[Haskell-cafe] [Alternative] change some/many semantics

[Antoine Latter]

On Wed, Dec 14, 2011 at 10:33 PM, Gregory Crosswhite
<gcrosswhite at gmail.com> wrote:
>
> 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