perlfaq7.pod

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=head1 NAME

perlfaq7 - Perl Language Issues ($Revision: 1.21 $, $Date: 1998/06/22 15:20:07 $)

=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?

There is no BNF, but you can paw your way through the yacc grammar in
perly.y in the source distribution if you're particularly brave.  The
grammar relies on very smart tokenizing code, so be prepared to
venture into toke.c as well.

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.

Although it has the same precedence as in C, Perl's C<?:> operator
produces an lvalue.  This assigns $x to either $a or $b, depending
on the trueness of $maybe:

    ($maybe ? $a : $b) = $x;

=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: 1.21 $=~/\d+/g;sprintf '%d.'.'%02d'x$#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

=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 pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations.
These are "typeglobs" - see L<perldata/"Typeglobs and Filehandles">
and especially L<perlsub/"Pass by Reference"> for more information.

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 C<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>;

=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