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

[Gregory Crosswhite]

On Dec 15, 2011, at 9:31 PM, malcolm.wallace wrote:

> I do not regard that as a change in their semantics - it is perfectly allowed already  Indeed, the instances of some/many that I write are already lazily-unfolding, wherever possible.  It all depends simply on whether your instance of Applicative is lazy or strict.

That makes sense.  So the problem is not with Alternative but with Maybe:  specifically, the problem is that there is no way to write a fully lazy instance of Applicative for Maybe since both arguments have to be reduced to WHNF before we can determine the WHNF of the result of applying (<$>), and this is why some/many cannot return lazily generated lists of results.

Put another way, the problem with Maybe computations is that if there is a failure at any point in the computation than *the entire computation fails*, and this means that you can't lazily generate a list of results using some/many because you can't tell whether your computation was a success or a failure until the entire infinite computation has been run;  the only solution to this problem is, as others have suggested, to build domain-specific knowledge about Maybe into the some/many methods of Alternative instance, which I think is one of the good solutions that has been brought up in this discussion.  :-)

Cheers,
Greg
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