$ 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, except for files, 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
$/) 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, seek and ``copying from STDIN to FILE''.
use strict). But a hash key consisting of a simple word (that isn't the name of a
defined subroutine) and the left-hand operand to the => operator both count as though they were quoted:
This is like this
------------ ---------------
$foo{line} $foo{"line"}
bar => stuff "bar" => stuff
The final semicolon in a block is optional, as is the final comma in a list. Good style (see perlsty) says to put them in except for one-liners:
if ($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);
$^W variable (documented in the perlvar manpage) controls runtime warnings for a block:
{
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.
$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 25
$person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try the perltoot manpage.
Here's a convenient template you might wish you use when starting your own module. Make sure to change the names appropriately.
package Some::Module; # assumes Some/Module.pm
use strict;
BEGIN {
use Exporter ();
use vars qw($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. $VERSION = (qw$Revision: 1.9 $)[1];
@ISA = qw(Exporter); @EXPORT = qw(&func1 &func2 &func3); %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
# your exported package globals go here,
# as well as any optionally exported functions
@EXPORT_OK = qw($Var1 %Hashit);
}
use vars @EXPORT_OK;
# non-exported package globals go here
use vars qw( @more $stuff );
# initalize 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.
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 is always 20 plus whatever $n you pass in, whereas
&$f2 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.
func( \$some_scalar );
func( \$some_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
*FH
or \*FH notation (``typeglobs'' - see the perldata manpage for more information), or create filehandles dynamically using the old
FileHandle or the new IO::File modules, both part of the standard Perl
distribution.
use Fcntl;
use IO::File;
my $fh = new IO::File $filename, O_WRONLY|O_APPEND;
or die "Can't append to $filename: $!";
func($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/"; # WRONG
or 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
particulary interesting.
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.
$x, and assigns a new value for the duration of the subroutine, 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.
my 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 global
Notice 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 also the perlsub manpage, which explains this all in more detail.
use strict "refs". So instead of $var, use ${'var'}.
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 explictly, 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 $main::var. Specifying the package
directly makes you hard-code its name, but it executes faster and avoids
running afoul of use strict "refs".
=. The
<FH> read operation, like so many of Perl's functions and operators, can tell
which context it was called in and behaves appropriately. In general, the
scalar function can
help. This function does nothing to the data itself (contrary to popular
myth) but rather tells its argument to behave in whatever its scalar
fashion is. If that function doesn't have a defined scalar behavior, this
of course doesn't help you (such as with sort).
To enforce scalar context in this particular case, however, you need merely omit the parentheses:
local($foo) = <FILE>; # WRONG
local($foo) = scalar(<FILE>); # ok
local $foo = <FILE>; # right
You should probably be using lexical variables anyway, although the issue is the same here:
my($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 Overriding Builtin Functions. There's also an example in Class/Template.
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 Overridden Methods.
&foo, you allow that function access to your current @_ values, plus you
by-pass prototypes. That means that the function doesn't get an empty @_,
it gets yours! While not strictly speaking a bug (hey, it's documented that
way in the perlsub manpage!), it would be hard to construe this a feature in most cases.
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.
Here's a simple example of a switch based on pattern matching. 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";
}
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] ) {
/Use of uninitialized value/ && do {
# promote warning to a fatal
die $_;
};
# other warning cases to catch could go here;
warn $_; }
};
print
ref to find out the class $object was blessed into).
Another possible reason for problems is because you've used the indirect
object syntax (eg, find Guru "Tom") on a class name before Perl has seen that such a package exists. It's
wisest to make sure your packages are all defined first, 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>) instead. Object notation is explained in the perlobj manpage.
my $packname = ref bless [];
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";
}