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

[Antoine Latter]

On Wed, Dec 14, 2011 at 9:58 PM, Gregory Crosswhite
<gcrosswhite at gmail.com> wrote:
> Hey everyone,
>
> This is even more out there than my previous posts, but the following just
> occurred to me:  is it absolutely necessary that some/many have produced the
> entire list of results before returning?  Couldn't we change their semantics
> so that the list of results is computed and/or extracted lazily?  For
> example, if this were the case, then we would have that (some (Just 1))
> returns an infinite list rather than running in an infinite loop (even
> without defining a special case "some" implementation), which is exactly
> what my intuition would expect.
>

Isn't this what Ross previously suggested? I think his suggested
instance methods for Maybe return the elements of the lists
incrementally.

> Of course, this is not a simple change at all because it would have to be
> done in such a way as to respect the ordering of actions --- that is, we
> can't have each action executed only when the corresponding element of the
> list demanded is forced, or else actions would undesirably interleave.  For
> example, in a parser when we use "many v" we expect everything matching v to
> be consumed by the time "many v" returns, but if instead "many v" only
> consumed as much of the input as we demanded from its result list then we
> might see a chunk of input matching v in another part of our parser despite
> having assumed we'd gotten rid of it, which would cause our parser to be
> broken.
>
> Nonetheless, if there were some way that we could use magic fairy dust to
> have the results returned by some and many be lazily generated lists, then
> this might solve many of the problems that come up with them performing
> infinite loops in cases where it seems like they shouldn't.  :-)
>
> Cheers,
> Greg
>
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