To define an anonymous subroutine at runtime:
To import subroutines:
To call subroutines:
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 values are implicit
references (predating
the perlref manpage
) 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.
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 sections below on L<"Private Variables via my() ``> and ''Temporary Values via local() ". To create protected environments for a set of functions in a separate package (and probably a separate file), see ``Packages''.
Example:
Example:
Use array assignment to a local list to name your formal arguments:
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 callers values.
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:
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:
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:
Do not, however, be tempted to do this:
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 ``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 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 &
$<
EM>subref()
or
&{$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.
A ``my'' declares the listed variables to be confined (lexically) to the enclosing block, subroutine, eval , or 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.
Unlike dynamic variables created by the ``local'' statement, 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).
(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:
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
both supply a list context to the righthand side, while
supplies a scalar context. But the following only declares one variable:
That has the same effect as
The declared variable is not introduced (is not visible) until after the current statement. Thus,
can be used to initialize the new $x with the value of the old $x, and the expression
is false unless the old $x happened to have the value 123.
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
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 doesn't happen until run time, so gets executed every time through a loop.
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:
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:
That will print out 20 and 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 to a 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 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:
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 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.
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.
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 pain program, or more likely, merely placing a BEGIN sub around it to make sure it gets executed before your program starts to run:
See the perlrun manpage about the BEGIN function.
Synopsis:
A
local()
modifies its listed variables to be local to the enclosing
block, (or subroutine,
eval{}
or
do
) and I 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
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:
Because
local()
is a run-time command, and so 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
as a scalar or an array. So
both supply a list context to the righthand side, while
supplies a scalar context.
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: When evaluated, the type glob 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:
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, since 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 ``Typeglobs''.
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:
Here's how you might write a function that returns a
list of keys occurring in all the hashes passed to it:
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
concatenating, then the normal calling convention is ok, although
a little expensive.
Where people get into trouble is here:
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.
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:
It turns out that you can actually do this also:
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.
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:
If you're planning on generating new filehandles, you could do this:
Although that will actually produce a small memory leak. See the bottom
of
open
for a somewhat cleaner way using the FileHandle
functions supplied with the POSIX package.
then mypush() takes arguments exactly like
push()
does. (This only works
for function calls that are visible at compile time, not indirect function
calls through a Here are the prototypes for some other functions that parse almost exactly
like the corresponding builtins.
Any backslashed prototype character must be passed something starting
with that character. 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, and *
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.
Note that the last three are syntactically distinguished by the lexer.
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
say
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:
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? (Nevermind.))))
And here's a reimplementation of grep:
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:
and someone has been calling it with an array or expression
returning a list:
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 elments
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 only be used in moderation
to make the world a better place.
Overriding may only be done by importing the name from a
module--ordinary predeclaration isn't good enough. However, the
Library modules should not in general export builtin names like ``open''
or ``chdir'' as part of their default @EXPORT list, since 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
and it would import the open override, but if they said
they would get the default imports without the overrides.
Most
In fact, if you preclare the functions you want to call that way, you don't
even need the parentheses:
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 Autoloader, the standard SelfLoader modules in SelfLoader, and
the document on adding C functions to perl code in
the perlxs manpage
.
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.]
*foo
. This is often known as a ``type glob'', since 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.
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; see
the perlref manpage
.
Prototypes
As of the 5.002 release of perl, if you declare
&$func
reference nor for method calls as described in
the perlobj manpage
.)
Overriding Builtin Functions
Many builtin functions may be overridden, though this should only be
tried occasionally and for good reason. Typically this might be
done by a package attempting to emulate missing builtin functionality
on a non-Unix system.
subs
pragma (compiler directive) lets you, in effect, predeclare subs
via the import syntax, and these names may then override the builtin ones:
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,
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...
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:
SEE ALSO
See
the perlref manpage
for more on references. 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.