Acorn User 10/AU_CD10.iso/Updates/Perl/Non-RPC/Docs/perlsub

Text File | 1999-04-17 | 48KB | 1,303 lines

NAME
    perlsub - Perl subroutines

SYNOPSIS
    To declare subroutines:

        sub NAME;                # A "forward" declaration.
        sub NAME(PROTO);        #  ditto, but with prototypes

        sub NAME BLOCK          # A declaration and a definition.
        sub NAME(PROTO) BLOCK #  ditto, but with prototypes


    To define an anonymous subroutine at runtime:

        $subref = sub BLOCK;        # no proto
        $subref = sub (PROTO) BLOCK;    # with proto


    To import subroutines:

        use PACKAGE qw(NAME1 NAME2 NAME3);


    To call subroutines:

        NAME(LIST);       # & is optional with parentheses.
        NAME LIST;       # Parentheses optional if predeclared/imported.
        &NAME;       # Makes current @_ visible to called subroutine.


DESCRIPTION
    Like many languages, Perl provides for user-defined subroutines.
    These may be located anywhere in the main program, loaded in
    from other files via the `do', `require', or `use' keywords, or
    even generated on the fly using `eval' or anonymous subroutines
    (closures). You can even call a function indirectly using a
    variable containing its name or a CODE reference to it.

    The Perl model for function call and return values is simple:
    all functions are passed as parameters one single flat list of
    scalars, and all functions likewise return to their caller one
    single flat list of scalars. Any arrays or hashes in these call
    and return lists will collapse, losing their identities--but you
    may always use pass-by-reference instead to avoid this. Both
    call and return lists may contain as many or as few scalar
    elements as you'd like. (Often a function without an explicit
    return statement is called a subroutine, but there's really no
    difference from the language's perspective.)

    Any arguments passed to the routine come in as the array `@_'.
    Thus if you called a function with two arguments, those would be
    stored in `$_[0]' and `$_[1]'. The array `@_' is a local array,
    but its elements are aliases for the actual scalar parameters.
    In particular, if an element `$_[0]' is updated, the
    corresponding argument is updated (or an error occurs if it is
    not updatable). If an argument is an array or hash element which
    did not exist when the function was called, that element is
    created only when (and if) it is modified or if a reference to
    it is taken. (Some earlier versions of Perl created the element
    whether or not it was assigned to.) Note that assigning to the
    whole array `@_' removes the aliasing, and does not update any
    arguments.

    The return value of the subroutine is the value of the last
    expression evaluated. Alternatively, a `return' statement may be
    used to exit the subroutine, optionally specifying the returned
    value, which will be evaluated in the appropriate context (list,
    scalar, or void) depending on the context of the subroutine
    call. If you specify no return value, the subroutine will return
    an empty list in a list context, an undefined value in a scalar
    context, or nothing in a void context. If you return one or more
    arrays and/or hashes, these will be flattened together into one
    large indistinguishable list.

    Perl does not have named formal parameters, but in practice all
    you do is assign to a `my()' list of these. Any variables you
    use in the function that aren't declared private are global
    variables. For the gory details on creating private variables,
    see the section on "Private Variables via my()" and the section
    on "Temporary Values via local()". To create protected
    environments for a set of functions in a separate package (and
    probably a separate file), see the section on "Packages" in the
    perlmod manpage.

    Example:

        sub max {
        my $max = shift(@_);
        foreach $foo (@_) {
            $max = $foo if $max < $foo;
        }
        return $max;
        }
        $bestday = max($mon,$tue,$wed,$thu,$fri);


    Example:

        # get a line, combining continuation lines
        #  that start with whitespace

        sub get_line {
        $thisline = $lookahead;  # GLOBAL VARIABLES!!
        LINE: while (defined($lookahead = <STDIN>)) {
            if ($lookahead =~ /^[ \t]/) {
            $thisline .= $lookahead;
            }
            else {
            last LINE;
            }
        }
        $thisline;
        }

        $lookahead = <STDIN>;    # get first line
        while ($_ = get_line()) {
        ...
        }


    Use array assignment to a local list to name your formal
    arguments:

        sub maybeset {
        my($key, $value) = @_;
        $Foo{$key} = $value unless $Foo{$key};
        }


    This also has the effect of turning call-by-reference into call-
    by-value, because the assignment copies the values. Otherwise a
    function is free to do in-place modifications of `@_' and change
    its caller's values.

        upcase_in($v1, $v2);  # this changes $v1 and $v2
        sub upcase_in {
        for (@_) { tr/a-z/A-Z/ }
        }


    You aren't allowed to modify constants in this way, of course.
    If an argument were actually literal and you tried to change it,
    you'd take a (presumably fatal) exception. For example, this
    won't work:

        upcase_in("frederick");


    It would be much safer if the `upcase_in()' function were
    written to return a copy of its parameters instead of changing
    them in place:

        ($v3, $v4) = upcase($v1, $v2);  # this doesn't
        sub upcase {
        return unless defined wantarray;  # void context, do nothing
        my @parms = @_;
        for (@parms) { tr/a-z/A-Z/ }
          return wantarray ? @parms : $parms[0];
        }


    Notice how this (unprototyped) function doesn't care whether it
    was passed real scalars or arrays. Perl will see everything as
    one big long flat `@_' parameter list. This is one of the ways
    where Perl's simple argument-passing style shines. The
    `upcase()' function would work perfectly well without changing
    the `upcase()' definition even if we fed it things like this:

        @newlist   = upcase(@list1, @list2);
        @newlist   = upcase( split /:/, $var );


    Do not, however, be tempted to do this:

        (@a, @b)   = upcase(@list1, @list2);


    Because like its flat incoming parameter list, the return list
    is also flat. So all you have managed to do here is stored
    everything in `@a' and made `@b' an empty list. See the Pass by
    Reference manpage for alternatives.

    A subroutine may be called using the "`&'" prefix. The "`&'" is
    optional in modern Perls, and so are the parentheses if the
    subroutine has been predeclared. (Note, however, that the "`&'"
    is *NOT* optional when you're just naming the subroutine, such
    as when it's used as an argument to `defined()' or `undef()'.
    Nor is it optional when you want to do an indirect subroutine
    call with a subroutine name or reference using the `&$subref()'
    or `&{$subref}()' constructs. See the perlref manpage for more
    on that.)

    Subroutines may be called recursively. If a subroutine is called
    using the "`&'" form, the argument list is optional, and if
    omitted, no `@_' array is set up for the subroutine: the `@_'
    array at the time of the call is visible to subroutine instead.
    This is an efficiency mechanism that new users may wish to
    avoid.

        &foo(1,2,3);    # pass three arguments
        foo(1,2,3);        # the same

        foo();        # pass a null list
        &foo();        # the same

        &foo;        # foo() get current args, like foo(@_) !!
        foo;        # like foo() IFF sub foo predeclared, else "foo"


    Not only does the "`&'" form make the argument list optional,
    but it also disables any prototype checking on the arguments you
    do provide. This is partly for historical reasons, and partly
    for having a convenient way to cheat if you know what you're
    doing. See the section on Prototypes below.

    Function whose names are in all upper case are reserved to the
    Perl core, just as are modules whose names are in all lower
    case. A function in all capitals is a loosely-held convention
    meaning it will be called indirectly by the run-time system
    itself. Functions that do special, pre-defined things are
    `BEGIN', `END', `AUTOLOAD', and `DESTROY'--plus all the
    functions mentioned in the perltie manpage. The 5.005 release
    adds `INIT' to this list.

  Private Variables via my()

    Synopsis:

        my $foo;            # declare $foo lexically local
        my (@wid, %get);     # declare list of variables local
        my $foo = "flurp";    # declare $foo lexical, and init it
        my @oof = @bar;    # declare @oof lexical, and init it


    A "`my'" declares the listed variables to be confined
    (lexically) to the enclosing block, conditional
    (`if/unless/elsif/else'), loop
    (`for/foreach/while/until/continue'), subroutine, `eval', or
    `do/require/use''d file. If more than one value is listed, the
    list must be placed in parentheses. All listed elements must be
    legal lvalues. Only alphanumeric identifiers may be lexically
    scoped--magical builtins like `$/' must currently be `local'ize
    with "`local'" instead.

    Unlike dynamic variables created by the "`local'" operator,
    lexical variables declared with "`my'" are totally hidden from
    the outside world, including any called subroutines (even if
    it's the same subroutine called from itself or elsewhere--every
    call gets its own copy).

    This doesn't mean that a `my()' variable declared in a
    statically *enclosing* lexical scope would be invisible. Only
    the dynamic scopes are cut off. For example, the `bumpx()'
    function below has access to the lexical `$x' variable because
    both the my and the sub occurred at the same scope, presumably
    the file scope.

        my $x = 10;
        sub bumpx { $x++ }


    (An `eval()', however, can see the lexical variables of the
    scope it is being evaluated in so long as the names aren't
    hidden by declarations within the `eval()' itself. See the
    perlref manpage.)

    The parameter list to `my()' may be assigned to if desired,
    which allows you to initialize your variables. (If no
    initializer is given for a particular variable, it is created
    with the undefined value.) Commonly this is used to name the
    parameters to a subroutine. Examples:

        $arg = "fred";      # "global" variable
        $n = cube_root(27);
        print "$arg thinks the root is $n\n";
     fred thinks the root is 3

        sub cube_root {
        my $arg = shift;  # name doesn't matter
        $arg **= 1/3;
        return $arg;
        }


    The "`my'" is simply a modifier on something you might assign
    to. So when you do assign to the variables in its argument list,
    the "`my'" doesn't change whether those variables are viewed as
    a scalar or an array. So

        my ($foo) = <STDIN>;        # WRONG?
        my @FOO = <STDIN>;


    both supply a list context to the right-hand side, while

        my $foo = <STDIN>;


    supplies a scalar context. But the following declares only one
    variable:

        my $foo, $bar = 1;            # WRONG


    That has the same effect as

        my $foo;
        $bar = 1;


    The declared variable is not introduced (is not visible) until
    after the current statement. Thus,

        my $x = $x;


    can be used to initialize the new $x with the value of the old
    `$x', and the expression

        my $x = 123 and $x == 123


    is false unless the old `$x' happened to have the value `123'.

    Lexical scopes of control structures are not bounded precisely
    by the braces that delimit their controlled blocks; control
    expressions are part of the scope, too. Thus in the loop

        while (defined(my $line = <>)) {
            $line = lc $line;
        } continue {
            print $line;
        }


    the scope of `$line' extends from its declaration throughout the
    rest of the loop construct (including the `continue' clause),
    but not beyond it. Similarly, in the conditional

        if ((my $answer = <STDIN>) =~ /^yes$/i) {
            user_agrees();
        } elsif ($answer =~ /^no$/i) {
            user_disagrees();
        } else {
        chomp $answer;
            die "'$answer' is neither 'yes' nor 'no'";
        }


    the scope of `$answer' extends from its declaration throughout
    the rest of the conditional (including `elsif' and `else'
    clauses, if any), but not beyond it.

    (None of the foregoing applies to `if/unless' or `while/until'
    modifiers appended to simple statements. Such modifiers are not
    control structures and have no effect on scoping.)

    The `foreach' loop defaults to scoping its index variable
    dynamically (in the manner of `local'; see below). However, if
    the index variable is prefixed with the keyword "`my'", then it
    is lexically scoped instead. Thus in the loop

        for my $i (1, 2, 3) {
            some_function();
        }


    the scope of `$i' extends to the end of the loop, but not beyond
    it, and so the value of `$i' is unavailable in
    `some_function()'.

    Some users may wish to encourage the use of lexically scoped
    variables. As an aid to catching implicit references to package
    variables, if you say

        use strict 'vars';


    then any variable reference from there to the end of the
    enclosing block must either refer to a lexical variable, or must
    be fully qualified with the package name. A compilation error
    results otherwise. An inner block may countermand this with "`no
    strict 'vars''".

    A `my()' has both a compile-time and a run-time effect. At
    compile time, the compiler takes notice of it; the principle
    usefulness of this is to quiet "`use strict 'vars''". The actual
    initialization is delayed until run time, so it gets executed
    appropriately; every time through a loop, for example.

    Variables declared with "`my'" are not part of any package and
    are therefore never fully qualified with the package name. In
    particular, you're not allowed to try to make a package variable
    (or other global) lexical:

        my $pack::var;    # ERROR!  Illegal syntax
        my $_;        # also illegal (currently)


    In fact, a dynamic variable (also known as package or global
    variables) are still accessible using the fully qualified `::'
    notation even while a lexical of the same name is also visible:

        package main;
        local $x = 10;
        my    $x = 20;
        print "$x and $::x\n";


    That will print out `20' and `10'.

    You may declare "`my'" variables at the outermost scope of a
    file to hide any such identifiers totally from the outside
    world. This is similar to C's static variables at the file
    level. To do this with a subroutine requires the use of a
    closure (anonymous function with lexical access). If a block
    (such as an `eval()', function, or `package') wants to create a
    private subroutine that cannot be called from outside that
    block, it can declare a lexical variable containing an anonymous
    sub reference:

        my $secret_version = '1.001-beta';
        my $secret_sub = sub { print $secret_version };
        &$secret_sub();


    As long as the reference is never returned by any function
    within the module, no outside module can see the subroutine,
    because its name is not in any package's symbol table. Remember
    that it's not *REALLY* called `$some_pack::secret_version' or
    anything; it's just `$secret_version', unqualified and
    unqualifiable.

    This does not work with object methods, however; all object
    methods have to be in the symbol table of some package to be
    found.

  Persistent Private Variables

    Just because a lexical variable is lexically (also called
    statically) scoped to its enclosing block, `eval', or `do' FILE,
    this doesn't mean that within a function it works like a C
    static. It normally works more like a C auto, but with implicit
    garbage collection.

    Unlike local variables in C or C++, Perl's lexical variables
    don't necessarily get recycled just because their scope has
    exited. If something more permanent is still aware of the
    lexical, it will stick around. So long as something else
    references a lexical, that lexical won't be freed--which is as
    it should be. You wouldn't want memory being free until you were
    done using it, or kept around once you were done. Automatic
    garbage collection takes care of this for you.

    This means that you can pass back or save away references to
    lexical variables, whereas to return a pointer to a C auto is a
    grave error. It also gives us a way to simulate C's function
    statics. Here's a mechanism for giving a function private
    variables with both lexical scoping and a static lifetime. If
    you do want to create something like C's static variables, just
    enclose the whole function in an extra block, and put the static
    variable outside the function but in the block.

        {
        my $secret_val = 0;
        sub gimme_another {
            return ++$secret_val;
        }
        }
        # $secret_val now becomes unreachable by the outside
        # world, but retains its value between calls to gimme_another


    If this function is being sourced in from a separate file via
    `require' or `use', then this is probably just fine. If it's all
    in the main program, you'll need to arrange for the `my()' to be
    executed early, either by putting the whole block above your
    main program, or more likely, placing merely a `BEGIN' sub
    around it to make sure it gets executed before your program
    starts to run:

        sub BEGIN {
        my $secret_val = 0;
        sub gimme_another {
            return ++$secret_val;
        }
        }


    See the section on "Package Constructors and Destructors" in the
    perlmod manpage about the `BEGIN' function.

    If declared at the outermost scope, the file scope, then
    lexicals work someone like C's file statics. They are available
    to all functions in that same file declared below them, but are
    inaccessible from outside of the file. This is sometimes used in
    modules to create private variables for the whole module.

  Temporary Values via local()

    NOTE: In general, you should be using "`my'" instead of
    "`local'", because it's faster and safer. Exceptions to this
    include the global punctuation variables, filehandles and
    formats, and direct manipulation of the Perl symbol table
    itself. Format variables often use "`local'" though, as do other
    variables whose current value must be visible to called
    subroutines.

    Synopsis:

        local $foo;                # declare $foo dynamically local
        local (@wid, %get);     # declare list of variables local
        local $foo = "flurp";    # declare $foo dynamic, and init it
        local @oof = @bar;        # declare @oof dynamic, and init it

        local *FH;            # localize $FH, @FH, %FH, &FH  ...
        local *merlyn = *randal;    # now $merlyn is really $randal, plus
                                    #     @merlyn is really @randal, etc
        local *merlyn = 'randal';    # SAME THING: promote 'randal' to *randal
        local *merlyn = \$randal;   # just alias $merlyn, not @merlyn etc


    A `local()' modifies its listed variables to be "local" to the
    enclosing block, `eval', or `do FILE'--and to *any subroutine
    called from within that block*. A `local()' just gives temporary
    values to global (meaning package) variables. It does not create
    a local variable. This is known as dynamic scoping. Lexical
    scoping is done with "`my'", which works more like C's auto
    declarations.

    If more than one variable is given to `local()', they must be
    placed in parentheses. All listed elements must be legal
    lvalues. This operator works by saving the current values of
    those variables in its argument list on a hidden stack and
    restoring them upon exiting the block, subroutine, or eval. This
    means that called subroutines can also reference the local
    variable, but not the global one. The argument list may be
    assigned to if desired, which allows you to initialize your
    local variables. (If no initializer is given for a particular
    variable, it is created with an undefined value.) Commonly this
    is used to name the parameters to a subroutine. Examples:

        for $i ( 0 .. 9 ) {
        $digits{$i} = $i;
        }
        # assume this function uses global %digits hash
        parse_num();

        # now temporarily add to %digits hash
        if ($base12) {
        # (NOTE: not claiming this is efficient!)
        local %digits  = (%digits, 't' => 10, 'e' => 11);
        parse_num();  # parse_num gets this new %digits!
        }
        # old %digits restored here


    Because `local()' is a run-time command, it gets executed every
    time through a loop. In releases of Perl previous to 5.0, this
    used more stack storage each time until the loop was exited.
    Perl now reclaims the space each time through, but it's still
    more efficient to declare your variables outside the loop.

    A `local' is simply a modifier on an lvalue expression. When you
    assign to a `local'ized variable, the `local' doesn't change
    whether its list is viewed as a scalar or an array. So

        local($foo) = <STDIN>;
        local @FOO = <STDIN>;


    both supply a list context to the right-hand side, while

        local $foo = <STDIN>;


    supplies a scalar context.

    A note about `local()' and composite types is in order.
    Something like `local(%foo)' works by temporarily placing a
    brand new hash in the symbol table. The old hash is left alone,
    but is hidden "behind" the new one.

    This means the old variable is completely invisible via the
    symbol table (i.e. the hash entry in the `*foo' typeglob) for
    the duration of the dynamic scope within which the `local()' was
    seen. This has the effect of allowing one to temporarily occlude
    any magic on composite types. For instance, this will briefly
    alter a tied hash to some other implementation:

        tie %ahash, 'APackage';
        [...]
        {
           local %ahash;
           tie %ahash, 'BPackage';
           [..called code will see %ahash tied to 'BPackage'..]
           {
              local %ahash;
              [..%ahash is a normal (untied) hash here..]
           }
        }
        [..%ahash back to its initial tied self again..]


    As another example, a custom implementation of `%ENV' might look
    like this:

        {
            local %ENV;
            tie %ENV, 'MyOwnEnv';
            [..do your own fancy %ENV manipulation here..]
        }
        [..normal %ENV behavior here..]


    It's also worth taking a moment to explain what happens when you
    `local'ize a member of a composite type (i.e. an array or hash
    element). In this case, the element is `local'ized *by name*.
    This means that when the scope of the `local()' ends, the saved
    value will be restored to the hash element whose key was named
    in the `local()', or the array element whose index was named in
    the `local()'. If that element was deleted while the `local()'
    was in effect (e.g. by a `delete()' from a hash or a `shift()'
    of an array), it will spring back into existence, possibly
    extending an array and filling in the skipped elements with
    `undef'. For instance, if you say

        %hash = ( 'This' => 'is', 'a' => 'test' );
        @ary  = ( 0..5 );
        {
             local($ary[5]) = 6;
             local($hash{'a'}) = 'drill';
             while (my $e = pop(@ary)) {
                 print "$e . . .\n";
                 last unless $e > 3;
             }
             if (@ary) {
                 $hash{'only a'} = 'test';
                 delete $hash{'a'};
             }
        }
        print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
        print "The array has ",scalar(@ary)," elements: ",
              join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";


    Perl will print

        6 . . .
        4 . . .
        3 . . .
        This is a test only a test.
        The array has 6 elements: 0, 1, 2, undef, undef, 5


    Note also that when you `local'ize a member of a composite type
    that does not exist previously, the value is treated as though
    it were in an lvalue context, i.e., it is first created and then
    `local'ized. The consequence of this is that the hash or array
    is in fact permanently modified. For instance, if you say

        %hash = ( 'This' => 'is', 'a' => 'test' );
        @ary  = ( 0..5 );
        {
            local($ary[8]) = 0;
            local($hash{'b'}) = 'whatever';
        }
        printf "%%hash has now %d keys, \@ary %d elements.\n",
            scalar(keys(%hash)), scalar(@ary);


    Perl will print

        %hash has now 3 keys, @ary 9 elements.


    The above behavior of local() on non-existent members of
    composite types is subject to change in future.

  Passing Symbol Table Entries (typeglobs)

    [Note: The mechanism described in this section was originally
    the only way to simulate pass-by-reference in older versions of
    Perl. While it still works fine in modern versions, the new
    reference mechanism is generally easier to work with. See
    below.]

    Sometimes you don't want to pass the value of an array to a
    subroutine but rather the name of it, so that the subroutine can
    modify the global copy of it rather than working with a local
    copy. In perl you can refer to all objects of a particular name
    by prefixing the name with a star: `*foo'. This is often known
    as a "typeglob", because the star on the front can be thought of
    as a wildcard match for all the funny prefix characters on
    variables and subroutines and such.

    When evaluated, the typeglob produces a scalar value that
    represents all the objects of that name, including any
    filehandle, format, or subroutine. When assigned to, it causes
    the name mentioned to refer to whatever "`*'" value was assigned
    to it. Example:

        sub doubleary {
        local(*someary) = @_;
        foreach $elem (@someary) {
            $elem *= 2;
        }
        }
        doubleary(*foo);
        doubleary(*bar);


    Note that scalars are already passed by reference, so you can
    modify scalar arguments without using this mechanism by
    referring explicitly to `$_[0]' etc. You can modify all the
    elements of an array by passing all the elements as scalars, but
    you have to use the `*' mechanism (or the equivalent reference
    mechanism) to `push', `pop', or change the size of an array. It
    will certainly be faster to pass the typeglob (or reference).

    Even if you don't want to modify an array, this mechanism is
    useful for passing multiple arrays in a single LIST, because
    normally the LIST mechanism will merge all the array values so
    that you can't extract out the individual arrays. For more on
    typeglobs, see the section on "Typeglobs and Filehandles" in the
    perldata manpage.

  When to Still Use local()

    Despite the existence of `my()', there are still three places
    where the `local()' operator still shines. In fact, in these
    three places, you *must* use `local' instead of `my'.

    1. You need to give a global variable a temporary value, especially `$_'.
        The global variables, like `@ARGV' or the punctuation
        variables, must be `local'ized with `local()'. This block
        reads in /etc/motd, and splits it up into chunks separated
        by lines of equal signs, which are placed in `@Fields'.

            {
            local @ARGV = ("/etc/motd");
                local $/ = undef;
                local $_ = <>;
            @Fields = split /^\s*=+\s*$/;
            }


        It particular, it's important to `local'ize `$_' in any
        routine that assigns to it. Look out for implicit
        assignments in `while' conditionals.

    2. You need to create a local file or directory handle or a local function.
        A function that needs a filehandle of its own must use
        `local()' uses `local()' on complete typeglob. This can be
        used to create new symbol table entries:

            sub ioqueue {
                local  (*READER, *WRITER);    # not my!
                pipe    (READER,  WRITER);    or die "pipe: $!";
                return (*READER, *WRITER);
            }
            ($head, $tail) = ioqueue();


        See the Symbol module for a way to create anonymous symbol
        table entries.

        Because assignment of a reference to a typeglob creates an
        alias, this can be used to create what is effectively a
        local function, or at least, a local alias.

            {
                local *grow = \&shrink; # only until this block exists
                grow();                 # really calls shrink()
            move();            # if move() grow()s, it shrink()s too
            }
            grow();            # get the real grow() again


        See the section on "Function Templates" in the perlref
        manpage for more about manipulating functions by name in
        this way.

    3. You want to temporarily change just one element of an array or hash.
        You can `local'ize just one element of an aggregate. Usually
        this is done on dynamics:

            {
            local $SIG{INT} = 'IGNORE';
            funct();                # uninterruptible
            }
            # interruptibility automatically restored here


        But it also works on lexically declared aggregates. Prior to
        5.005, this operation could on occasion misbehave.


  Pass by Reference

    If you want to pass more than one array or hash into a function-
    -or return them from it--and have them maintain their integrity,
    then you're going to have to use an explicit pass-by-reference.
    Before you do that, you need to understand references as
    detailed in the perlref manpage. This section may not make much
    sense to you otherwise.

    Here are a few simple examples. First, let's pass in several
    arrays to a function and have it `pop' all of then, return a new
    list of all their former last elements:

        @tailings = popmany ( \@a, \@b, \@c, \@d );

        sub popmany {
        my $aref;
        my @retlist = ();
        foreach $aref ( @_ ) {
            push @retlist, pop @$aref;
        }
        return @retlist;
        }


    Here's how you might write a function that returns a list of
    keys occurring in all the hashes passed to it:

        @common = inter( \%foo, \%bar, \%joe );
        sub inter {
        my ($k, $href, %seen); # locals
        foreach $href (@_) {
            while ( $k = each %$href ) {
            $seen{$k}++;
            }
        }
        return grep { $seen{$_} == @_ } keys %seen;
        }


    So far, we're using just the normal list return mechanism. What
    happens if you want to pass or return a hash? Well, if you're
    using only one of them, or you don't mind them concatenating,
    then the normal calling convention is ok, although a little
    expensive.

    Where people get into trouble is here:

        (@a, @b) = func(@c, @d);
    or
        (%a, %b) = func(%c, %d);


    That syntax simply won't work. It sets just `@a' or `%a' and
    clears the `@b' or `%b'. Plus the function didn't get passed
    into two separate arrays or hashes: it got one long list in
    `@_', as always.

    If you can arrange for everyone to deal with this through
    references, it's cleaner code, although not so nice to look at.
    Here's a function that takes two array references as arguments,
    returning the two array elements in order of how many elements
    they have in them:

        ($aref, $bref) = func(\@c, \@d);
        print "@$aref has more than @$bref\n";
        sub func {
        my ($cref, $dref) = @_;
        if (@$cref > @$dref) {
            return ($cref, $dref);
        } else {
            return ($dref, $cref);
        }
        }


    It turns out that you can actually do this also:

        (*a, *b) = func(\@c, \@d);
        print "@a has more than @b\n";
        sub func {
        local (*c, *d) = @_;
        if (@c > @d) {
            return (\@c, \@d);
        } else {
            return (\@d, \@c);
        }
        }


    Here we're using the typeglobs to do symbol table aliasing. It's
    a tad subtle, though, and also won't work if you're using `my()'
    variables, because only globals (well, and `local()'s) are in
    the symbol table.

    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 `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>;
        }


    Another way to do this is using `*HANDLE{IO}', see the perlref
    manpage for usage and caveats.

    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;
        }


    Although that will actually produce a small memory leak. See the
    bottom of the "open()" entry in the perlfunc manpage for a
    somewhat cleaner way using the `IO::Handle' package.

  Prototypes

    As of the 5.002 release of perl, if you declare

        sub mypush (\@@)


    then `mypush()' takes arguments exactly like `push()' does. The
    declaration of the function to be called must be visible at
    compile time. The prototype affects only the interpretation of
    new-style calls to the function, where new-style is defined as
    not using the `&' character. In other words, if you call it like
    a builtin function, then it behaves like a builtin function. If
    you call it like an old-fashioned subroutine, then it behaves
    like an old-fashioned subroutine. It naturally falls out from
    this rule that prototypes have no influence on subroutine
    references like `\&foo' or on indirect subroutine calls like
    `&{$subref}' or `$subref->()'.

    Method calls are not influenced by prototypes either, because
    the function to be called is indeterminate at compile time,
    because it depends on inheritance.

    Because the intent is primarily to let you define subroutines
    that work like builtin commands, here are the prototypes for
    some other functions that parse almost exactly like the
    corresponding builtins.

        Declared as            Called as

        sub mylink ($$)         mylink $old, $new
        sub myvec ($$$)         myvec $var, $offset, 1
        sub myindex ($$;$)         myindex &getstring, "substr"
        sub mysyswrite ($$$;$)   mysyswrite $buf, 0, length($buf) - $off, $off
        sub myreverse (@)         myreverse $a, $b, $c
        sub myjoin ($@)         myjoin ":", $a, $b, $c
        sub mypop (\@)         mypop @array
        sub mysplice (\@$$@)     mysplice @array, @array, 0, @pushme
        sub mykeys (\%)         mykeys %{$hashref}
        sub myopen (*;$)         myopen HANDLE, $name
        sub mypipe (**)         mypipe READHANDLE, WRITEHANDLE
        sub mygrep (&@)         mygrep { /foo/ } $a, $b, $c
        sub myrand ($)         myrand 42
        sub mytime ()         mytime


    Any backslashed prototype character represents an actual
    argument that absolutely must start with that character. The
    value passed to the subroutine (as part of `@_') will be a
    reference to the actual argument given in the subroutine call,
    obtained by applying `\' to that argument.

    Unbackslashed prototype characters have special meanings. Any
    unbackslashed `@' or `%' eats all the rest of the arguments, and
    forces list context. An argument represented by `$' forces
    scalar context. An `&' requires an anonymous subroutine, which,
    if passed as the first argument, does not require the "`sub'"
    keyword or a subsequent comma. A `*' allows the subroutine to
    accept a bareword, constant, scalar expression, typeglob, or a
    reference to a typeglob in that slot. The value will be
    available to the subroutine either as a simple scalar, or (in
    the latter two cases) as a reference to the typeglob.

    A semicolon separates mandatory arguments from optional
    arguments. (It is redundant before `@' or `%'.)

    Note how the last three examples above are treated specially by
    the parser. `mygrep()' is parsed as a true list operator,
    `myrand()' is parsed as a true unary operator with unary
    precedence the same as `rand()', and `mytime()' is truly without
    arguments, just like `time()'. That is, if you say

        mytime +2;


    you'll get `mytime() + 2', not `mytime(2)', which is how it
    would be parsed without the prototype.

    The interesting thing about `&' is that you can generate new
    syntax with it:

        sub try (&@) {
        my($try,$catch) = @_;
        eval { &$try };
        if ($@) {
            local $_ = $@;
            &$catch;
        }
        }
        sub catch (&) { $_[0] }

        try {
        die "phooey";
        } catch {
        /phooey/ and print "unphooey\n";
        };


    That prints `"unphooey"'. (Yes, there are still unresolved
    issues having to do with the visibility of `@_'. I'm ignoring
    that question for the moment. (But note that if we make `@_'
    lexically scoped, those anonymous subroutines can act like
    closures... (Gee, is this sounding a little Lispish? (Never
    mind.))))

    And here's a reimplementation of `grep':

        sub mygrep (&@) {
        my $code = shift;
        my @result;
        foreach $_ (@_) {
            push(@result, $_) if &$code;
        }
        @result;
        }


    Some folks would prefer full alphanumeric prototypes.
    Alphanumerics have been intentionally left out of prototypes for
    the express purpose of someday in the future adding named,
    formal parameters. The current mechanism's main goal is to let
    module writers provide better diagnostics for module users.
    Larry feels the notation quite understandable to Perl
    programmers, and that it will not intrude greatly upon the meat
    of the module, nor make it harder to read. The line noise is
    visually encapsulated into a small pill that's easy to swallow.

    It's probably best to prototype new functions, not retrofit
    prototyping into older ones. That's because you must be
    especially careful about silent impositions of differing list
    versus scalar contexts. For example, if you decide that a
    function should take just one parameter, like this:

        sub func ($) {
        my $n = shift;
        print "you gave me $n\n";
        }


    and someone has been calling it with an array or expression
    returning a list:

        func(@foo);
        func( split /:/ );


    Then you've just supplied an automatic `scalar()' in front of
    their argument, which can be more than a bit surprising. The old
    `@foo' which used to hold one thing doesn't get passed in.
    Instead, the `func()' now gets passed in `1', that is, the
    number of elements in `@foo'. And the `split()' gets called in a
    scalar context and starts scribbling on your `@_' parameter
    list.

    This is all very powerful, of course, and should be used only in
    moderation to make the world a better place.

  Constant Functions

    Functions with a prototype of `()' are potential candidates for
    inlining. If the result after optimization and constant folding
    is either a constant or a lexically-scoped scalar which has no
    other references, then it will be used in place of function
    calls made without `&' or `do'. Calls made using `&' or `do' are
    never inlined. (See constant.pm for an easy way to declare most
    constants.)

    The following functions would all be inlined:

        sub pi ()        { 3.14159 }        # Not exact, but close.
        sub PI ()        { 4 * atan2 1, 1 }    # As good as it gets,
                            # and it's inlined, too!
        sub ST_DEV ()    { 0 }
        sub ST_INO ()    { 1 }

        sub FLAG_FOO ()    { 1 << 8 }
        sub FLAG_BAR ()    { 1 << 9 }
        sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

        sub OPT_BAZ ()    { not (0x1B58 & FLAG_MASK) }
        sub BAZ_VAL () {
        if (OPT_BAZ) {
            return 23;
        }
        else {
            return 42;
        }
        }

        sub N () { int(BAZ_VAL) / 3 }
        BEGIN {
        my $prod = 1;
        for (1..N) { $prod *= $_ }
        sub N_FACTORIAL () { $prod }
        }


    If you redefine a subroutine that was eligible for inlining,
    you'll get a mandatory warning. (You can use this warning to
    tell whether or not a particular subroutine is considered
    constant.) The warning is considered severe enough not to be
    optional because previously compiled invocations of the function
    will still be using the old value of the function. If you need
    to be able to redefine the subroutine you need to ensure that it
    isn't inlined, either by dropping the `()' prototype (which
    changes the calling semantics, so beware) or by thwarting the
    inlining mechanism in some other way, such as

        sub not_inlined () {
            23 if $];
        }


  Overriding Builtin Functions

    Many builtin functions may be overridden, though this should be
    tried only occasionally and for good reason. Typically this
    might be done by a package attempting to emulate missing builtin
    functionality on a non-Unix system.

    Overriding may be done only by importing the name from a module-
    -ordinary predeclaration isn't good enough. However, the `subs'
    pragma (compiler directive) lets you, in effect, predeclare subs
    via the import syntax, and these names may then override the
    builtin ones:

        use subs 'chdir', 'chroot', 'chmod', 'chown';
        chdir $somewhere;
        sub chdir { ... }


    To unambiguously refer to the builtin form, one may precede the
    builtin name with the special package qualifier `CORE::'. For
    example, saying `CORE::open()' will always refer to the builtin
    `open()', even if the current package has imported some other
    subroutine called `&open()' from elsewhere.

    Library modules should not in general export builtin names like
    "`open'" or "`chdir'" as part of their default `@EXPORT' list,
    because these may sneak into someone else's namespace and change
    the semantics unexpectedly. Instead, if the module adds the name
    to the `@EXPORT_OK' list, then it's possible for a user to
    import the name explicitly, but not implicitly. That is, they
    could say

        use Module 'open';


    and it would import the `open' override, but if they said

        use Module;


    they would get the default imports without the overrides.

    The foregoing mechanism for overriding builtins is restricted,
    quite deliberately, to the package that requests the import.
    There is a second method that is sometimes applicable when you
    wish to override a builtin everywhere, without regard to
    namespace boundaries. This is achieved by importing a sub into
    the special namespace `CORE::GLOBAL::'. Here is an example that
    quite brazenly replaces the `glob' operator with something that
    understands regular expressions.

        package REGlob;
        require Exporter;
        @ISA = 'Exporter';
        @EXPORT_OK = 'glob';

        sub import {
        my $pkg = shift;
        return unless @_;
        my $sym = shift;
        my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
        $pkg->export($where, $sym, @_);
        }

        sub glob {
        my $pat = shift;
        my @got;
        local(*D);
        if (opendir D, '.') { @got = grep /$pat/, readdir D; closedir D; }
        @got;
        }
        1;


    And here's how it could be (ab)used:

        #use REGlob 'GLOBAL_glob';        # override glob() in ALL namespaces
        package Foo;
        use REGlob 'glob';            # override glob() in Foo:: only
        print for <^[a-z_]+\.pm\$>;        # show all pragmatic modules


    Note that the initial comment shows a contrived, even dangerous
    example. By overriding `glob' globally, you would be forcing the
    new (and subversive) behavior for the `glob' operator for every
    namespace, without the complete cognizance or cooperation of the
    modules that own those namespaces. Naturally, this should be
    done with extreme caution--if it must be done at all.

    The `REGlob' example above does not implement all the support
    needed to cleanly override perl's `glob' operator. The builtin
    `glob' has different behaviors depending on whether it appears
    in a scalar or list context, but our `REGlob' doesn't. Indeed,
    many perl builtins have such context sensitive behaviors, and
    these must be adequately supported by a properly written
    override. For a fully functional example of overriding `glob',
    study the implementation of `File::DosGlob' in the standard
    library.

  Autoloading

    If you call a subroutine that is undefined, you would ordinarily
    get an immediate fatal error complaining that the subroutine
    doesn't exist. (Likewise for subroutines being used as methods,
    when the method doesn't exist in any base class of the class
    package.) If, however, there is an `AUTOLOAD' subroutine defined
    in the package or packages that were searched for the original
    subroutine, then that `AUTOLOAD' subroutine is called with the
    arguments that would have been passed to the original
    subroutine. The fully qualified name of the original subroutine
    magically appears in the `$AUTOLOAD' variable in the same
    package as the `AUTOLOAD' routine. The name is not passed as an
    ordinary argument because, er, well, just because, that's why...

    Most `AUTOLOAD' routines will load in a definition for the
    subroutine in question using eval, and then execute that
    subroutine using a special form of "goto" that erases the stack
    frame of the `AUTOLOAD' routine without a trace. (See the
    standard `AutoLoader' module, for example.) But an `AUTOLOAD'
    routine can also just emulate the routine and never define it.
    For example, let's pretend that a function that wasn't defined
    should just call `system()' with those arguments. All you'd do
    is this:

        sub AUTOLOAD {
        my $program = $AUTOLOAD;
        $program =~ s/.*:://;
        system($program, @_);
        }
        date();
        who('am', 'i');
        ls('-l');


    In fact, if you predeclare the functions you want to call that
    way, you don't even need the parentheses:

        use subs qw(date who ls);
        date;
        who "am", "i";
        ls -l;


    A more complete example of this is the standard Shell module,
    which can treat undefined subroutine calls as calls to Unix
    programs.

    Mechanisms are available for modules writers to help split the
    modules up into autoloadable files. See the standard AutoLoader
    module described in the AutoLoader manpage and in the AutoSplit
    manpage, the standard SelfLoader modules in the SelfLoader
    manpage, and the document on adding C functions to perl code in
    the perlxs manpage.

SEE ALSO
    See the perlref manpage for more about references and closures.
    See the perlxs manpage if you'd like to learn about calling C
    subroutines from perl. See the perlmod manpage to learn about
    bundling up your functions in separate files.