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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