In the words of Chaim Frenkel: ``Perl's grammar can not be reduced to BNF. The work of parsing perl is distributed between yacc, the lexer, smoke and mirrors.''
$ for scalar values (number, string or reference) @ for arrays % for hashes (associative arrays) * for all types of that symbol name. In version 4 you used them like pointers, but in modern perls you can just use references.While there are a few places where you don't actually need these type specifiers, you should always use them.
A couple of others that you're likely to encounter that aren't really type specifiers are:
<> are used for inputting a record from a filehandle. \ takes a reference to something.Note that <FILE> is neither the type specifier for files nor the name of the handle. It is the <> operator applied to the handle FILE. It reads one line (well, record - see the section on $/ in the perlvar manpage) from the handle FILE in scalar context, or all lines in list context. When performing open, close, or any other operation besides <> on files, or even talking about the handle, do not use the brackets. These are correct: eof(FH), seek(FH, 0, 2) and ``copying from STDIN to FILE''.
This is like this ------------ --------------- $foo{line} $foo{"line"} bar => stuff "bar" => stuffThe final semicolon in a block is optional, as is the final comma in a list. Good style (see the perlstyle manpage) says to put them in except for one-liners:
if ($whoops) { exit 1 } @nums = (1, 2, 3);
if ($whoops) { exit 1; } @lines = ( "There Beren came from mountains cold", "And lost he wandered under leaves", );
$dir = (getpwnam($user))[7];Another way is to use undef as an element on the left-hand-side:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
{ local $^W = 0; # temporarily turn off warnings $a = $b + $c; # I know these might be undef }Note that like all the punctuation variables, you cannot currently use my() on $^W, only local().
A new use warnings pragma is in the works to provide finer control over all this. The curious should check the perl5-porters mailing list archives for details.
A common mistake is to write:
unlink $file || die "snafu";This gets interpreted as:
unlink ($file || die "snafu");To avoid this problem, either put in extra parentheses or use the super low precedence or operator:
(unlink $file) || die "snafu"; unlink $file or die "snafu";The ``English'' operators (and, or, xor, and not) deliberately have precedence lower than that of list operators for just such situations as the one above.
Another operator with surprising precedence is exponentiation. It binds more tightly even than unary minus, making -2**2 product a negative not a positive four. It is also right-associating, meaning that 2**3**2 is two raised to the ninth power, not eight squared.
Although it has the same precedence as in C, Perl's ?: operator produces an lvalue. This assigns $x to either $a or $b, depending on the trueness of $maybe:
($maybe ? $a : $b) = $x;
$person = {}; # new anonymous hash $person->{AGE} = 24; # set field AGE to 24 $person->{NAME} = "Nat"; # set field NAME to "Nat"If you're looking for something a bit more rigorous, try the perltoot manpage.
Here's a convenient template you might wish you use when starting your own module. Make sure to change the names appropriately.
package Some::Module; # assumes Some/Module.pm
use strict;
BEGIN { use Exporter (); use vars qw($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: 1.21 $=~/\d+/g;sprintf '%d.'.'%02d'x$#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); } use vars @EXPORT_OK;
# non-exported package globals go here use vars qw( @more $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
sub is_tainted { return ! eval { join('',@_), kill 0; 1; }; }This is not -w clean, however. There is no -w clean way to detect taintedness - take this as a hint that you should untaint all possibly-tainted data.
Closure is a computer science term with a precise but hard-to-explain meaning. Closures are implemented in Perl as anonymous subroutines with lasting references to lexical variables outside their own scopes. These lexicals magically refer to the variables that were around when the subroutine was defined (deep binding).
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 example. For more information on closures, check out any textbook on functional programming. Scheme is a language that not only supports but encourages closures.
Here's a classic function-generating function:
sub add_function_generator { return sub { shift + shift }; }
$add_sub = add_function_generator(); $sum = $add_sub->(4,5); # $sum is 9 now.The closure works as a function template with some customization slots left out to be filled later. The anonymous subroutine returned by add_function_generator() isn't technically a closure because it refers to no lexicals outside its own scope.
Contrast this with the following make_adder() function, in which the returned anonymous function contains a reference to a lexical variable outside the scope of that function itself. Such a reference requires that Perl return a proper closure, thus locking in for all time the value that the lexical had when the function was created.
sub make_adder { my $addpiece = shift; return sub { shift + $addpiece }; }
$f1 = make_adder(20); $f2 = make_adder(555);Now &$f1($n) is always 20 plus whatever $n you pass in, whereas &$f2($n) is always 555 plus whatever $n you pass in. The $addpiece in the closure sticks around.
Closures are often used for less esoteric purposes. For example, when you want to pass in a bit of code into a function:
my $line; timeout( 30, sub { $line = <STDIN> } );If the code to execute had been passed in as a string, '$line = <STDIN>', there would have been no way for the hypothetical timeout() function to access the lexical variable $line back in its caller's scope.
my $f = "foo"; sub T { while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } } T; print "Finally $f\n";The $f that has ``bar'' added to it three times should be a new $f (my $f should create a new local variable each time through the loop). It isn't, however. This is a bug, and will be fixed.
func( \$some_scalar );
func( \$some_array ); func( [ 1 .. 10 ] );
func( \%some_hash ); func( { this => 10, that => 20 } );
func( \&some_func ); func( sub { $_[0] ** $_[1] } );
Here's an excerpt:
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 use strict 'refs'. For example:
splutter(\*STDOUT); sub splutter { my $fh = shift; print $fh "her um well a hmmm\n"; }
$rec = get_rec(\*STDIN); sub get_rec { my $fh = shift; return scalar <$fh>; }If you're planning on generating new filehandles, you could do this:
sub openit { my $name = shift; local *FH; return open (FH, $path) ? *FH : undef; } $fh = openit('< /etc/motd'); print <$fh>;
sub compare($$) { my ($val1, $regexp) = @_; my $retval = eval { $val =~ /$regexp/ }; die if $@; return $retval; }
$match = compare("old McDonald", q/d.*D/);Make sure you never say something like this:
return eval "\$val =~ /$regexp/"; # WRONGor someone can sneak shell escapes into the regexp due to the double interpolation of the eval and the double-quoted string. For example:
$pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
eval "\$string =~ /$pattern_of_evil/";Those preferring to be very, very clever might see the O'Reilly book, Mastering Regular Expressions, by Jeffrey Friedl. Page 273's Build_MatchMany_Function() is particularly interesting. A complete citation of this book is given in the perlfaq2 manpage.
call_a_lot(10, $some_obj, "methname") sub call_a_lot { my ($count, $widget, $trick) = @_; for (my $i = 0; $i < $count; $i++) { $widget->$trick(); } }Or you can use a closure to bundle up the object and its method call and arguments:
my $whatnot = sub { $some_obj->obfuscate(@args) }; func($whatnot); sub func { my $code = shift; &$code(); }You could also investigate the can() method in the UNIVERSAL class (part of the standard perl distribution).
Here's code to implement a function-private variable:
BEGIN { my $counter = 42; sub prev_counter { return --$counter } sub next_counter { return $counter++ } }Now prev_counter() and next_counter() share a private variable $counter 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:
package Pax; my $started = scalar(localtime(time()));
sub begun { return $started }When use Pax or require Pax loads this module, the variable will be initialized. It won't get garbage-collected the way most variables going out of scope do, because the begun() function cares about it, but no one else can get it. It is not called $Pax::started because its scope is unrelated to the package. It's scoped to the file. You could conceivably have several packages in that same file all accessing the same private variable, but another file with the same package couldn't get to it.
See the section on Peristent Private Variables in the perlsub manpage for details.
my($x) creates a new variable that is only visible in the current subroutine. This is done at compile-time, so is called lexical or static scoping. my() always affects private variables, also called lexical variables or (improperly) static(ly scoped) variables.
For instance:
sub visible { print "var has value $var\n"; }
sub dynamic { local $var = 'local'; # new temporary value for the still-global visible(); # variable called $var }
sub lexical { my $var = 'private'; # new private variable, $var visible(); # (invisible outside of sub scope) }
$var = 'global';
visible(); # prints global dynamic(); # prints local lexical(); # prints globalNotice how at no point does the value ``private'' get printed. That's because $var only has that value within the block of the lexical() function, and it is hidden from called subroutine.
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 the section on Private Variables via my() in the perlsub manpage and the section on Temporary Values via local() in the perlsub manpage for excruciating details.
local $var = "global"; my $var = "lexical";
print "lexical is $var\n";
no strict 'refs'; print "global is ${'var'}\n";If you know your package, you can just mention it explicitly, as in $Some_Pack::var. Note that the notation $::var is not the dynamic $var in the current package, but rather the one in the main package, as though you had written $main::var. Specifying the package directly makes you hard-code its name, but it executes faster and avoids running afoul of use strict "refs".
To enforce scalar context in this particular case, however, you need merely omit the parentheses:
local($foo) = <FILE>; # WRONG local($foo) = scalar(<FILE>); # ok local $foo = <FILE>; # rightYou should probably be using lexical variables anyway, although the issue is the same here:
my($foo) = <FILE>; # WRONG my $foo = <FILE>; # right
If you want to override a predefined function, such as open(), then you'll have to import the new definition from a different module. See the section on Overriding Builtin Functions in the perlsub manpage. There's also an example in the section on Class::Template in the perltoot manpage.
If you want to overload a Perl operator, such as + or **, then you'll want to use the use overload pragma, documented in the overload manpage.
If you're talking about obscuring method calls in parent classes, see the section on Overridden Methods in the perltoot manpage.
When you call your function as &foo(), then you do get a new @_, but prototyping is still circumvented.
Normally, you want to call a function using foo(). You may only omit the parentheses if the function is already known to the compiler because it already saw the definition (use but not require), or via a forward reference or use subs declaration. Even in this case, you get a clean @_ without any of the old values leaking through where they don't belong.
The general answer is to write a construct like this:
for ($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 in $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do { print_scalar($$ref); last SWITCH; };
/ARRAY/ && do { print_array(@$ref); last SWITCH; };
/HASH/ && do { print_hash(%$ref); last SWITCH; };
/CODE/ && do { warn "can't print function ref"; last SWITCH; };
# DEFAULT
warn "User defined type skipped";
}See perlsyn/"Basic BLOCKs and Switch Statements" for many other examples in this style.
Sometimes you should change the positions of the constant and the variable. 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 one takes precedence over another, as "SEND" has precedence over "STOP" here:
chomp($answer = <>); if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } 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.
my %commands = ( "happy" => \&joy, "sad", => \&sullen, "done" => sub { die "See ya!" }, "mad" => \&angry, );
print "How are you? "; chomp($string = <STDIN>); if ($commands{$string}) { $commands{$string}->(); } else { print "No such command: $string\n"; }
When it comes to undefined variables that would trigger a warning under -w, you can use a handler to trap the pseudo-signal __WARN__ like this:
$SIG{__WARN__} = sub {
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 $_; }
};
Another possible reason for problems is because you've used the indirect object syntax (eg, 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 use statement instead of require. If not, make sure to use arrow notation (eg, Guru->find("Samy")) instead. Object notation is explained in the perlobj manpage.
Make sure to read about creating modules in the perlmod manpage and the perils of indirect objects in the section on WARNING in the perlobj manpage.
my $packname = __PACKAGE__;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):
sub amethod { my $self = shift; my $class = ref($self) || $self; warn "called me from a $class object"; }
# program is here
=for nobody This paragraph is commented out
# program continues
=begin comment text
all of this stuff
here will be ignored by everyone
=end comment text
=cutThis 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.
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 outside of that package require that special arrangements be made with copyright holder.
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.