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Re: Context tutorial

by ikegami (Pope)
on Jan 25, 2009 at 04:09 UTC ( #738748=note: print w/ replies, xml ) Need Help??


in reply to Context tutorial

To force a list context where there wouldn't be one otherwise, assign to an empty list.

Then how come Example 4 is

my ($x) = localtime(); # Example 4

and not

my $x = () = localtime();

I've never see assigning to an empty list used to force a list context. Assigning to an empty list is used to count the number of elements in a list (which has nothing to do with context).

In the case of badly written code, assigning to the empty list can differentiate between void and list context, but it never makes sense to use it to differentiate between scalar and list context as you show.

To force list context, one adds parens around the LHS of the assignment or use a list slice.

my ($x) = f(); # first element of list is assigned my $x = ( f() )[0]; # same, but overkill my $x = ( f() )[-1]; # last element of list is assigned my $x = ( f() )[0] # logical-or of the first elements || ( g() )[0]; # returned by f() and by g()

Update: Added example.


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Re^2: Context tutorial
by shmem (Canon) on Jan 25, 2009 at 14:20 UTC
    I've never see assigning to an empty list used to force a list context. Assigning to an empty list is used to count the number of elements in a list (which has nothing to do with context).

    It has to do with context. Assigning to the empty list forces list context upon the RHS. It returns the empty list in list context, and the number of elements of the RHS in scalar context. Consider:

    print my $x = ( 'a', 'b', 'c' ); # c print my @x = () = ( 'a', 'b', 'c' ); # nothing print my $x = () = ( 'a', 'b', 'c' ); # 3 print my $x = @a = ( 'a', 'b', 'c' ); # 3 print scalar ( 'a', 'b', 'c' ); # c print +() = ( 'a', 'b', 'c' ); # nothing print scalar ( () = ( 'a', 'b', 'c' ) ); # 3

    update: to make it clearer...

    sub bar { local $\; print "bar:"; print wantarray ? "list - " : "scalar - "; return ( "d", "e" ); } sub foo { local $\; print "foo:"; print wantarray ? "list - " : "scalar - "; return ( "c", bar ); } $\ = "\n"; print ( 'a', 'b', foo ); # adcde print scalar ( 'a', 'b', foo ); # e print +() = ( 'a', 'b', foo ); # nothing print scalar ( () = ( 'a', 'b', foo ) ); # 5 __END__ foo:list - bar:list - abcde foo:scalar - bar:scalar - e foo:list - bar:list - foo:list - bar:list - 5

      Assigning to the empty list forces list context upon the RHS.

      So does system, but you don't see me using system to force list context. Your code supports what I say:

      print ( 'a', 'b', foo ); # adcde print +() = ( 'a', 'b', foo ); # nothing

      Assigning to the empty list does more than force context. It's not a good choice to force list context.

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