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'll 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'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.)

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 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.