To define an anonymous subroutine at runtime:
- $subref = sub BLOCK;
+ $subref = sub BLOCK; # no proto
+ $subref = sub (PROTO) BLOCK; # with proto
To import subroutines:
To call subroutines:
- NAME(LIST); # & is optional with parens.
- NAME LIST; # Parens optional if predeclared/imported.
- &NAME; # Passes current @_ to subroutine.
+ NAME(LIST); # & is optional with parentheses.
+ NAME LIST; # Parentheses optional if predeclared/imported.
+ &NAME; # Makes current @_ visible to called subroutine.
=head1 DESCRIPTION
via the C<do>, C<require>, or C<use> keywords, or even generated on the
fly using C<eval> or anonymous subroutines (closures). You can even call
a function indirectly using a variable containing its name or a CODE reference
-to it, as in C<$var = \&function>.
+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
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
+Any arguments passed to the routine come in as the array C<@_>. 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<$_[1]>. The array C<@_> is a local array, but its elements are
+aliases for the actual scalar parameters. In particular, if an element
+C<$_[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 C<@_> 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 C<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
+assign to a C<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
L<"Private Variables via my()"> and L<"Temporary Values via local()">.
sub get_line {
$thisline = $lookahead; # GLOBAL VARIABLES!!
- LINE: while ($lookahead = <STDIN>) {
+ LINE: while (defined($lookahead = <STDIN>)) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
}
This also has the effect of turning call-by-reference into call-by-value,
-since the assignment copies the values. Otherwise a function is free to
-do in-place modifications of @_ and change its caller's values.
+because the assignment copies the values. Otherwise a function is free to
+do in-place modifications of C<@_> and change its caller's values.
upcase_in($v1, $v2); # this changes $v1 and $v2
sub upcase_in {
- for (@_) { tr/a-z/A-Z/ }
- }
+ for (@_) { tr/a-z/A-Z/ }
+ }
You aren't allowed to modify constants in this way, of course. If an
argument were actually literal and you tried to change it, you'd take a
upcase_in("frederick");
-It would be much safer if the upcase_in() function
+It would be much safer if the C<upcase_in()> function
were written to return a copy of its parameters instead
of changing them in place:
($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 : $parms[0];
- }
+ }
Notice how this (unprototyped) function doesn't care whether it was passed
-real scalars or arrays. Perl will see everything as one big long flat @_
+real scalars or arrays. Perl will see everything as one big long flat C<@_>
parameter list. This is one of the ways where Perl's simple
-argument-passing style shines. The upcase() function would work perfectly
-well without changing the upcase() definition even if we fed it things
+argument-passing style shines. The C<upcase()> function would work perfectly
+well without changing the C<upcase()> definition even if we fed it things
like this:
@newlist = upcase(@list1, @list2);
(@a, @b) = upcase(@list1, @list2);
Because like its flat incoming parameter list, the return list is also
-flat. So all you have managed to do here is stored everything in @a and
-made @b an empty list. See L</"Pass by Reference"> for alternatives.
-
-A subroutine may be called using the "&" prefix. The "&" is optional in
-Perl 5, and so are the parens if the subroutine has been predeclared.
-(Note, however, that the "&" is I<NOT> optional when you're just naming
-the subroutine, such as when it's used as an argument to defined() or
-undef(). Nor is it optional when you want to do an indirect subroutine
-call with a subroutine name or reference using the C<&$subref()> or
-C<&{$subref}()> constructs. See L<perlref> for more on that.)
+flat. So all you have managed to do here is stored everything in C<@a> and
+made C<@b> an empty list. See L<Pass by Reference> for alternatives.
+
+A subroutine may be called using the "C<&>" prefix. The "C<&>" is optional
+in modern Perls, and so are the parentheses if the subroutine has been
+predeclared. (Note, however, that the "C<&>" is I<NOT> optional when
+you're just naming the subroutine, such as when it's used as an
+argument to C<defined()> or C<undef()>. Nor is it optional when you want to
+do an indirect subroutine call with a subroutine name or reference
+using the C<&$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
-set up for the subroutine: the @_ array at the time of the call is
+the "C<&>" form, the argument list is optional, and if omitted, no C<@_> array is
+set up for the subroutine: the C<@_> 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(); # 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
+Not only does the "C<&>" 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.
-=head2 Private Variables via my()
+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 C<BEGIN>, C<END>, C<AUTOLOAD>, and C<DESTROY>--plus all the
+functions mentioned in L<perltie>. The 5.005 release adds C<INIT>
+to this list.
+
+=head2 Private Variables via C<my()>
Synopsis:
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, subroutine, C<eval>, or C<do/require/use>'d file. If
-more than one value is listed, the list must be placed in parens. All
-listed elements must be legal lvalues. Only alphanumeric identifiers may
-be lexically scoped--magical builtins like $/ must currently be localized with
-"local" instead.
+A "C<my>" declares the listed variables to be confined (lexically) to the
+enclosing block, conditional (C<if/unless/elsif/else>), loop
+(C<for/foreach/while/until/continue>), subroutine, C<eval>, or
+C<do/require/use>'d file. If more than one value is listed, the list
+must be placed in parentheses. All listed elements must be legal lvalues.
+Only alphanumeric identifiers may be lexically scoped--magical
+builtins like C<$/> must currently be C<local>ize with "C<local>" instead.
-Unlike dynamic variables created by the "local" statement, lexical
-variables declared with "my" are totally hidden from the outside world,
+Unlike dynamic variables created by the "C<local>" operator, lexical
+variables declared with "C<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).
-(An eval(), however, can see the lexical variables of the scope it is
+This doesn't mean that a C<my()> variable declared in a statically
+I<enclosing> lexical scope would be invisible. Only the dynamic scopes
+are cut off. For example, the C<bumpx()> function below has access to the
+lexical C<$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 C<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 L<perlref>.)
+the C<eval()> itself. See L<perlref>.)
-The parameter list to my() may be assigned to if desired, which allows you
+The parameter list to C<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:
my $arg = shift; # name doesn't matter
$arg **= 1/3;
return $arg;
- }
+ }
-The "my" is simply a modifier on something you might assign to. So when
-you do assign to the variables in its argument list, the "my" doesn't
-change whether those variables is viewed as a scalar or an array. So
+The "C<my>" is simply a modifier on something you might assign to. So when
+you do assign to the variables in its argument list, the "C<my>" doesn't
+change whether those variables are viewed as a scalar or an array. So
- my ($foo) = <STDIN>;
+ my ($foo) = <STDIN>; # WRONG?
my @FOO = <STDIN>;
-both supply a list context to the righthand side, while
+both supply a list context to the right-hand side, while
my $foo = <STDIN>;
-supplies a scalar context. But the following only declares one variable:
+supplies a scalar context. But the following declares only one variable:
- my $foo, $bar = 1;
+ my $foo, $bar = 1; # WRONG
That has the same effect as
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 $x with the value of the old C<$x>, and
the expression
my $x = 123 and $x == 123
-is false unless the old $x happened to have the value 123.
+is false unless the old C<$x> happened to have the value C<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 $line;
+ }
+
+the scope of C<$line> extends from its declaration throughout the rest of
+the loop construct (including the C<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 C<$answer> extends from its declaration throughout the rest
+of the conditional (including C<elsif> and C<else> clauses, if any),
+but not beyond it.
+
+(None of the foregoing applies to C<if/unless> or C<while/until>
+modifiers appended to simple statements. Such modifiers are not
+control structures and have no effect on scoping.)
+
+The C<foreach> loop defaults to scoping its index variable dynamically
+(in the manner of C<local>; see below). However, if the index
+variable is prefixed with the keyword "C<my>", then it is lexically
+scoped instead. Thus in the loop
+
+ for my $i (1, 2, 3) {
+ some_function();
+ }
+
+the scope of C<$i> extends to the end of the loop, but not beyond it, and
+so the value of C<$i> is unavailable in C<some_function()>.
Some users may wish to encourage the use of lexically scoped variables.
As an aid to catching implicit references to package variables,
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 S<"no strict 'vars'">.
+otherwise. An inner block may countermand this with S<"C<no strict 'vars'>">.
-A my() has both a compile-time and a run-time effect. At compile time,
+A C<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 S<"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
+Variables declared with "C<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 $_; # 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
+are still accessible using the fully qualified C<::> notation even while a
lexical of the same name is also visible:
package main;
my $x = 20;
print "$x and $::x\n";
-That will print out 20 and 10.
+That will print out C<20> and C<10>.
-You may declare "my" variables at the outer most scope of a file to
-totally hide any such identifiers from the outside world. This is similar
+You may declare "C<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). If a block (such as
-an eval(), function, or C<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:
+requires the use of a closure (anonymous function with lexical access).
+If a block (such as an C<eval()>, function, or C<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, since its name is not in
+module, no outside module can see the subroutine, because its name is not in
any package's symbol table. Remember that it's not I<REALLY> called
-$some_pack::secret_version or anything; it's just $secret_version,
+C<$some_pack::secret_version> or anything; it's just C<$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.
-Just because the lexical variable is lexically (also called statically)
-scoped doesn't mean that within a function it works like a C static. It
-normally works more like a C auto. But 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.
+=head2 Peristent Private Variables
+
+Just because a lexical variable is lexically (also called statically)
+scoped to its enclosing block, C<eval>, or C<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;
+ 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 C<my()>
to be executed early, either by putting the whole block above
-your pain program, or more likely, merely placing a BEGIN
+your main program, or more likely, placing merely a C<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<perlmod/"Package Constructors and Destructors"> about the C<BEGIN> function.
-See L<perlrun> 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.
=head2 Temporary Values via local()
-B<NOTE>: In general, you should be using "my" instead of "local", because
+B<NOTE>: In general, you should be using "C<my>" instead of "C<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
+symbol table itself. Format variables often use "C<local>" though, as do
other variables whose current value must be visible to called
subroutines.
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
-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.
+A C<local()> modifies its listed variables to be "local" to the enclosing
+block, C<eval>, or C<do FILE>--and to I<any subroutine called from within that block>.
+A C<local()> just gives temporary values to global (meaning package)
+variables. It does B<not> create a local variable. This is known as
+dynamic scoping. Lexical scoping is done with "C<my>", which works more
+like C's auto declarations.
-If more than one variable is given to local(), they must be placed in
-parens. All listed elements must be legal lvalues. This operator works
+If more than one variable is given to C<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) {
}
# old %digits restored here
-Because local() is a run-time command, it gets executed every time
+Because C<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 localized variable, the local doesn't change whether its list is viewed
+A C<local> is simply a modifier on an lvalue expression. When you assign to
+a C<local>ized variable, the C<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 righthand side, while
+both supply a list context to the right-hand side, while
local $foo = <STDIN>;
supplies a scalar context.
+A note about C<local()> and composite types is in order. Something
+like C<local(%foo)> works by temporarily placing a brand new hash in
+the symbol table. The old hash is left alone, but is hidden "behind"
+the new one.
+
+This means the old variable is completely invisible via the symbol
+table (i.e. the hash entry in the C<*foo> typeglob) for the duration
+of the dynamic scope within which the C<local()> was seen. This
+has the effect of allowing one to temporarily occlude any magic on
+composite types. For instance, this will briefly alter a tied
+hash to some other implementation:
+
+ tie %ahash, 'APackage';
+ [...]
+ {
+ local %ahash;
+ tie %ahash, 'BPackage';
+ [..called code will see %ahash tied to 'BPackage'..]
+ {
+ local %ahash;
+ [..%ahash is a normal (untied) hash here..]
+ }
+ }
+ [..%ahash back to its initial tied self again..]
+
+As another example, a custom implementation of C<%ENV> might look
+like this:
+
+ {
+ local %ENV;
+ tie %ENV, 'MyOwnEnv';
+ [..do your own fancy %ENV manipulation here..]
+ }
+ [..normal %ENV behavior here..]
+
+It's also worth taking a moment to explain what happens when you
+C<local>ize a member of a composite type (i.e. an array or hash element).
+In this case, the element is C<local>ized I<by name>. This means that
+when the scope of the C<local()> ends, the saved value will be
+restored to the hash element whose key was named in the C<local()>, or
+the array element whose index was named in the C<local()>. If that
+element was deleted while the C<local()> was in effect (e.g. by a
+C<delete()> from a hash or a C<shift()> of an array), it will spring
+back into existence, possibly extending an array and filling in the
+skipped elements with C<undef>. For instance, if you say
+
+ %hash = ( 'This' => 'is', 'a' => 'test' );
+ @ary = ( 0..5 );
+ {
+ local($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 $_ ? $_ : 'undef' } @ary),"\n";
+
+Perl will print
+
+ 6 . . .
+ 4 . . .
+ 3 . . .
+ This is a test only a test.
+ The array has 6 elements: 0, 1, 2, undef, undef, 5
+
=head2 Passing Symbol Table Entries (typeglobs)
[Note: The mechanism described in this section was originally the only
but rather the name of it, so that the subroutine can modify the global
copy of it rather than working with a local copy. In perl you can
refer to all objects of a particular name by prefixing the name
-with a star: C<*foo>. This is often known as a "type glob", since the
+with a star: C<*foo>. This is often known as a "typeglob", because the
star on the front can be thought of as a wildcard match for all the
funny prefix characters on variables and subroutines and such.
-When evaluated, the type glob produces a scalar value that represents
-all the objects of that name, including any filehandle, format or
+When evaluated, the typeglob produces a scalar value that represents
+all the objects of that name, including any filehandle, format, or
subroutine. When assigned to, it causes the name mentioned to refer to
-whatever "*" value was assigned to it. Example:
+whatever "C<*>" value was assigned to it. Example:
sub doubleary {
local(*someary) = @_;
Note that scalars are already passed by reference, so you can modify
scalar arguments without using this mechanism by referring explicitly
to C<$_[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
+all the elements as scalars, but you have to use the C<*> mechanism (or
+the equivalent reference mechanism) to C<push>, C<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, since normally the LIST
+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">.
+the individual arrays. For more on typeglobs, see
+L<perldata/"Typeglobs and Filehandles">.
+
+=head2 When to Still Use local()
+
+Despite the existence of C<my()>, there are still three places where the
+C<local()> operator still shines. In fact, in these three places, you
+I<must> use C<local> instead of C<my>.
+
+=over
+
+=item 1. You need to give a global variable a temporary value, especially C<$_>.
+
+The global variables, like C<@ARGV> or the punctuation variables, must be
+C<local>ized with C<local()>. This block reads in F</etc/motd>, and splits
+it up into chunks separated by lines of equal signs, which are placed
+in C<@Fields>.
+
+ {
+ local @ARGV = ("/etc/motd");
+ local $/ = undef;
+ local $_ = <>;
+ @Fields = split /^\s*=+\s*$/;
+ }
+
+It particular, it's important to C<local>ize C<$_> in any routine that assigns
+to it. Look out for implicit assignments in C<while> conditionals.
+
+=item 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 C<local()> uses
+C<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 L<perlref/"Function Templates"> for more about manipulating
+functions by name in this way.
+
+=item 3. You want to temporarily change just one element of an array or hash.
+
+You can C<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.
+
+=back
=head2 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 L<perlref>.
+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 L<perlref>.
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
+arrays to a function and have it C<pop> all of then, return a new
list of all their former last elements:
@tailings = popmany ( \@a, \@b, \@c, \@d );
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 just using the normal list return mechanism.
-What happens if you want to pass or return a hash? Well,
-if you're only using one of them, or you don't mind them
+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.
+a little expensive.
Where people get into trouble is here:
or
(%a, %b) = func(%c, %d);
-That syntax simply won't work. It just sets @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.
+That syntax simply won't work. It sets just C<@a> or C<%a> and clears the C<@b> or
+C<%b>. Plus the function didn't get passed into two separate arrays or
+hashes: it got one long list in C<@_>, 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
return ($cref, $dref);
} else {
return ($dref, $cref);
- }
- }
+ }
+ }
It turns out that you can actually do this also:
return (\@c, \@d);
} else {
return (\@d, \@c);
- }
- }
+ }
+ }
Here we're using the typeglobs to do symbol table aliasing. It's
-a tad subtle, though, and also won't work if you're using my()
-variables, since only globals (well, and local()s) are in the symbol table.
+a tad subtle, though, and also won't work if you're using C<my()>
+variables, because only globals (well, and C<local()>s) are in the symbol table.
If you're passing around filehandles, you could usually just use the bare
-typeglob, like *STDOUT, but typeglobs references would be better because
-they'll still work properly under C<use strict 'refs'>. For example:
+typeglob, like C<*STDOUT>, but typeglobs references would be better because
+they'll still work properly under S<C<use strict 'refs'>>. For example:
splutter(\*STDOUT);
sub splutter {
return scalar <$fh>;
}
+Another way to do this is using C<*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;
- }
+ return open (FH, $path) ? *FH : undef;
+ }
Although that will actually produce a small memory leak. See the bottom
-of L<perlfunc/open()> for a somewhat cleaner way using the FileHandle
-functions supplied with the POSIX package.
+of L<perlfunc/open()> for a somewhat cleaner way using the C<IO::Handle>
+package.
=head2 Prototypes
sub mypush (\@@)
-then mypush() takes arguments exactly like push() does. The declaration
+then C<mypush()> takes arguments exactly like C<push()> does. The declaration
of the function to be called must be visible at compile time. The prototype
-only affects the interpretation of new-style calls to the function, where
+affects only the interpretation of new-style calls to the function, where
new-style is defined as not using the C<&> character. In other words,
if you call it like a builtin function, then it behaves like a builtin
function. If you call it like an old-fashioned subroutine, then it
like C<\&foo> or on indirect subroutine calls like C<&{$subref}>.
Method calls are not influenced by prototypes either, because the
-function to be called is indeterminate at compile time, since it depends
+function to be called is indeterminate at compile time, because it depends
on inheritance.
-Since the intent is primarily to let you define subroutines that work
+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 (\@$$@) mysplice @array,@array,0,@pushme
- sub mykeys (\%) mykeys %{$hashref}
- sub myopen (*;$) myopen HANDLE, $name
- sub mypipe (**) mypipe READHANDLE, WRITEHANDLE
- sub mygrep (&@) mygrep { /foo/ } $a,$b,$c
- sub myrand ($) myrand 42
- sub mytime () mytime
+ sub mylink ($$) 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
C<\> 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
-* does whatever it has to do to turn the argument into a reference to a
+unbackslashed C<@> or C<%> eats all the rest of the arguments, and forces
+list context. An argument represented by C<$> forces scalar context. An
+C<&> requires an anonymous subroutine, which, if passed as the first
+argument, does not require the "C<sub>" keyword or a subsequent comma. A
+C<*> does whatever it has to do to turn the argument into a reference to a
symbol table entry.
A semicolon separates mandatory arguments from optional arguments.
-(It is redundant before @ or %.)
+(It is redundant before C<@> or C<%>.)
Note how the last three examples above are treated specially by the parser.
-mygrep() is parsed as a true list operator, myrand() is parsed as a
-true unary operator with unary precedence the same as rand(), and
-mytime() is truly argumentless, just like time(). That is, if you
+C<mygrep()> is parsed as a true list operator, C<myrand()> is parsed as a
+true unary operator with unary precedence the same as C<rand()>, and
+C<mytime()> is truly without arguments, just like C<time()>. That is, if you
say
mytime +2;
-you'll get mytime() + 2, not mytime(2), which is how it would be parsed
+you'll get C<mytime() + 2>, not C<mytime(2)>, which is how it would be parsed
without the prototype.
-The interesting thing about & is that you can generate new syntax with it:
+The interesting thing about C<&> is that you can generate new syntax with it:
sub try (&@) {
my($try,$catch) = @_;
&$catch;
}
}
- sub catch (&) { @_ }
+ sub catch (&) { $_[0] }
try {
die "phooey";
/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
+That prints C<"unphooey">. (Yes, there are still unresolved
+issues having to do with the visibility of C<@_>. I'm ignoring that
+question for the moment. (But note that if we make C<@_> lexically
scoped, those anonymous subroutines can act like closures... (Gee,
-is this sounding a little Lispish? (Nevermind.))))
+is this sounding a little Lispish? (Never mind.))))
-And here's a reimplementation of grep:
+And here's a reimplementation of C<grep>:
sub mygrep (&@) {
my $code = shift;
sub func ($) {
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
+Then you've just supplied an automatic C<scalar()> in front of their
+argument, which can be more than a bit surprising. The old C<@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 elments
-in @foo. And the split() gets called in a scalar context and
-starts scribbling on your @_ parameter list.
+the C<func()> now gets passed in C<1>, that is, the number of elements
+in C<@foo>. And the C<split()> gets called in a scalar context and
+starts scribbling on your C<@_> parameter list.
+
+This is all very powerful, of course, and should be used only in moderation
+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 F<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 }
+ }
-This is all very powerful, of course, and should only be used in moderation
-to make the world a better place.
+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 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
-Many builtin functions may be overridden, though this should only be
-tried occasionally and for good reason. Typically this might be
+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 only be done by importing the name from a
+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, predeclare subs
via the import syntax, and these names may then override the builtin ones:
chdir $somewhere;
sub chdir { ... }
-Library modules should not in general export builtin names like "open"
-or "chdir" as part of their default @EXPORT list, since these may
+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 "C<open>"
+or "C<chdir>" as part of their default C<@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
+Instead, if the module adds the name to the C<@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
+and it would import the C<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 C<CORE::GLOBAL::>. Here is an
+example that quite brazenly replaces the C<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 C<glob> globally, you would be forcing the new (and
+subversive) behavior for the C<glob> operator for B<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 C<REGlob> example above does not implement all the support needed to
+cleanly override perl's C<glob> operator. The builtin C<glob> has
+different behaviors depending on whether it appears in a scalar or list
+context, but our C<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
+C<glob>, study the implementation of C<File::DosGlob> in the standard
+library.
+
+
=head2 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 of the base classes of the class package.) If,
+doesn't exist in any base class of the class package.) If,
however, there is an C<AUTOLOAD> subroutine defined in the package or
packages that were searched for the original subroutine, then that
C<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 $AUTOLOAD variable in the
+original subroutine magically appears in the C<$AUTOLOAD> variable in the
same package as the C<AUTOLOAD> routine. The name is not passed as an
ordinary argument because, er, well, just because, that's why...
without a trace. (See the standard C<AutoLoader> module, for example.)
But an C<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:
+should just call C<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 preclare 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
-separate files.
+See L<perlref> for more about references and closures. 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 separate files.
projects / p5sagit/p5-mst-13.2.git / blobdiff