To call subroutines:
NAME(LIST); # & is optional with parentheses.
- NAME LIST; # Parentheses optional if pre-declared/imported.
+ NAME LIST; # Parentheses optional if predeclared/imported.
&NAME; # Passes current @_ to subroutine.
=head1 DESCRIPTION
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 C<$_[0]>
-and C<$_[1]>. The array @_ is a local array, but its values are implicit
-references (predating L<perlref>) to the actual scalar parameters. The
-return value of the subroutine is the value of the last expression
-evaluated. Alternatively, a return statement may be used to specify the
-returned value and exit the subroutine. If you return one or more arrays
-and/or hashes, these will be flattened together into one large
-indistinguishable list.
+and C<$_[1]>. The array @_ is a local array, but its elements are
+aliases for the actual scalar parameters. In particular, if an element
+C<$_[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
sub get_line {
$thisline = $lookahead; # GLOBAL VARIABLES!!
- LINE: while ($lookahead = <STDIN>) {
+ LINE: while (defined($lookahead = <STDIN>)) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
- for (@_) { tr/a-z/A-Z/ }
- }
+ for (@_) { tr/a-z/A-Z/ }
+ }
You aren't allowed to modify constants in this way, of course. If an
argument were actually literal and you tried to change it, you'd take a
upcase_in("frederick");
-It would be much safer if the upcase_in() function
+It would be much safer if the upcase_in() function
were written to return a copy of its parameters instead
of changing them in place:
($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/ }
- # wantarray checks if we were called in list context
+ for (@parms) { tr/a-z/A-Z/ }
return wantarray ? @parms : $parms[0];
- }
+ }
Notice how this (unprototyped) function doesn't care whether it was passed
real scalars or arrays. Perl will see everything as one big long flat @_
flat. So all you have managed to do here is stored everything in @a and
made @b an empty list. See L</"Pass by Reference"> for alternatives.
-A subroutine may be called using the "&" prefix. The "&" is optional in
-Perl 5, and so are the parentheses if the subroutine has been pre-declared.
-(Note, however, that the "&" is I<NOT> optional when you're just naming
-the subroutine, such as when it's used as an argument to defined() or
-undef(). Nor is it optional when you want to do an indirect subroutine
-call with a subroutine name or reference using the C<&$subref()> or
-C<&{$subref}()> constructs. See L<perlref> for more on that.)
+A subroutine may be called using the "&" prefix. The "&" is optional
+in modern Perls, and so are the parentheses if the subroutine has been
+predeclared. (Note, however, that the "&" is I<NOT> optional when
+you're just naming the subroutine, such as when it's used as an
+argument to defined() or undef(). Nor is it optional when you want to
+do an indirect subroutine call with a subroutine name or reference
+using the C<&$subref()> or C<&{$subref}()> constructs. See L<perlref>
+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
&foo(); # the same
&foo; # foo() get current args, like foo(@_) !!
- foo; # like foo() IFF sub foo pre-declared, else "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
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
my $x = $x;
-can be used to initialize the new $x with the value of the old $x, and
+can be used to initialize the new $x with the value of the old $x, and
the expression
my $x = 123 and $x == 123
braces that delimit their controlled blocks; control expressions are
part of the scope, too. Thus in the loop
- while (my $line = <>) {
+ while (defined(my $line = <>)) {
$line = lc $line;
} continue {
print $line;
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 C<use strict 'vars'>. The actual initialization doesn't happen
-until run time, so gets executed every time through a loop.
+quiet C<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
static variable outside the function but in the block.
{
- my $secret_val = 0;
+ 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
+If this function is being sourced in from a separate file
via C<require> or C<use>, then this is probably just fine. If it's
-all in the main program, you'll need to arrange for the my()
+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 pain program, or more likely, placing merely a BEGIN
+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;
+ my $secret_val = 0;
sub gimme_another {
return ++$secret_val;
- }
- }
+ }
+ }
See L<perlrun> about the BEGIN function.
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
+ local *merlyn = \$randal; # just alias $merlyn, not @merlyn etc
A local() modifies its listed variables to be local to the enclosing
-block, (or subroutine, C<eval{}> or C<do>) and I<any called from
+block, (or subroutine, C<eval{}>, or C<do>) and I<any called from
within that block>. A local() just gives temporary values to global
(meaning package) variables. 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
+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
for $i ( 0 .. 9 ) {
$digits{$i} = $i;
- }
+ }
# assume this function uses global %digits hash
- parse_num();
+ parse_num();
# now temporarily add to %digits hash
if ($base12) {
supplies a scalar context.
+A note about C<local()> and composite types is in order. Something
+like C<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 C<*foo> typeglob) for the duration
+of the dynamic scope within which the C<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 C<%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
+localize a member of a composite type (i.e. an array or hash element).
+In this case, the element is localized I<by name>. This means that
+when the scope of the C<local()> ends, the saved value will be
+restored to the hash element whose key was named in the C<local()>, or
+the array element whose index was named in the C<local()>. If that
+element was deleted while the C<local()> was in effect (e.g. by a
+C<delete()> from a hash or a C<shift()> of an array), it will spring
+back into existence, possibly extending an array and filling in the
+skipped elements with C<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 $_ ? $_ : 'undef' } @ary),"\n";
+
+Perl will print
+
+ 6 . . .
+ 4 . . .
+ 3 . . .
+ This is a test only a test.
+ The array has 6 elements: 0, 1, 2, undef, undef, 5
+
+In short, be careful when manipulating the containers for composite types
+whose elements have been localized.
+
=head2 Passing Symbol Table Entries (typeglobs)
[Note: The mechanism described in this section was originally the only
funny prefix characters on variables and subroutines and such.
When evaluated, the typeglob produces a scalar value that represents
-all the objects of that name, including any filehandle, format or
+all the objects of that name, including any filehandle, format, or
subroutine. When assigned to, it causes the name mentioned to refer to
whatever "*" value was assigned to it. Example:
scalar arguments without using this mechanism by referring explicitly
to C<$_[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
+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
-L<perldata/"Typeglobs and FileHandles">.
+L<perldata/"Typeglobs and Filehandles">.
=head2 Pass by Reference
my @retlist = ();
foreach $aref ( @_ ) {
push @retlist, pop @$aref;
- }
+ }
return @retlist;
- }
+ }
-Here's how you might write a function that returns a
+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 );
+ @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
+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.
+a little expensive.
Where people get into trouble is here:
return ($cref, $dref);
} else {
return ($dref, $cref);
- }
- }
+ }
+ }
It turns out that you can actually do this also:
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 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>;
+ }
+
+Another way to do this is using *HANDLE{IO}, see L<perlref> 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, $path) ? *FH : undef;
+ }
+
+Although that will actually produce a small memory leak. See the bottom
+of L<perlfunc/open()> for a somewhat cleaner way using the IO::Handle
+package.
+
=head2 Prototypes
As of the 5.002 release of perl, if you declare
sub func ($) {
my $n = shift;
print "you gave me $n\n";
- }
+ }
and someone has been calling it with an array or expression
returning a list:
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.
+to make the world a better place.
+
+=head2 Constant Functions
+
+Functions with a prototype of C<()> 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 C<&> or C<do>. Calls made using C<&> or C<do> are never
+inlined. (See constant.pm for an easy way to declare most
+constants.)
+
+All of the following functions would 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 which 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 C<()> 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 $];
+ }
=head2 Overriding Builtin Functions
Overriding may be done only by importing the name from a
module--ordinary predeclaration isn't good enough. However, the
-C<subs> pragma (compiler directive) lets you, in effect, pre-declare subs
+C<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 C<CORE::>. For example,
+saying C<CORE::open()> will always refer to the builtin C<open()>, even
+if the current package has imported some other subroutine called
+C<&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.
they would get the default imports without the overrides.
+Note that such overriding is restricted to the package that requests
+the import. Some means of "globally" overriding builtins may become
+available in future.
+
=head2 Autoloading
If you call a subroutine that is undefined, you would ordinarily get an
my $program = $AUTOLOAD;
$program =~ s/.*:://;
system($program, @_);
- }
+ }
date();
who('am', 'i');
ls('-l');
-In fact, if you pre-declare the functions you want to call that way, you don't
+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);
=head1 SEE ALSO
See L<perlref> for more on references. See L<perlxs> if you'd
-like to learn about calling C subroutines from perl. See
-L<perlmod> to learn about bundling up your functions in
+like to learn about calling C subroutines from perl. See
+L<perlmod> to learn about bundling up your functions in
separate files.