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 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 @_
A subroutine may be called using the "&" prefix. The "&" is optional
in modern Perls, and so are the parentheses if the subroutine has been
-pre-declared. (Note, however, that the "&" is I<NOT> optional when
+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
&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;
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
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) {
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()
sub openit {
my $name = shift;
local *FH;
- return open (FH, $path) ? \*FH : undef;
- }
+ return open (FH, $path) ? *FH : undef;
+ }
Although that will actually produce a small memory leak. See the bottom
of L<perlfunc/open()> for a somewhat cleaner way using the IO::Handle
sub func ($) {
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 a
-constant then it will be used in place of new-style calls to the
-function. Old-style calls (that is, calls made using C<&>) are not
-affected.
+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 }
+ 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 () { 1 }
+
+ sub OPT_BAZ () { not (0x1B58 & FLAG_MASK) }
sub BAZ_VAL () {
if (OPT_BAZ) {
return 23;
}
}
+ 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
(which changes the calling semantics, so beware) or by thwarting the
inlining mechanism in some other way, such as
- my $dummy;
sub not_inlined () {
- $dummy || 23
+ 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';
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.
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