what should be output of perl -we 'print do { foreach (1, 2) { 1; } }'

[p5pRT]

Migrated from rt.perl.org#1085 (status was 'resolved')

Searchable as RT1085$

[p5pRT]

From [email protected]

If this were true, then

perl -we "print do { 17; foreach (1,2) { 1; } }"

would print 17. But it prints

  Useless use of a constant in void context at -e line 1.

I consider this (no return of foreach) as a bug, and report it as such.

Ilya

[p5pRT]

From @schwern

If Data​::Dumper is to be believed, it returns an empty string.

$ perl -MData​::Dumper -wle "@​a = do { foreach (1,2) { 1; } }; print Dumper \@​a"
$VAR1 = [
  ''
  ];

Different from an empty do block.

$ perl -MData​::Dumper -wle "@​a = do { }; print Dumper \@​a"
$VAR1 = [];

This explains why do { 17; foreach ... } complains about a useless constant.

A while loop returns 0.

$ perl -MData​::Dumper -wle '$foo = 42; sub foo { $bar = 2; $foo++; while ($bar--) { 1;
} } print Dumper [foo]'
$VAR1 = [
  0
  ];

Changing this behavior can cause serious backwards compat issues. Consider the
options​:

1) Have foreach be "null" so the statement before it is what is returned from the block.

That will alter subroutines such as​:

  sub foo {
  $foo++;
  foreach (...) { ... }
  }

Currently this will return an empty string. If we change the foreach behavior, it will
return the value of $foo.

2) Have foreach return its last evaluated expression in its block.

Similar problem. Any subroutine ending in a foreach loop will suddenly start returning
values.

I suppose it should be documented, but at this point it would likely be better to simply
declare it undefined rather than paint ourselves into a corner by documenting that it
returns false. Perhaps to say simply that for purposes of the "last evaluated expression"
rule, a loop is an expression and its value is undefined.

[p5pRT]

From [email protected]

On Tue Jul 15 19​:20​:10 2003, schwern wrote​:

A while loop returns 0.

Actually, that has changed​: in my perl-5.10.0, while loop returns an
empty string as well.

So the situation is more consistent now.

It's almost ten years since the ticket was started, almost 6 years since
it was noted that foreach returns an empty string. Let's call it a
tradition and close the ticket.

[p5pRT]

[email protected] - Status changed from 'open' to 'resolved'