On Wed, Dec 14, 2011 at 10:33 PM, Gregory Crosswhite
On Dec 15, 2011, at 2:13 PM, Antoine Latter wrote:
Isn't this what Ross previously suggested? I think his suggested instance methods for Maybe return the elements of the lists incrementally.
Yes and no. Yes, his excellent suggestion is one of my favorite ideas for what we should do with Alternative that I have seen so far and was the inspiration for my proposal, but no it is not the same idea at all. Whereas his suggestion keeps the types and generic definitions of some and many the way that they are but overrides them manually to work for types such as Maybe, my proposal is that we instead change the types and generic definitions of some and many themselves so that they automatically do the right thing for the Maybe and List types.
Unless the Alternative and Applicative type classes offer class methods to guarantee laziness, we'll have a hard time writing functions with those guarantees. Such is the cost of parametric polymorphism! But also the beauty - in the absence of any laziness guaranteeing functions on the class, we are promised that a polymorphic function can't be using magic fairy dust behind our back. We could add some sort of laziness guaranteeing combinators to the class interface, but that would restrict its membership even further. Or we could not use 'some' and 'many' with list and maybe :-) Antoine