Hey everyone,
First, thank you all for this great discussion! I unfortunately have been home
due to health concerns which gets really boring after a while, so being able to
participate in such a fun intellectual exercise like has really been making my
day. :-D Sorry that this has resulted in such a flood of commentary on the
list!
Antoine Latter has pointed out to me that (using my own words here) essentially
entire parser libraries are built around the assumption that many and some are
sensibly defined the way that they are, and that as a result much of their
functionality simply doesn't make sense for Maybe and [].
So maybe the best approach to take really is to remove the instance for Maybe
and [] from Alternative. After all, some and many simply are not well-behaved
for them, and if you are using Alternative you are expecting them to be
well-behaved.
Now, on the other hand, one might argue: but Maybe and [] have well-defined
functions for empty and <|>, so since some and many are defined in terms of
these operations, shouldn't that make Maybe and [] natural instances of
Alternative anyway?
And *this* is where Haskell separates its way from other languages. In others
language we may very well just say, "Well, good point, why not make them
instances of Alternative, and simply not worry about the fact that some and
many don't behave well --- just don't use them like that!"
But in Haskell we don't do things this way. When we make something be an
instance of a typeclass, we want that to *mean* something. In the case of
Alternative, we want, among other things, for it to mean that our type has
sensible meanings for some and many --- and if Maybe and [] simply do not meet
this criteria, then THEN THEY DESERVE TO BE CAST OUT!
I know, I know, I can hear you all shouting: This is blasphemy! This is
madness!
Madness? This... IS HASKELL!
But on a more serious note, it turns out that we *already* have a typeclass
that does everything that Alternative does but without the "some" and "many"
baggage: it's called "Monoid"! So we can already get all of the features that
we need (and most likely have been using anyway) by using the Monoid instances
for Maybe and [] and just forgetting about the existence of Alternative
entirely.
So at the end of the day... what is the point of even making Maybe and []
instances of Alternative?
Cheers,
Greg
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