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NAMEWant - A generalisation of "wantarray"SYNOPSISuse 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 } DESCRIPTIONThis 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.Top-level contexts:The three kinds of top-level context are well known:
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:
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 = ; $z = 3", which may not be what you were expecting. The reason is that the assignment is in scalar context, so the comma operator is in scalar context too, and discards all values but the last. You can fix it by writing "(list) = (1,2,3);" instead. If your lvalue subroutine is used on the left of an assignment statement, it's in ASSIGN context. If ASSIGN is the only argument to "want()", then it returns a reference to an array of the value(s) of the right-hand side. In this case, you should return with the "lnoreturn" function, rather than an ordinary "return". This makes it very easy to write lvalue subroutines which do clever things: use Want; use strict; sub backstr :lvalue { if (want(qw'LVALUE ASSIGN')) { my ($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. 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 $format = *{foo()}{FORMAT} # GLOB reference expected You can check this using conditionals like "if (want('CODE'))". There is also a function "wantref()" which returns one of the strings "CODE", "HASH", "ARRAY", "GLOB", "SCALAR" or "OBJECT"; or the empty string if a reference is not expected. Because "want('SCALAR')" is already used to select ordinary scalar context, you have to use "want('REFSCALAR')" to find out if a SCALAR reference is expected. Or you could use "want('REF') eq 'SCALAR'" of course. Be warned that "want('ARRAY')" is a very different thing from "wantarray()". Item countSometimes in list context the caller is expecting a particular number of items to be returned:my ($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 contextSometimes 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
EXAMPLESuse 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 ARGUMENTSThe 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.
EXPORTThe "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". INTERFACEThis 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. SUBTLETIESThere 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. BUGS* Doesn't work from inside a tie-handler. AUTHORRobin Houston, <robin@cpan.org>Thanks to Damian Conway for encouragement and good suggestions, and Father Chrysostomos for a patch. SEE ALSO
COPYRIGHTCopyright (c) 2001-2012, 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.
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