Question

The perldocs say the following about last:

last cannot be used to exit a block that returns a value such as eval {}, sub {}, or do {}, and should not be used to exit a grep() or map() operation.

Why should it be avoided in a grep() or map()? I’m especially curious about map since it is an alternative to the foreach construct. The docs seem to insist on not doing something without describing the consequences.

Answer 1

First, realise that grep and map are transparent to next, last and redo, just like if is.

$ perl -e'
   print "A";
   for (8,9) {
      print "B";
      print map { print "C"; next; print "D"; $_ } 1,2,3;
      print "E";
   }
   print "F\n";
'
ABCBCF

But the docs doesn’t say last cannot be used to exit map, it says it shouldn’t used. It’s not clear what that means.

• Don’t use them thinking they will affect map and grep?
• Don’t use them that way because your reader might think they affect map and grep?
• Don’t use them that way because it can leave Perl in a wonky state?
• Don’t use them that way because the behaviour might change in the future?

I don’t know.

I don’t think it’s #3 because I don’t know of any bad side-effects of using next, last and redo to leave a map or grep callback.

The funny thing, leaving map and grep using a loop construct doesn’t even warn even though leaving a sub in the same fashion does.

$ perl -wE'while (1) { map { last; } 1; }'

$ perl -wE'while (1) { sub { last; }->(); }'
Exiting subroutine via last at -e line 1.