module.pm - - use strict; - use warnings; - - BEGIN { - use Exporter (); - our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); - - ## set the version for version checking; uncomment to use - ## $VERSION = 1.00; - - # 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}; - - @ISA = qw(Exporter); - @EXPORT = qw(&func1 &func2 &func3); - %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], - - # your exported package globals go here, - # as well as any optionally exported functions - @EXPORT_OK = qw($Var1 %Hashit); - } - our @EXPORT_OK; - - # exported package globals go here - our $Var1; - our %Hashit; - - # non-exported package globals go here - our @more; - our $stuff; - - # initialize package globals, first exported ones - $Var1 = ''; - %Hashit = (); - - # then the others (which are still accessible as $Some::Module::stuff) - $stuff = ''; - @more = (); - - # all file-scoped lexicals must be created before - # the functions below that use them. - - # file-private lexicals go here - my $priv_var = ''; - my %secret_hash = (); - - # here's a file-private function as a closure, - # callable as &$priv_func; it cannot be prototyped. - my $priv_func = sub { - # stuff goes here. - }; - - # make all your functions, whether exported or not; - # remember to put something interesting in the {} stubs - sub func1 {} # no prototype - sub func2() {} # proto'd void - sub func3($$) {} # proto'd to 2 scalars - - # this one isn't exported, but could be called! - sub func4(\%) {} # proto'd to 1 hash ref - - END { } # module clean-up code here (global destructor) - - 1; # modules must return true - -The h2xs program will create stubs for all the important stuff for you: +The C program will create stubs for all the important stuff for you: % h2xs -XA -n My::Module +The C<-X> switch tells C that you are not using C extension +code. The C<-A> switch tells C that you are not using the +AutoLoader, and the C<-n> switch specifies the name of the module. +See L for more details. + =head2 How do I create a class? See L for an introduction to classes and objects, as well as @@ -249,17 +188,9 @@ L and L. =head2 How can I tell if a variable is tainted? -See L. Here's an -example (which doesn't use any system calls, because the kill() -is given no processes to signal): - - sub is_tainted { - return ! eval { join('',@_), kill 0; 1; }; - } - -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 -all possibly-tainted data. +You can use the tainted() function of the Scalar::Util module, available +from CPAN (or included with Perl since release 5.8.0). +See also L. =head2 What's a closure? @@ -372,37 +303,21 @@ reference to an existing or anonymous variable or function: =item Passing Filehandles -To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. -These are "typeglobs"--see L -and especially L for more information. - -Here's an excerpt: +As of Perl 5.6, you can represent filehandles with scalar variables +which you treat as any other scalar. -If you're passing around filehandles, you could usually just use the bare -typeglob, like *STDOUT, but typeglobs references would be better because -they'll still work properly under C. For example: + open my $fh, $filename or die "Cannot open $filename! $!"; + func( $fh ); - splutter(\*STDOUT); - sub splutter { - my $fh = shift; - print $fh "her um well a hmmm\n"; - } + sub func { + my $passed_fh = shift; - $rec = get_rec(\*STDIN); - sub get_rec { - my $fh = shift; - return scalar <$fh>; - } + my $line = <$fh>; + } -If you're planning on generating new filehandles, you could do this: - - sub openit { - my $path = shift; - local *FH; - return open (FH, $path) ? *FH : undef; - } - $fh = openit('< /etc/motd'); - print <$fh>; +Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations. +These are "typeglobs"--see L +and especially L for more information. =item Passing Regexes @@ -560,28 +475,38 @@ In summary, local() doesn't make what you think of as private, local variables. It gives a global variable a temporary value. my() is what you're looking for if you want private variables. -See L and +See L and L for excruciating details. =head2 How can I access a dynamic variable while a similarly named lexical is in scope? -You can do this via symbolic references, provided you haven't set -C. So instead of $var, use C<${'var'}>. +If you know your package, you can just mention it explicitly, as in +$Some_Pack::var. Note that the notation $::var is B the dynamic $var +in the current package, but rather the one in the "main" package, as +though you had written $main::var. - local $var = "global"; - my $var = "lexical"; + use vars '$var'; + local $var = "global"; + my $var = "lexical"; - print "lexical is $var\n"; + print "lexical is $var\n"; + print "global is $main::var\n"; - no strict 'refs'; - print "global is ${'var'}\n"; +Alternatively you can use the compiler directive our() to bring a +dynamic variable into the current lexical scope. -If you know your package, you can just mention it explicitly, as in -$Some_Pack::var. Note that the notation $::var is I the dynamic -$var in the current package, but rather the one in the C
-package, as though you had written $main::var. Specifying the package -directly makes you hard-code its name, but it executes faster and -avoids running afoul of C. + require 5.006; # our() did not exist before 5.6 + use vars '$var'; + + local $var = "global"; + my $var = "lexical"; + + print "lexical is $var\n"; + + { + our $var; + print "global is $var\n"; + } =head2 What's the difference between deep and shallow binding? @@ -594,7 +519,7 @@ However, dynamic variables (aka global, local, or package variables) are effectively shallowly bound. Consider this just one more reason not to use them. See the answer to L<"What's a closure?">. -=head2 Why doesn't "my($foo) = ;" work right? +=head2 Why doesn't "my($foo) = EFILEE;" work right? C and C give list context to the right hand side of C<=>. The read operation, like so many of Perl's @@ -657,22 +582,32 @@ where they don't belong. This is explained in more depth in the L. 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: + + use Switch; -The general answer is to write a construct like this: +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 elsif (/pat2/) { } # do something else elsif (/pat3/) { } # do something else else { } # default - } + } Here's a simple example of a switch based on pattern matching, this time lined up in a way to make it look more like a switch statement. -We'll do a multi-way conditional based on the type of reference stored +We'll do a multiway conditional based on the type of reference stored in $whatchamacallit: SWITCH: for (ref $whatchamacallit) { @@ -705,7 +640,7 @@ in $whatchamacallit: } -See C for many other +See C for many other examples in this style. Sometimes you should change the positions of the constant and the variable. @@ -723,7 +658,7 @@ C<"STOP"> here: elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } -A totally different approach is to create a hash of function references. +A totally different approach is to create a hash of function references. my %commands = ( "happy" => \&joy, @@ -738,33 +673,18 @@ A totally different approach is to create a hash of function references. $commands{$string}->(); } else { print "No such command: $string\n"; - } + } -=head2 How can I catch accesses to undefined variables/functions/methods? +=head2 How can I catch accesses to undefined variables, functions, or methods? The AUTOLOAD method, discussed in L and L, lets you capture calls to undefined functions and methods. When it comes to undefined variables that would trigger a warning -under C<-w>, you can use a handler to trap the pseudo-signal -C<__WARN__> like this: - - $SIG{__WARN__} = sub { +under C, you can promote the warning to an error. - for ( $_[0] ) { # voici un switch statement - - /Use of uninitialized value/ && do { - # promote warning to a fatal - die $_; - }; - - # other warning cases to catch could go here; - - warn $_; - } - - }; + use warnings FATAL => qw(uninitialized); =head2 Why can't a method included in this same file be found? @@ -784,7 +704,7 @@ C<< Guru->find("Samy") >>) instead. Object notation is explained in L. Make sure to read about creating modules in L and -the perils of indirect objects in L. +the perils of indirect objects in L. =head2 How can I find out my current package? @@ -805,7 +725,8 @@ not necessarily the same as the one in which you were compiled): =head2 How can I comment out a large block of perl code? -Use embedded POD to discard it: +You can use embedded POD to discard it. The =for directive +lasts until the next paragraph (two consecutive newlines). # program is here @@ -814,6 +735,9 @@ Use embedded POD to discard it: # program continues +The =begin and =end directives can contain multiple +paragraphs. + =begin comment text all of this stuff @@ -823,11 +747,12 @@ Use embedded POD to discard it: =end comment text - =cut +The pod directives cannot go just anywhere. You must put a +pod directive where the parser is expecting a new statement, +not just in the middle of an expression or some other +arbitrary s grammar production. -This can't go just anywhere. You have to put a pod directive where -the parser is expecting a new statement, not just in the middle -of an expression or some other arbitrary yacc grammar production. +See L for more details. =head2 How do I clear a package? @@ -836,7 +761,7 @@ Use this code, provided by Mark-Jason Dominus: sub scrub_package { no strict 'refs'; my $pack = shift; - die "Shouldn't delete main package" + die "Shouldn't delete main package" if $pack eq "" || $pack eq "main"; my $stash = *{$pack . '::'}{HASH}; my $name; @@ -851,7 +776,7 @@ Use this code, provided by Mark-Jason Dominus: } } -Or, if you're using a recent release of Perl, you can +Or, if you're using a recent release of Perl, you can just use the Symbol::delete_package() function instead. =head2 How can I use a variable as a variable name? @@ -883,7 +808,7 @@ 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 use your own hash or a real reference instead. - $fred = 23; + $USER_VARS{"fred"} = 23; $varname = "fred"; $USER_VARS{$varname}++; # not $$varname++ @@ -919,7 +844,7 @@ wanted to use another scalar variable to refer to those by name. $name = "fred"; $$name{WIFE} = "wilma"; # set %fred - $name = "barney"; + $name = "barney"; $$name{WIFE} = "betty"; # set %barney This is still a symbolic reference, and is still saddled with the @@ -943,7 +868,7 @@ can play around with the symbol table. For example: for my $name (@colors) { no strict 'refs'; # renege for the block *$name = sub { "@_" }; - } + } All those functions (red(), blue(), green(), etc.) appear to be separate, but the real code in the closure actually was compiled only once. @@ -954,17 +879,38 @@ 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. +=head2 What does "bad interpreter" mean? + +The "bad interpreter" message comes from the shell, not perl. The +actual message may vary depending on your platform, shell, and locale +settings. + +If you see "bad interpreter - no such file or directory", the first +line in your perl script (the "shebang" line) does not contain the +right path to perl (or any other program capable of running scripts). +Sometimes this happens when you move the script from one machine to +another and each machine has a different path to perl---/usr/bin/perl +versus /usr/local/bin/perl for instance. + +If you see "bad interpreter: Permission denied", you need to make your +script executable. + +In either case, you should still be able to run the scripts with perl +explicitly: + + % perl script.pl + +If you get a message like "perl: command not found", perl is not in +your PATH, which might also mean that the location of perl is not +where you expect it so you need to adjust your shebang line. + =head1 AUTHOR AND COPYRIGHT -Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. +Copyright (c) 1997-2002 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 -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