[Haskell-cafe] Splitting off many/some from Alternative

[John Meacham]

Yes, they are major pains for frisby, which is a parser but needs to
be cleverer about recursion, the many and some that come with
applicative actually cause infinite loops.

    John

On Sun, Dec 11, 2011 at 9:18 PM, Gregory Crosswhite
<gcrosswhite at gmail.com> wrote:
> Hey everyone,
>
> I am sure that it is too late to do more than idly speculate about this, but
> could we split the some/many methods out from Alternative?  They simply
> don't make sense except in a subset of possible Alternatives --- in most
> cases they just result in an infinite loop.  That is to say, it is not just
> that they are extraneous functionality, but most of the time they are
> *impossible* functionality to even implement!  In a way, it is a lie to be
> including them in Alternative since people making use of the class might
> incorrectly (but quite reasonably!) assume that any type that is an instance
> of Alternative *has* a well-defined some and many method, when this is
> actually the exception rather than the rule.
>
> It is only recently that I have been able to grok what some and many are
> even about (I think), and they seem to only make sense in cases where
> executing the Alternative action results in a portion of some input being
> consumed or not consumed.  "some v" means "consume at least one v and return
> the list of items consumed or fail", and "many v" means "consume zero or
> more v and return the list of items consumed or the empty list of none are
> consume".  It thus makes sense for there to be some subclass of Alternative
> called something like "Consumptive" that contains these methods.  The
> majority of "Alternative" instances would no longer have these methods, and
> again that would actually be an improvement since in such cases some/many
> were unworkable and did nothing but cause infinite loops anyway.
>
> Normally it would be unthinkable to even consider such a change to the base
> libraries, but I suspect that there are not more than a couple of packages
> out there that make active use of the some/many instances of Alternative;
>  it is really only parser packages that need some/many, and users most
> likely use the versions included with the packages themselves rather than
> the Alternative version.  Could we verify that this is the case, and if so
> split them away?  I thought I heard a trick whereby people were able to grep
> all the source on Hackage, but I can't remember what it was.  :-)
>
> Just a thought.  :-)
>
> Thanks,
> Greg
>
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