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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