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Re^2: Context tutorial

by shmem (Canon)
on Jan 25, 2009 at 14:20 UTC ( #738776=note: print w/ replies, xml ) Need Help??


in reply to Re: Context tutorial
in thread Context tutorial

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


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Re^3: Context tutorial
by ikegami (Pope) on Jan 25, 2009 at 17:15 UTC

    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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