splitpod broken in 5.004_01

[p5sagit/p5-mst-13.2.git] / pod / perlfaq7.pod

=head1 NAME

perlfaq7 - Perl Language Issues ($Revision: 1.18 $, $Date: 1997/04/24 22:44:14 $)

=head1 DESCRIPTION

This section deals with general Perl language issues that don't
clearly fit into any of the other sections.

=head2 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."

=head2 What are all these $@%* punctuation signs, and how do I know when to use them?

They are type specifiers, as detailed in L<perldata>:

    $ 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 E<lt>FILEE<gt> is I<neither> the type specifier for files
nor the name of the handle.  It is the C<E<lt>E<gt>> operator applied
to the handle FILE.  It reads one line (well, record - see
L<perlvar/$/>) from the handle FILE in scalar context, or I<all> lines
in list context.  When performing open, close, or any other operation
besides C<E<lt>E<gt>> on files, or even talking about the handle, do
I<not> use the brackets.  These are correct: C<eof(FH)>, C<seek(FH, 0,
2)> and "copying from STDIN to FILE".

=head2 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 C<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 C<=E<gt>> 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 L<perlstyle>) 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",
    );

=head2 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);

=head2 How do I temporarily block warnings?

The C<$^W> variable (documented in L<perlvar>) 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 C<$^W>, only local().

A new C<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.

=head2 What's an extension?

A way of calling compiled C code from Perl.  Reading L<perlxstut>
is a good place to learn more about extensions.

=head2 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
L<perlop>.

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 C<or> operator:

    (unlink $file) || die "snafu";
    unlink $file or die "snafu";

The "English" operators (C<and>, C<or>, C<xor>, and C<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 C<-2**2> product a
negative not a positive four.  It is also right-associating, meaning
that C<2**3**2> is two raised to the ninth power, not eight squared.

=head2 How do I declare/create a structure?

In general, you don't "declare" a structure.  Just use a (probably
anonymous) hash reference.  See L<perlref> and L<perldsc> 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 L<perltoot>.

=head2 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 L<perlmod>.  You'll also find L<Exporter> helpful.  If
you're writing a C or mixed-language module with both C and Perl, then
you should study L<perlxstut>.

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

=head2 How do I create a class?

See L<perltoot> for an introduction to classes and objects, as well as
L<perlobj> and L<perlbot>.

=head2 How can I tell if a variable is tainted?

See L<perlsec/"Laundering and Detecting Tainted Data">.  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.

=head2 What's a closure?

Closures are documented in L<perlref>.

I<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 I<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 C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
C<&$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, C<'$line =
E<lt>STDINE<gt>'>, there would have been no way for the hypothetical
timeout() function to access the lexical variable $line back in its
caller's scope.

=head2 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 $f that has "bar" added to it three times should be a new C<$f>
(C<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.

=head2 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 L<perlsub/"Pass by Reference"> for this particular
question, and L<perlref> for information on references.

=over 4

=item 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] }   );

=item Passing Filehandles

To create filehandles you can pass to subroutines, you can use C<*FH>
or C<\*FH> notation ("typeglobs" - see L<perldata> 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);

=item 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,
I<Mastering Regular Expressions>, by Jeffrey Friedl.  Page 273's
Build_MatchMany_Function() is particularly interesting.  A complete
citation of this book is given in L<perlfaq2>.

=item 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).

=back

=head2 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 C<use Pax> or C<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.

=head2 What's the difference between dynamic and lexical (static) scoping?  Between local() and my()?

C<local($x)> saves away the old value of the global variable C<$x>,
and assigns a new value for the duration of the subroutine, I<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.

C<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 L<perlsub>, which explains this all in more detail.

=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<use strict "refs">.  So instead of $var, use C<${'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 I<not> the dynamic
$var in the current package, but rather the one in the C<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 C<use strict "refs">.

=head2 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 L<"What's a closure?">.

=head2 Why doesn't "local($foo) = <FILE>;" work right?

C<local()> gives list context to the right hand side of C<=>.  The
E<lt>FHE<gt> 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

=head2 How do I redefine a builtin function, operator, or method?

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 L<perlsub/"Overriding Builtin Functions">.  There's
also an example in L<perltoot/"Class::Template">.

If you want to overload a Perl operator, such as C<+> or C<**>,
then you'll want to use the C<use overload> pragma, documented
in L<overload>.

If you're talking about obscuring method calls in parent classes,
see L<perltoot/"Overridden Methods">.

=head2 What's the difference between calling a function as &foo and foo()?

When you call a function as C<&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 L<perlsub>), it
would be hard to consider this a feature in most cases.

When you call your function as C<&foo()>, then you do get a new @_,
but prototyping is still circumvented.

Normally, you want to call a function using C<foo()>.  You may only
omit the parentheses if the function is already known to the compiler
because it already saw the definition (C<use> but not C<require>),
or via a forward reference or C<use subs> declaration.  Even in this
case, you get a clean @_ without any of the old values leaking through
where they don't belong.

=head2 How do I create a switch or case statement?

This is explained in more depth in the L<perlsyn>.  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";

    }

=head2 How can I catch accesses to undefined variables/functions/methods?

The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and
L<perltoot/"AUTOLOAD: Proxy Methods">, 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 {

	for ( $_[0] ) {

	    /Use of uninitialized value/  && do {
		# promote warning to a fatal
		die $_;
	    };

	    # other warning cases to catch could go here;

	    warn $_;
	}

    };

=head2 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 L<perltoot> for details on these.  You may also use C<print
ref($object)> to find out the class C<$object> was blessed into.

Another possible reason for problems is because you've used the
indirect object syntax (eg, C<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 C<use> statement instead of
C<require>.  If not, make sure to use arrow notation (eg,
C<Guru->find("Samy")>) instead.  Object notation is explained in
L<perlobj>.

=head2 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";
    }

=head2 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

    =cut

=head1 AUTHOR AND COPYRIGHT

Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved.  See L<perlfaq> for distribution information.