Want - A generalisation of wantarray
use Want;
sub foo :lvalue {
if (want(qw'LVALUE ASSIGN')) {
print "We have been assigned ", want('ASSIGN');
lnoreturn;
}
elsif (want('LIST')) {
rreturn (1, 2, 3);
}
elsif (want('BOOL')) {
rreturn 0;
}
elsif (want(qw'SCALAR !REF')) {
rreturn 23;
}
elsif (want('HASH')) {
rreturn { foo => 17, bar => 23 };
}
return; # You have to put this at the end to keep the compiler happy
}
This module generalises the mechanism of the wantarray function, allowing a function to determine in some detail how its return value is going to be immediately used.
The three kinds of top-level context are well known:
foo();, or the last component of a compound statement which is itself in
void context, such as $test || foo();n. Be warned that the last statement
of a subroutine will be in whatever context the subroutine was called in, because
the result is implicitly returned.
The return value is being treated as a scalar value of some sort:
my $x = foo();
$y += foo();
print "123" x foo();
print scalar foo();
warn foo()->{23};
...etc...
The return value is treated as a list of values:
my @x = foo();
my ($x) = foo();
() = foo(); # even though the results are discarded
print foo();
bar(foo()); # unless the bar subroutine has a prototype
print @hash{foo()}; # (hash slice)
...etc...
The introduction of lvalue subroutines in Perl 5.6 has created a new type of contextual information, which is independent of those listed above. When an lvalue subroutine is called, it can either be called in the ordinary way (so that its result is treated as an ordinary value, an rvalue); or else it can be called so that its result is considered updatable, an lvalue.
These rather arcane terms (lvalue and rvalue) are easier to remember if you
know why they are so called. If you consider a simple assignment statement
left = right, then the left-hand side is an lvalue and the right-hand
side is an rvalue.
So (for lvalue subroutines only) there are two new types of context:
The caller is definitely not trying to assign to the result:
foo(); my $x = foo(); ...etc...
If the sub is declared without the :lvalue attribute, then it will
always be in RVALUE context.
If you need to return values from an lvalue subroutine in RVALUE context,
you should use the rreturn function rather than an ordinary return.
Otherwise you'll probably get a compile-time error in perl 5.6.1 and later.
Either the caller is directly assigning to the result of the sub call:
foo() = $x; foo() = (1, 1, 2, 3, 5, 8);
or the caller is making a reference to the result, which might be assigned to later:
my $ref = \(foo()); # Could now have: $$ref = 99; # Note that this example imposes LIST context on the sub call. # So we're taking a reference to the first element to be # returned _in list context_. # If we want to call the function in scalar context, we can # do it like this: my $ref = \(scalar foo());
or else the result of the function call is being used as part of the argument list for another function call:
bar(foo()); # Will *always* call foo in lvalue context, # (provided that foo is an C<:lvalue> sub) # regardless of what bar actually does.
The reason for this last case is that bar might be a sub which modifies its arguments. They're rare in contemporary Perl code, but perfectly possible:
sub bar {
$_[0] = 23;
}
(This is really a throwback to Perl 4, which didn't support explicit references.)
The commonest use of lvalue subroutines is with the assignment statement:
size() = 12; (list()) = (1..10);
A useful motto to remember when thinking about assignment statements is context comes from the left. Consider code like this:
my ($x, $y, $z);
sub list () :lvalue { ($x, $y, $z) }
list = (1, 2, 3);
print "\$x = $x; \$y = $y; \$z = $z\n";
This prints $x = ; $y = ; $z = 3, which may not be what you were expecting.
The reason is that the assignment is in scalar context, so the comma operator
is in scalar context too, and discards all values but the last. You can fix
it by writing (list) = (1,2,3); instead.
If your lvalue subroutine is used on the left of an assignment statement,
it's in ASSIGN context. If ASSIGN is the only argument to want(), then
it returns a reference to an array of the value(s) of the right-hand side.
In this case, you should return with the lnoreturn function, rather than
an ordinary return.
This makes it very easy to write lvalue subroutines which do clever things:
use Want;
use strict;
sub backstr :lvalue {
if (want(qw'LVALUE ASSIGN')) {
my ($a) = want('ASSIGN');
$_[0] = reverse $a;
lnoreturn;
}
elsif (want('RVALUE')) {
rreturn scalar reverse $_[0];
}
else {
carp("Not in ASSIGN context");
}
return
}
print "foo -> ", backstr("foo"), "\n"; # foo -> oof
backstr(my $robin) = "nibor";
print "\$robin is now $robin\n"; # $robin is now robin
Notice that you need to put a (meaningless) return statement at the end of the function, otherwise you will get the error Can't modify non-lvalue subroutine call in lvalue subroutine return.
The only way to write that backstr function without using Want is to return
a tied variable which is tied to a custom class.
Sometimes in scalar context the caller is expecting a reference of some sort to be returned:
print foo()->(); # CODE reference expected
print foo()->{bar}; # HASH reference expected
print foo()->[23]; # ARRAY reference expected
print ${foo()}; # SCALAR reference expected
print foo()->bar(); # OBJECT reference expected
my $format = *{foo()}{FORMAT} # GLOB reference expected
You can check this using conditionals like if (want('CODE')).
There is also a function wantref() which returns one of the strings
"CODE", "HASH", "ARRAY", "GLOB", "SCALAR" or "OBJECT"; or the empty string
if a reference is not expected.
Because want('SCALAR') is already used to select ordinary scalar context,
you have to use want('REFSCALAR') to find out if a SCALAR reference is
expected. Or you could use want('REF') eq 'SCALAR' of course.
Be warned that want('ARRAY') is a very different thing from wantarray().
Sometimes in list context the caller is expecting a particular number of items to be returned:
my ($x, $y) = foo(); # foo is expected to return two items
If you pass a number to the want function, then it will return true or false
according to whether at least that many items are wanted. So if we are in the
definition of a sub which is being called as above, then:
want(1) returns true
want(2) returns true
want(3) returns false
Sometimes there is no limit to the number of items that might be used:
my @x = foo();
do_something_with( foo() );
In this case, want(2), want(100), want(1E9) and so on will all return
true; and so will want('Infinity').
The howmany function can be used to find out how many items are wanted.
If the context is scalar, then want(1) returns true and howmany() returns
1. If you want to check whether your result is being assigned to a singleton
list, you can say if (want('LIST', 1)) { ... }.
Sometimes the caller is only interested in the truth or falsity of a function's return value:
if (everything_is_okay()) {
# Carry on
}
print (foo() ? "ok\n" : "not ok\n");
In the following example, all subroutine calls are in BOOL context:
my $x = ( (foo() && !bar()) xor (baz() || quux()) );
Boolean context, like the reference contexts above, is considered to be a subcontext of SCALAR.
This is the primary interface to this module, and should suffice for most purposes. You pass it a list of context specifiers, and the return value is true whenever all of the specifiers hold.
want('LVALUE', 'SCALAR'); # Are we in scalar lvalue context?
want('RVALUE', 3); # Are at least three rvalues wanted?
want('ARRAY'); # Is the return value used as an array ref?
You can also prefix a specifier with an exclamation mark to indicate that you don't want it to be true
want(2, '!3'); # Caller wants exactly two items.
want(qw'REF !CODE !GLOB'); # Expecting a reference that
# isn't a CODE or GLOB ref.
want(100, '!Infinity'); # Expecting at least 100 items,
# but there is a limit.
If the REF keyword is the only parameter passed, then the type of reference will be
returned. This is just a synonym for the wantref function: it's included because
you might find it useful if you don't want to pollute your namespace by importing
several functions, and to conform to Damian Conway's suggestion in RFC 21.
Finally, the keyword COUNT can be used, provided that it's the only keyword
you pass. Mixing COUNT with other keywords is an error. This is a synonym for the
howmany function.
A full list of the permitted keyword is in the ARGUMENTS section below.
return from inside an lvalue subroutine when
you know that you're in RVALUE context. If you try to use a normal return,
you'll get a compile-time error in Perl 5.6.1 and above unless you return an
lvalue.
Use this function instead of return from inside an lvalue subroutine when
you're in ASSIGN context and you've used want('ASSIGN') to carry out the
appropriate action.
If you use rreturn or lnoreturn, then you have to put a bare return;
at the very end of your lvalue subroutine, in order to stop the Perl compiler
from complaining. Think of it as akin to the 1; that you have to put at the
end of a module.
Returns the expectation count, i.e. the number of items expected. If the expectation count is undefined, that indicates that an unlimited number of items might be used (e.g. the return value is being assigned to an array). In void context the expectation count is zero, and in scalar context it is one.
The same as want('COUNT').
Returns the type of reference which the caller is expecting, or the empty string if the caller isn't expecting a reference immediately.
The same as want('REF').
use Carp 'croak';
use Want 'howmany';
sub numbers {
my $count = howmany();
croak("Can't make an infinite list") if !defined($count);
return (1..$count);
}
my ($one, $two, $three) = numbers();
use Want 'want';
sub pi () {
if (want('ARRAY')) {
return [3, 1, 4, 1, 5, 9];
}
elsif (want('LIST')) {
return (3, 1, 4, 1, 5, 9);
}
else {
return 3;
}
}
print pi->[2]; # prints 4
print ((pi)[3]); # prints 1
The permitted arguments to the want function are listed below.
The list is structured so that sub-contexts appear below the context that they
are part of.
SCALAR
REF
LIST
LVALUE
The want and rreturn functions are exported by default.
The wantref and/or howmany functions can also be imported:
use Want qw'want howmany';
If you don't import these functions, you must qualify their names as (e.g.)
Want::wantref.
This module is still under development, and the public interface may change in
future versions. The want function can now be regarded as stable.
I'd be interested to know how you're using this module.
There are two different levels of BOOL context. Pure boolean context
occurs in conditional expressions, and the operands of the xor and !/not
operators.
Pure boolean context also propagates down through the && and || operators.
However, consider an expression like my $x = foo() && "yes". The subroutine
is called in pseudo-boolean context - its return value isn't entirely
ignored, because the undefined value, the empty string and the integer 0 are
all false.
At the moment want('BOOL') is true in either pure or pseudo boolean
context. Let me know if this is a problem.
* Doesn't work from inside a tie-handler.
Robin Houston, <robin@cpan.org>
Thanks to Damian Conway for encouragement and good suggestions.
Copyright (c) 2001-2006, Robin Houston. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the same terms as Perl itself.