PERLFAQ7(1)
NNAAMMEE
perlfaq7 - Perl Language Issues ($Revision: 1.18 $, $Date:
1997/04/24 22:44:14 $)
DDEESSCCRRIIPPTTIIOONN
This section deals with general Perl language issues that
don't clearly fit into any of the other sections.
CCaann II ggeett aa BBNNFF//yyaacccc//RREE ffoorr tthhee PPeerrll llaanngguuaaggee??
No, 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."
WWhhaatt aarree aallll tthheessee $$@@%%** ppuunnccttuuaattiioonn ssiiggnnss,, aanndd hhooww ddoo II
kknnooww wwhheenn ttoo uussee tthheemm??
They are type specifiers, as detailed in the perldata
manpage:
$ 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".
DDoo II aallwwaayyss//nneevveerr hhaavvee ttoo qquuoottee mmyy ssttrriinnggss oorr uussee
sseemmiiccoolloonnss aanndd ccoommmmaass??
Normally, a bareword doesn't need to be quoted, but in
most cases probably should be (and must be under 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 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",
);
HHooww ddoo II sskkiipp ssoommee rreettuurrnn vvaalluueess??
One way is to treat the return values as a list and index
into it:
$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);
HHooww ddoo II tteemmppoorraarriillyy bblloocckk wwaarrnniinnggss??
The $^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.
WWhhaatt''ss aann eexxtteennssiioonn??
A way of calling compiled C code from Perl. Reading the
perlxstut manpage is a good place to learn more about
extensions.
WWhhyy ddoo PPeerrll ooppeerraattoorrss hhaavvee ddiiffffeerreenntt pprreecceeddeennccee tthhaann CC
ooppeerraattoorrss??
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 called "list operators" and appear as such in the
precedence table in the perlop manpage.
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.
HHooww ddoo II ddeeccllaarree//ccrreeaattee aa ssttrruuccttuurree??
In general, you don't "declare" a structure. Just use a
(probably anonymous) hash reference. See the perlref
manpage and the perldsc manpage for details. Here's an
example:
$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.
HHooww ddoo II ccrreeaattee aa mmoodduullee??
A module is a package that lives in a file of the same
name. For example, the Hello::There module would live in
Hello/There.pm. For details, read the perlmod manpage.
You'll also find the Exporter manpage helpful. If you're
writing a C or mixed-language module with both C and Perl,
then you should study the perlxstut 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: 1.18 $=~/\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
HHooww ddoo II ccrreeaattee aa ccllaassss??
See the perltoot manpage for an introduction to classes
and objects, as well as the perlobj manpage and the
perlbot manpage.
HHooww ccaann II tteellll iiff aa vvaarriiaabbllee iiss ttaaiinntteedd??
See the section on Laundering and Detecting Tainted Data
in the perlsec manpage. 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 -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.
WWhhaatt''ss aa cclloossuurree??
Closures are documented in the perlref manpage.
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.
WWhhaatt iiss vvaarriiaabbllee ssuuiicciiddee aanndd hhooww ccaann II pprreevveenntt iitt??
Variable suicide is when you (temporarily or permanently)
lose the value of a variable. It is caused by scoping
through my() and local() interacting with either closures
or aliased foreach() interator variables and subroutine
arguments. It used to be easy to inadvertently lose a
variable's value this way, but now it's much harder. Take
this code:
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.
HHooww ccaann II ppaassss//rreettuurrnn aa {{FFuunnccttiioonn,, FFiilleeHHaannddllee,, AArrrraayy,,
HHaasshh,, MMeetthhoodd,, RReeggeexxpp}}??
With the exception of regexps, you need to pass references
to these objects. See the section on Pass by Reference in
the perlsub manpage for this particular question, and the
perlref manpage for information on references.
Passing Variables and Functions
Regular variables and functions are quite easy: just
pass in a reference to an existing or anonymous
variable or function:
func( \$some_scalar );
func( \$some_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
Passing Filehandles
To create filehandles you can pass to subroutines, you
can use *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);
Passing Regexps
To pass regexps around, you'll need to either use one
of the highly experimental regular expression modules
from CPAN (Nick Ing-Simmons's Regexp or Ilya
Zakharevich's Devel::Regexp), pass around strings and
use an exception-trapping eval, or else be be very,
very clever. Here's an example of how to pass in a
string to be regexp compared:
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 particularly interesting. A complete citation of
this book is given in the perlfaq2 manpage.
Passing Methods
To pass an object method into a subroutine, you can do
this:
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).
HHooww ddoo II ccrreeaattee aa ssttaattiicc vvaarriiaabbllee??
As with most things in Perl, TMTOWTDI. What is a "static
variable" in other languages could be either a function-
private variable (visible only within a single function,
retaining its value between calls to that function), or a
file-private variable (visible only to functions within
the file it was declared in) in Perl.
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.
WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn ddyynnaammiicc aanndd lleexxiiccaall ((ssttaattiicc))
ssccooppiinngg?? BBeettwweeeenn local() and my()?
local($x) saves away the old value of the global variable
$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($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 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.
HHooww ccaann II aacccceessss aa ddyynnaammiicc vvaarriiaabbllee wwhhiillee aa ssiimmiillaarrllyy
nnaammeedd lleexxiiccaall iiss iinn ssccooppee??
You can do this via symbolic references, provided you
haven't set 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
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".
WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn ddeeeepp aanndd sshhaallllooww bbiinnddiinngg??
In deep binding, lexical variables mentioned in anonymous
subroutines are the same ones that were in scope when the
subroutine was created. In shallow binding, they are
whichever variables with the same names happen to be in
scope when the subroutine is called. Perl always uses
deep binding of lexical variables (i.e., those created
with my()). 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 the section on What's a closure?.
WWhhyy ddooeessnn''tt """"local($foo) = <FILE>;"" work right?
local() gives list context to the right hand side of =.
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
HHooww ddoo II rreeddeeffiinnee aa bbuuiillttiinn ffuunnccttiioonn,, ooppeerraattoorr,, oorr mmeetthhoodd??
Why do you want to do that? :-)
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.
WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn ccaalllliinngg aa ffuunnccttiioonn aass &&ffoooo
aanndd foo()?
When you call a function as &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 strictly speaking a
bug (it's documented that way in the perlsub manpage), it
would be hard to consider 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.
HHooww ddoo II ccrreeaattee aa sswwiittcchh oorr ccaassee ssttaatteemmeenntt??
This is explained in more depth in the the perlsyn
manpage. Briefly, there's no official case statement,
because of the variety of tests possible in Perl (numeric
comparison, string comparison, glob comparison, regexp
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.
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";
}
HHooww ccaann II ccaattcchh aacccceesssseess ttoo uunnddeeffiinneedd
vvaarriiaabblleess//ffuunnccttiioonnss//mmeetthhooddss??
The AUTOLOAD method, discussed in the section on
Autoloading in the perlsub manpage and the section on
AUTOLOAD: Proxy Methods in the perltoot manpage, lets you
capture calls to undefined functions and methods.
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 $_;
}
};
WWhhyy ccaann''tt aa mmeetthhoodd iinncclluuddeedd iinn tthhiiss ssaammee ffiillee bbee ffoouunndd??
Some possible reasons: your inheritance is getting
confused, you've misspelled the method name, or the object
is of the wrong type. Check out the perltoot manpage for
details on these. You may also use print ref($object) 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 "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.
HHooww ccaann II ffiinndd oouutt mmyy ccuurrrreenntt ppaacckkaaggee??
If you're just a random program, you can do this to find
out what the currently compiled package is:
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";
}
HHooww ccaann II ccoommmmeenntt oouutt aa llaarrggee bblloocckk ooff ppeerrll ccooddee??
Use embedded POD to discard it:
# 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
=cut
AAUUTTHHOORR AANNDD CCOOPPYYRRIIGGHHTT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved. See the perlfaq manpage for
distribution information.