Re^5: reset particular variable

Actually, I seem to recall that there exists an obscure case in which return; and return (); differ materially..

Yes, Perl "bare return" does things that might not be expected. Basically, I figure it is a bad idea to use a "bare" "return;" and I just don't do it. The below code is a simple demo... "return" will return something unless you tell it to return "nothing". As I mentioned before, "returning nothing" is useful in map{}, but for a normal function, I would return "undef".

#!/usr/bin/perl -w
use strict;

sub x
{
   my $a =5;
}

print x; #prints 5
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Update: This part was wrong...goof in my testing! got a bit confused while doing this quickly...Oooops...I'll have to look at some other code I did awhile ago to find a better example...I think I have one somewhere. Anyway the idea of returning $x or () from within map{} is right idea to "skip" a value in output list. sub x1 { my $a =5; return; } print x1; #still prints 5! Whoa! #"return'" returns value of last true statement. #if you don't specify something else for it to return.
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sub x2
{
   my $a =5;
   return ();
}

print "x2:",x2; #prints "x2:" , ie nothing from sub x2
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Update: I don't see any basic disagreement or argument, but perhaps a misunderstanding of the application for this:

But my questions in Re^2: reset particular variable were in the context of a discussion of differences between the $var = (); and $var = undef; et al statements, differences I still don't see.

@outputlist = map{..code...}@inputlist;
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If I can "skip a value", instead of returning "undef" from the map, then say @outputlist could have fewer items in the list than @inputlist. Later when @outputlist is processed, there doesn't have to be code that says "check for undefined value", because there won't be any.

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Re^6: reset particular variable by Anonymous Monk on Aug 21, 2009 at 13:56 UTC
Sorry marshall, but you're confusing explicit return with implicit return, ie. x1 doesn't still print 5 `sub fo { 5 } # implicit_return sub fox { 5; return } # explicit return print fo; print fox; __END__ 5` [download] perldoc -f return	[reply] [d/l]
Re^7: reset particular variable by Marshall (Canon) on Aug 21, 2009 at 15:09 UTC
aside from the fact that your code doesn't work under -w and strict, your point is valid. I have amended my post above. Something is not right with my example. Well, stuff happens. Thanks for pointing this out!	[reply]
Re^8: reset particular variable by Anonymous Monk on Aug 22, 2009 at 00:39 UTC
aside from the fact that your code doesn't work under -w and strict, your point is valid. Come come now, did I put strict/warnings in my 4 lines of code?	[reply]
Re^6: reset particular variable by AnomalousMonk (Archbishop) on Aug 21, 2009 at 16:21 UTC
(I've seen Marshall's updated post, so this post is a bit redundant; but what the heck, I've already done the work, may as well cash the check.) Based on the example code below, I'm willing to go out on a limb and say there is no difference between the behavior of the `return;` and `return ();` statements in list or in scalar context; that is, although they behave differently in list versus scalar context, they behave differently in exactly the same way. (The example code uses the `$x = 1 + scalar @_;` business to try to make absolutely sure that the 'statement evaluating to true' is not just optimized away by the compiler.) >perl -wMstrict -le "my $x; sub Sbr { $x = 1 + scalar @_; return } sub Sel { $x = 1 + scalar @_; return () } my ($s, @ra); $s = Sbr(); print 'scalar bare return ', defined $s ? '' : 'UN', 'defined'; print qq{scalar context bare return: '$s'}; $s = Sel(); print 'scalar empty list ', defined $s ? '' : 'UN', 'defined'; print qq{scalar context empty list: '$s'}; @ra = Sbr(); print qq{list context bare return: (@ra)}; @ra = Sel(); print qq{list context empty list: (@ra)}; " scalar bare return UNdefined Use of uninitialized value in concatenation (.) or string at ... scalar context bare return: '' scalar empty list UNdefined Use of uninitialized value in concatenation (.) or string at ... scalar context empty list: '' list context bare return: () list context empty list: () [download] So I guess it comes down to the questions: Is there any difference in the behavior of the `return;` and `return();` and `return ();` statements that does not arise from a context difference? Do they behave exactly the same way in list context? Do they behave exactly the same way in scalar context? My answers would be: no; yes; yes. (And BTW: I completely agree with you about the importance of returning an empty list in, e.g., a `map` statement in which you wish to truncate the output list.)	[reply] [d/l] [select]
Re^7: reset particular variable by Marshall (Canon) on Aug 21, 2009 at 23:31 UTC
I suspect that you are right. I will have to "noodle on this a bit" to completely convince myself, but I suspect that it is true. As often happens in these threads, we have gone a long way from the original question about "reset"! I believe that there is consensus that package scoped variables and reset is not the best way although this is part of the language. I was surprised, but this IS truly a language construct. However, just because "it works", doesn't mean that it is a good idea - smaller scoped lexical variables is the "right" idea. Then we got onto map{} and how to suppress an "undef" value which is also a good idea. Now we are onto something that hasn't come up in the last few years of my coding. Almost all my subs{} return something and that result is expected and used or I consider this a coding error. Subs that don't return things are signal handlers or subs that modify a reference that is passed in. Basically I don't try to check return values from subs that aren't expected to return anything as a lvalue and I don't have much experience checking for this situation in a normal sub. I figure that until proved otherwise AnomalousMonk has it right! Great work! Thanks!	[reply]
Re^8: reset particular variable by Anonymous Monk on Aug 22, 2009 at 00:55 UTC
I believe that there is consensus that package scoped variables and reset is not the best way although this is part of the language. reset is a holdover from perl4, no-one uses it anymore	[reply]