PERLFAQ7(1) PERLFAQ7(1)
NAME
perlfaq7 - Perl Language Issues ($Revision: 1.18 $, $Date: 1997/04/24
22:44:14 $)
DESCRIPTION
This section deals with general Perl language issues that don't clearly
fit into any of the other sections.
Can I get a BNF/yacc/RE for the Perl language?
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."
What are all these $@%* punctuation signs, and how do I know when to use
them?
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".
Do I always/never have to quote my strings or use semicolons and commas?
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:
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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",
);
How do I skip some return values?
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);
How do I temporarily block warnings?
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.
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What's an extension?
A way of calling compiled C code from Perl. Reading the perlxstut
manpage is a good place to learn more about extensions.
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
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.
How do I declare/create a structure?
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
$person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try the perltoot
manpage.
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How do I create a module?
A module is a package that lives in a file of the same name. For
example, the Hello::There module would live in Hello/There.pm. For
details, read the perlmod manpage. You'll also find the Exporter manpage
helpful. If you're writing a C or mixed-language module with both C and
Perl, then you should study the perlxstut manpage.
Here's a convenient template you might wish you use when starting your
own module. Make sure to change the names appropriately.
package Some::Module; # assumes Some/Module.pm
use strict;
BEGIN {
use Exporter ();
use vars qw($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.
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# 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
How do I create a class?
See the perltoot manpage for an introduction to classes and objects, as
well as the perlobj manpage and the perlbot manpage.
How can I tell if a variable is tainted?
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.
What's a closure?
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).
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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.
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What is variable suicide and how can I prevent it?
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.
How can I pass/return a {Function, FileHandle, Array, Hash, Method,
Regexp}?
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.
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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:
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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).
How do I create a static variable?
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 $Pax::started because its scope is
unrelated to the package. It's scoped to the file. You could
conceivably have several packages in that same file all accessing the
same private variable, but another file with the same package couldn't
get to it.
What's the difference between dynamic and lexical (static) scoping?
Between local() and my()?
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local($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.
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 use
strict "refs". So instead of $var, use ${'var'}.
local $var = "global";
my $var = "lexical";
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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 $main::var. Specifying the package directly makes you
hard-code its name, but it executes faster and avoids running afoul of
use strict "refs".
What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous subroutines are
the same ones that were in scope when the subroutine was created. In
shallow binding, they are whichever variables with the same names happen
to be in scope when the subroutine is called. Perl always uses deep
binding of lexical variables (i.e., those created with my()). However,
dynamic variables (aka global, local, or package variables) are
effectively shallowly bound. Consider this just one more reason not to
use them. See the answer to the section on What's a closure?.
Why doesn't "local($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
How do I redefine a builtin function, operator, or method?
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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.
What's the difference between calling a function as &foo and 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.
How do I create a switch or case statement?
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";
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/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";
}
How can I catch accesses to undefined variables/functions/methods?
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 $_;
}
};
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Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting confused, you've
misspelled the method name, or the object is of the wrong type. Check
out the perltoot manpage for details on these. You may also use print
ref($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.
How can I find out my current package?
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";
}
How can I comment out a large block of perl code?
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
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=cut
AUTHOR AND COPYRIGHT
Copyright (c) 1997 Tom Christiansen and Nathan Torkington. All rights
reserved. See the perlfaq manpage for distribution information.
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