Odd behavior of given {when (... or ...) ...}

Polynomial has asked for the wisdom of the Perl Monks concerning the following question:

perlsyn says that in the when clause of a given block, or should cause "the test" to be applied recursively to the first argument. What happens to the second, then? Shouldn't the implicit smart-matching magic apply to both operands, as with and? I ran into this strangeness when I used something like given ($x) {when ('a' or 'b') {foo();}}, thinking it would work like foo() if $x ~~ 'a' or $x ~~ 'b'. Instead, it appears to be mysteriously inconsistent. The following code:

sub test
   {my $x = shift;
    given ($x)
      {when ('a' or 'b') {say 'Y';}
       default {say 'N';}}
    given ($x)
      {when ('a' or $_ ~~ 'b') {say 'Y';}
       default {say 'N';}}
    given ($x)
      {when ($_ ~~ 'a' or 'b') {say 'Y';}
       default {say 'N';}}}

test('a');
test('b');
[download]

prints:

Y
Y
Y
N
N
Y
[download]

Can anyone explain this? I'm using a perl version 5.10.0 that I built for i686-linux-thread-multi-ld, if it makes a difference.

Comment on Odd behavior of given {when (... or ...) ...} Select or Download Code

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Re: Odd behavior of given {when (... or ...) ...} by ikegami (Patriarch) on Jun 22, 2008 at 15:41 UTC
Since `'a'` is true, `('a' or ...anything...)` is the same as `'a'`. Therefore, `when ('a' or 'b')` is the same as `when ('a')` `when ('a' or $_ ~~ 'b')` is the same as `when ('a')` The third one is wrong too. It'll say yes no matter what `$x` contains since `($_ ~~ 'a' or 'b')` is always true. `test('c')` [download] gives `N N Y` [download]	[reply] [d/l] [select]
Re^2: Odd behavior of given {when (... or ...) ...} by ysth (Canon) on Jun 22, 2008 at 20:51 UTC
No, or and and are intended to work differently in a when() clause. See the (vague and/or confusing) documentation cited by the OP. -- Online Fortune Cookie Search	[reply]
Re: Odd behavior of given {when (... or ...) ...} by massa (Hermit) on Jun 23, 2008 at 12:50 UTC
perlsyn says that it only uses the 'smart match both sides of OR/AND/\|\|/&&' _if_ both sides are prone to smart matching, i.e., they do not contain """ . a subroutine or method call . a regular expression match, i.e. `/REGEX/` or `$foo =~ /REGEX/`, or a negated regular expression match `$foo !~ /REGEX/` . a comparison such as `$_ < 10` or `$x eq "abc"` (or of course `$_ ~~ $c`) . `defined(...)`, `exists(...)`, or `eof(...)` . A negated expression `!(...)` or `not (...)`, or a logical exclusive-or `(...) xor (...)` """	[reply] [d/l] [select]
Re^2: Odd behavior of given {when (... or ...) ...} by Polynomial (Acolyte) on Jun 25, 2008 at 11:08 UTC
Right, and as string literals are none of those things, I'm pretty sure they should get converted to `$_ ~~ 'foo'`. I guess I'll file a bug report.	[reply]
Re: Odd behavior of given {when (... or ...) ...} by Anonymous Monk on Jun 23, 2008 at 08:43 UTC
Don't you have to `use feature "switch";`?	[reply] [d/l]
Re^2: Odd behavior of given {when (... or ...) ...} by Polynomial (Acolyte) on Jun 23, 2008 at 12:03 UTC
I just `use 5.10.0`, but yes, in order for my example to work you'll need to enable the switch feature one way or another.	[reply]