> On 7 Apr 2017, at 13:49, brian d foy (via RT) <perl6-bugs-follo...@perl.org> > wrote: > > # New Ticket Created by "brian d foy" > # Please include the string: [perl #131114] > # in the subject line of all future correspondence about this issue. > # <URL: https://rt.perl.org/Ticket/Display.html?id=131114 > > > > # using Rakudo 2017.01 > > Should junctions only care about unique values? > > For example, does 3 need to be in the junction twice? > >> my $j = any( 1, 2, 3, 3 ) > any(1, 2, 3, 3)
I would argue yes for two reasons: 1. What determines when something is unique? This is typically done by using the .WHICH method on the object and using the resulting string as a key in a hash. But that *can* be very expensive for some types of objects, like sets, bags, buffers, etc. Imposing a uniqueness check on junctions would cause a severe degradation in performance of junctions. Even if it can be done at compile time. 2. Junctions *can* have side effects, if they contain Callables and you call the junction. Having identical Callables in there may produce different side-effects. > I ran into this when I was playing around with something like > this where I ended up with a junction has the same value repeated: > >> $j > 5 > any(False, False, False, False) > > Curiously, I expected the result would be either True or False > rather than another junction. Now, Booleanising a junction *does* collapse: $ 6 'my $j = any(1,2,3); say ($j > 5).Bool' False The REPL doesn’t booleanize, and thus you see the whole junction: $ 6 'my $j = any(1,2,3); say $j > 5' any(False, False, False) > Consider how this propagates: > >> $j > any( 5, 2 ) > any(any(False, False), any(False, False), any(False, True), > any(False, True)) > > As a boolean value, this is merely a baroque True. Since junctions > are not introspective, it doesn't need to remember how it got there. Again, the REPL is not booleanizing: $ 6 'my $j = any(1,2,3); say $j > any( 5, 2 )' any(any(False, False), any(False, False), any(False, True)) If you do, it does *exactly* what you expect: $ 6 'my $j = any(1,2,3); say ($j > any( 5, 2 )).Bool’ True or: $ 6 'my $j = any(1,2,3); say ?($j > any( 5, 2 ))’ True I think this ticket can be closed.
