NAME

perlfaq7 - General Perl Language Issues ($Revision: 10100 $)

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?

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."

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 subroutines (aka functions, procedures, methods)
        * for all types of that symbol name.  In version 4 you used them like
          pointers, but in modern perls you can just use references.

There are couple of other symbols that you're likely to encounter that aren't really type specifiers:

        <> 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 $/ 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 when 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:

        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);

You can also use a list slice to select only the elements that you need:

        ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];

How do I temporarily block warnings?

If you are running Perl 5.6.0 or better, the use warnings pragma allows fine control of what warning are produced. See the perllexwarn manpage for more details.

        {
        no warnings;          # temporarily turn off warnings
        $a = $b + $c;         # I know these might be undef
        }

Additionally, you can enable and disable categories of warnings. You turn off the categories you want to ignore and you can still get other categories of warnings. See the perllexwarn manpage for the complete details, including the category names and hierarchy.

        {
        no warnings 'uninitialized';
        $a = $b + $c;
        }

If you have an older version of Perl, 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().

What's an extension?

An extension is 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.

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

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

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 L<perltoot>.

How do I create a module?

(contributed by brian d foy)

the perlmod manpage, the perlmodlib manpage, the perlmodstyle manpage explain modules in all the gory details. the perlnewmod manpage gives a brief overview of the process along with a couple of suggestions about style.

If you need to include C code or C library interfaces in your module, you'll need h2xs. h2xs will create the module distribution structure and the initial interface files you'll need. the perlxs manpage and the perlxstut manpage explain the details.

If you don't need to use C code, other tools such as ExtUtils::ModuleMaker and Module::Starter, can help you create a skeleton module distribution.

You may also want to see Sam Tregar's "Writing Perl Modules for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 ) which is the best hands-on guide to creating module distributions.

How do I adopt or take over a module already on CPAN?

(contributed by brian d foy)

The easiest way to take over a module is to have the current module maintainer either make you a co-maintainer or transfer the module to you.

If you can't reach the author for some reason (e.g. email bounces), the PAUSE admins at modules@perl.org can help. The PAUSE admins treat each case individually.

Get a login for the Perl Authors Upload Server (PAUSE) if you don't already have one: http://pause.perl.org

Write to modules@perl.org explaining what you did to contact the current maintainer. The PAUSE admins will also try to reach the maintainer.

Post a public message in a heavily trafficked site announcing your intention to take over the module.

Wait a bit. The PAUSE admins don't want to act too quickly in case the current maintainer is on holiday. If there's no response to private communication or the public post, a PAUSE admin can transfer it to you.

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?

You can use the tainted() function of the Scalar::Util module, available from CPAN (or included with Perl since release 5.8.0). See also Laundering and Detecting Tainted Data in the perlsec manpage.

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. Usually, 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 are most often used in programming languages 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 non-closure 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 anonymous subroutine returned by add_function_generator() isn't technically a closure because it refers to no lexicals outside its own scope. Using a closure gives you a function template with some customization slots left out to be filled later.

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.

Another use for a closure is to make a variable private to a named subroutine, e.g. a counter that gets initialized at creation time of the sub and can only be modified from within the sub. This is sometimes used with a BEGIN block in package files to make sure a variable doesn't get meddled with during the lifetime of the package:

        BEGIN {
                my $id = 0;
                sub next_id { ++$id }
        }

This is discussed in more detail in the perlsub manpage, see the entry on Persistent Private Variables.

What is variable suicide and how can I prevent it?

This problem was fixed in perl 5.004_05, so preventing it means upgrading your version of perl. ;)

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() iterator 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