[Haskell-cafe] Two-continuation `monads' and MonadMinus

oleg at pobox.com oleg at pobox.com
Sat Jul 7 05:32:11 EDT 2007

> I was initially skeptical about defining Foldable for the direct-style
> LogicT transformer, but now I suspect that it is definable.
> Now that I think about it, you're losing the ability to work with monad
> transformers.

I think you're right. The particular point of msplit is that it is a
monad transformer method and provides certain guarantees on the order
and occurrences of _effects_. It seems the interface of Foldable, at
brief glance, makes no assurances as to how foldr may behave. The
implementation of foldr may decide to `look ahead' of the foldable
data structure for optimization purposes and only then start invoking
the folding function. That is sound so long as `looking ahead' does
not entail any effect. The function `msplit' is designed to operate on
a `data structure' where examining each element may be accompanied by
an effect. It becomes important then not to look ahead and be careful
not to re-do an effect if we need to re-examine some previously seen
value. In short, msplit in LogicT does provide guarantees about the
effect. The Foldable interface leaves this issue unspecified, or so it
seems. So, they can't be considered equivalent.

And speaking of non-determinism in general, it seem from experience
FBackTrackT is a quite a better solver: it finds solutions where List
and similar monads fail to find them; compared to the breadth-first
search (which is too, complete), FBackTrackT takes far less
resources. So, here too FBackTrackT finds solutions where
breadth-first search just runs out of memory.

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