RE: [perl #78626] Patch: support higher arity in reduce()

Hi Moritz,

Thanks for the explanation, I  appreciate it and I can certainly appreciate the difficulty. 

(Sorry for not getting back sooner, I missed your email until now.) It's a bit of a shame since I expect most of the time .reduce will be called with a closure. If the general solution can be had, that is mighty impressive. I sort of oppose the notion that you can't provide a useful feature because the rarely more useful general case is too difficult to implement right now (if it was my task I'd be rejecting multis—forever) but I write a lot of Perl 5 too, and I don't feel regret when I have to use a utility module to make up for something that doesn't suit me and I'm happy to do the same in this case in Perl 6 for as long as is necessary.

Best Regards,
Stephen

P. S. Thanks for all your work on Perl 6—that goes for everyone involved. It's a great language whichever way you slice it.

> Subject: Re: [perl #78626] Patch: support higher arity in reduce()
> From: perl6-bugs-followup@perl.org
> To: smosher_@hotmail.com
> Date: Thu, 13 Oct 2011 02:02:28 -0700
> 
> Am 13.10.2011 06:01, schrieb smosher@loveandhotsauce.net:
> > That's fine, I think it was already shot down owing to the supposedly
> > dubious utility of supporting higher arities in .reduce()
> >
> > By the way, I submitted the patch because the error message had (and
> > still has) the "for now" wording when you try any other arity. But after
> > I submitted the patch I was asked to defend non-binary reduce. After
> > scratching my head I might have replied with some mumblings about
> > continued fractions or something, but I don't understand why I was asked
> > while that particular wording is still in place—either it's binary only
> > or it isn't, or perhaps it's just not specced.
> >
> > I can appreciate the added complication in checking when a non-binary
> > reduce should actually terminate, and other concerns. If I knew that the
> > functionality was desired, I'd be happy to submit another patch. At the
> > moment a change in the wording in src/core/List.pm might be advisable,
> > line 296: "fail('can only reduce with arity 2 for now')", dropping the
> > "for now" if the functionality is not in fact desired.
> 
> Let me provide some context for the "for now" here. I'm not defending 
> that phrase (and will remove it), just want to provide some food for 
> thought.
> 
> Back when you wrote the patch, we did find higher arity reduce 
> desirable, and I think some of us are still emotionally attached to the 
> idea.
> 
> The problem is that the complexity of signatures makes it basically 
> impossible to figure out which candidate to call if the reducer is a 
> multi, and thus to decide how many arguments to pass.
> 
> The idea back then was to somhow specific a mechanism for supplying a 
> (possibly lazy) stream of arguments to a set of multi candidates, and 
> have the call eat up an apropriate amount of arguments. Such a mechansim 
> would be useful in other places as well (such as 'for' and 'map' with 
> blocks of higher arity), and it would be a very nice anaolgy to regexes. 
> And like the famous "longest token matching" [1] in regexes, we'd expect 
> such a mechanism to be mostly greedy, ie the candidate that binds the 
> most arguments wins, unless other circumstances prevent that from 
> happening (you see, it's rather vague).
> 
> I have no idea how feasible such a mechanism is, and if it will be 
> included in Perl 6 at all. But I'm sure that if it will be defined, it 
> will be on a much lower level than reduce -- probably in the multi 
> dispatcher directly.
> 
> Up to now such a spec hasn't happened, but the idea keeps simmering in 
> the back of the minds of several Perl 6 hackers.
> 
> Cheers,
> Moritz
> 
> 
 		 	   		  

• RE: [perl #78626] Patch: support higher arity in reduce() by s mosher