NAME
Wanted - Extended caller context detection
SYNOPSIS
use Wanted;
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 };
}
# You have to put this at the end to keep the compiler happy
return;
}
foo() = 23; # Assign context
@x = foo(); # List context
if( foo() ) # Boolean context
{
print( "Not reached\n" );
}
Also works in threads, where the context is set at thread creation.
require threads;
# In scalar context
my $thr = threads->create(sub
{
return( want('SCALAR') );
});
my $is_scalar = $thr->join; # true
# or
my $thr = threads->create({ context => 'scalar' }, sub
{
return( want('SCALAR') );
});
my $is_scalar = $thr->join; # true
my( $thr ) = threads->create(sub
{
return( want('LIST') );
});
my @list_result = $thr->join;
# $list_result[0] is true
# or
my $thr = threads->create({ context => 'list' }, sub
{
return( want('LIST') );
});
my @list_result = $thr->join;
# $list_result[0] is true
# Force the context by being explicit:
my $thr = threads->create({ context => 'void' }, sub
{
return( want('VOID') ? 1 : 0 );
});
my $is_void = $thr_void->join; # undef
VERSION
v0.1.0
DESCRIPTION
This XS module generalises the mechanism of the wantarray function,
allowing a function to determine in detail how its return value is going
to be used.
It is a fork from the module Want, by Robin Houston, that is not updated
anymore since 2016, and that throws a segmentation fault when called
from the last line of a thread, or from a tie method, or from the last
line of a mod_perl handler, when there is a lack of context.
To install this module, run the following commands:
perl Makefile.PL
make
make test
make install
Top-level contexts:
The three kinds of top-level context are well known:
VOID
The return value is not being used in any way. It could be an entire
statement like "foo();", or the last component of a compound
statement which is itself in void context, such as "$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.
SCALAR
The return value is being treated as a scalar value of some sort:
my $x = foo();
$y += foo();
print "123" x foo();
print scalar foo();
warn foo()->{23};
# ...etc...
LIST
The return value is treated as a list of values:
my @x = foo();
my ($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...
Lvalue subroutines:
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:
RVALUE
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 will probably get a compile-time
error in perl 5.6.1 and later.
LVALUE
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 are 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 are rare in contemporary Perl code, but
perfectly possible:
sub bar {
$_[0] = 23;
}
(This is really a throwback to Perl 4, which did not support
explicit references.)
Assignment context:
The commonest use of lvalue subroutines is with the assignment
statement:
size() = 12;
(list()) = (1..10);
A useful motto to remember when thinking about assignment statements is
*context comes from the left*. Consider code like this:
my ($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 = ; $z = 3", which may not be what you were
expecting. The reason is that the assignment is in scalar context, so
the comma operator is in scalar context too, and discards all values but
the last. You can fix it by writing "(list) = (1,2,3);" instead.
If your lvalue subroutine is used on the left of an assignment
statement, it is in ASSIGN context. If ASSIGN is the only argument to
"want()", then it returns a reference to an array of the value(s) of the
right-hand side.
In this case, you should return with the "lnoreturn" function, rather
than an ordinary return.
This makes it very easy to write lvalue subroutines which do clever
things:
use Wanted;
use strict;
sub backstr :lvalue {
if (want(qw'LVALUE ASSIGN')) {
my ($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.
Reference context:
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 $format = *{foo()}{FORMAT} # GLOB reference expected
You can check this using conditionals like "if (want('CODE'))". There is
also a function "wantref()" which returns one of the strings "CODE",
"HASH", "ARRAY", "GLOB", "SCALAR" or "OBJECT"; or the empty string if a
reference is not expected.
Because "want('SCALAR')" is already used to select ordinary scalar
context, you have to use "want('REFSCALAR')" to find out if a SCALAR
reference is expected. Or you could use "want('REF') eq 'SCALAR'" of
course.
Be warned that "want('ARRAY')" is a very different thing from
"wantarray()".
Item count
Sometimes in list context the caller is expecting a particular number of
items to be returned:
my ($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)) { ...
}".
Boolean context
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".
FUNCTIONS
want(SPECIFIERS)
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 do not want it to be true
want(2, '!3'); # Caller wants exactly two items.
want(qw'REF !CODE !GLOB'); # Expecting a reference that is not 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 is included because you might find it useful if you do not
want to pollute your namespace by importing several functions, and to
conform to Damian Conway's suggestion in RFC 21
<http://dev.perl.org/rfc/21.html>.
Finally, the keyword "COUNT" can be used, provided that it is 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.
rreturn
Use this function instead of return from inside an lvalue subroutine
when you know that you are in "RVALUE" context. If you try to use a
normal return, you will get a compile-time error in Perl 5.6.1 and above
unless you return an lvalue. (Note: this is no longer true in Perl 5.16,
where an ordinary return will once again work.)
lnoreturn
Use this function instead of "return" from inside an lvalue subroutine
when you are in "ASSIGN" context and you have 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. (Note: this is no longer true in
Perl 5.16.)
howmany()
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')".
wantref()
Returns the type of reference which the caller is expecting, or the
empty string if the caller is not expecting a reference immediately.
The same as "want('REF')".
context
* "context()"
Returns a string representing the current calling context, such as
"VOID", "SCALAR", "LIST", "BOOL", "CODE", "HASH", "ARRAY", "GLOB",
"REFSCALAR", "ASSIGN", or "OBJECT". This function provides a
convenient way to determine the context without manually checking
multiple conditions using "want".
* Arguments
None.
* Returns
A string indicating the current context, such as "VOID",
"SCALAR", "LIST", "BOOL", "CODE", "HASH", "ARRAY", "GLOB",
"REFSCALAR", "ASSIGN", or "OBJECT". Returns "VOID" if the
context cannot be determined.
* Example
sub test_context
{
my $ctx = context();
print "Current context: $ctx\n";
}
test_context(); # Prints: Current context: VOID
my $x = test_context(); # Prints: Current context: SCALAR
my @x = test_context(); # Prints: Current context: LIST
INTERNAL FUNCTIONS
The following functions are internal to "Wanted" and are not intended
for public use. They are documented here for reference but should not be
relied upon in user code, as their behaviour or availability may change
in future releases.
wantassign
* "wantassign($uplevel)"
Checks if the current context is an lvalue assignment context
("ASSIGN") at the specified $uplevel in the call stack. Returns an
array reference containing the values being assigned if in "ASSIGN"
context, or "undef" if not. In boolean context (e.g., when
"want('BOOL')" is true), it returns a boolean indicating whether an
assignment is occurring.
This function is used internally by "want('ASSIGN')" to handle
lvalue assignments in subroutines marked with the ":lvalue"
attribute.
* Arguments
* $uplevel
An integer specifying how many levels up the call stack to
check the context. Typically set to 1 to check the immediate
caller.
* Returns
* In list or scalar context: An array reference containing the
values from the right-hand side of the assignment, or
"undef" if not in "ASSIGN" context.
* In boolean context: A boolean indicating whether the context
is an "ASSIGN" context.
* Example
sub assign_test :lvalue
{
if( want('LVALUE', 'ASSIGN') )
{
my $values = wantassign(1);
print "Assigned: @$values\n";
lnoreturn;
}
return;
}
assign_test() = 42; # Prints: Assigned: 42
want_assign
* "want_assign($level)"
A low-level XS function that retrieves the values being assigned in
an lvalue assignment context ("ASSIGN") at the specified $level in
the call stack.
Returns an array reference containing the values from the right-hand
side of the assignment, or "undef" if not in an "ASSIGN" context.
This function is used internally by "wantassign" to fetch assignment
values, which wantassign then processes based on the caller's
context (e.g., scalar, list, or boolean context). It supports
"want('ASSIGN')" indirectly through wantassign.
* Arguments
* $level
An integer specifying how many levels up the call stack to
check the context.
* Returns
* An array reference containing the values from the right-hand
side of the assignment, or "undef" if not in an "ASSIGN"
context.
* Example
This function is typically called by "wantassign", but for
illustrative purposes:
sub assign_test :lvalue
{
if( want('LVALUE', 'ASSIGN') )
{
my $values = want_assign( bump_level(1) );
print "Assigned: @$values\n";
lnoreturn;
}
return;
}
assign_test() = 42; # Prints: Assigned: 42
want_boolean
* "want_boolean($level)"
Checks if the context at the specified $level in the call stack is a
boolean context ("BOOL"). Returns true if the caller is evaluating
the return value as a boolean (e.g., in conditionals like "if",
"while", or with logical operators like "&&" or "||").
This function is used internally to support "want('BOOL')".
* Arguments
* $level
An integer specifying how many levels up the call stack to
check the context.
* Returns
* A boolean (true or false) indicating whether the context is
a boolean context.
* Example
sub bool_test
{
if( want_boolean(1) )
{
print "In boolean context\n";
return(1);
}
return(0);
}
if( bool_test() )
{
# Prints: In boolean context
}
want_count
* "want_count($level)"
Returns the number of items expected by the caller at the specified
$level in the call stack. Used internally to support "want('COUNT')"
and "howmany".
* Arguments
* $level
An integer specifying how many levels up the call stack to
check the context.
* Returns
* An integer representing the number of items expected, or -1
if an unlimited number of items is expected (e.g., in list
context with no fixed limit).
* Example
sub count_test
{
my $count = want_count(1);
print "Caller expects $count items\n";
}
my( $a, $b ) = count_test(); # Prints: Caller expects 2 items
want_lvalue
* "want_lvalue($uplevel)"
Checks if the context at the specified $uplevel in the call stack is
an lvalue context for a subroutine marked with the ":lvalue"
attribute. Returns true if the subroutine is called in a context
where its return value can be assigned to.
This function is used internally to support "want('LVALUE')" and
"want('RVALUE')".
* Arguments
* $uplevel
An integer specifying how many levels up the call stack to
check the context.
* Returns
* A boolean (true or false) indicating whether the context is
an lvalue context.
* Example
sub lvalue_test :lvalue
{
if( want_lvalue(1) )
{
print "In lvalue context\n";
}
my $var;
}
lvalue_test() = 42; # Prints: In lvalue context
EXAMPLES
use Wanted 'howmany';
sub numbers
{
my $count = howmany();
die( "Cannot make an infinite list" ) if( !defined( $count ) );
return( 1..$count );
}
my( $one, $two, $three ) = numbers();
use Wanted '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
ARGUMENTS
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.
* VOID
* SCALAR
* REF
* REFSCALAR
* CODE
* HASH
* ARRAY
* GLOB
* OBJECT
* BOOL
* LIST
* COUNT
* <number>
* Infinity
* LVALUE
* ASSIGN
* RVALUE
EXPORT
The "want" and "rreturn" functions are exported by default.
The "wantref" and/or "howmany" functions can also be imported:
use Wanted qw( want howmany );
If you do not import these functions, you must qualify their names as
(e.g.) "Wanted::wantref".
SUBTLETIES
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 is
not 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.
CREDITS
Robin Houston, <robin@cpan.org> wrote the original module Want on which
this is based.
Also, credits to Grok from xAI <https://x.ai> for its support in
updating the XS code, providing unit tests, and helping resolve several
bugs from the original Want module.
AUTHOR
Jacques Deguest <jack@deguest.jp>
SEE ALSO
"wantarray" in perlfunc, Perl6 RFC 21, by Damian Conway
<http://dev.perl.org/rfc/21.html>
Contextual::Call, Contextual::Diag, Contextual::Return
COPYRIGHT & LICENSE
Copyright(c) 2025 DEGUEST Pte. Ltd.
Portions copyright (c) 2001-2016, Robin Houston.
All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.