On Sep 12, 2014 2:35 PM, "Joachim Breitner"
Interesting. I assumed that some wrap.unwrap=id law would hold, or at least some moral approximation (e.g. disregarding bottoms in an acceptable manner). But if the wrappers have to do arbitrary stuff that can arbitrarily interact with how the producer calls them, this becomes a bit less appealing.
No, nothing pleasant like that, I'm afraid. isoSimple is like that of course, but once it gets to foldl, the fusion rule is handing the builder a wrap/unwrap pair that isn't even close to that.
2. Somewhat related to the above, this framework has a huge amount of "wiggle room". There is very little to guide the choice of Wrap type.
I guess that would be resolved by time and experience, if we’d employ that scheme. But maybe we don’t.
The only way I would imagine would be if it turned out there were a few types that could be composed somehow. But when I, experimentally, applied Dan Doel's scanl wrapper type combined with Simple to (!!), I just got wrong answers.
Do you have any ideas?
Directly related to foldrW, no.
About list fusion and foldl in general, some half-baked.
I once experimented with a magic "oneShot :: (a -> b) -> (a -> b)" function, semantically the identity, but tell the compiler not to share the result of the computation. Using that in the definition of foldl-as-foldr, one can get the same effect as Call Arity, but a bit more reliable. I need to investigate if that solves the sumConcatInits problem.
How does that work exactly? Where do you stick the oneShot/why is it valid?
Another idea, probably with the same effect: What happens if we extend build :: (forall b. (a -> b -> b) -> b -> b) -> [a] to buildI :: (forall b. (a -> b -> b) -> b -> (b -> b) -> b) -> [a] where the extra argument is the identity, but magically „improves values of type b“. So with
enum = buildI $ \c n imp -> go 0 where go i = imp $ case i of 100 -> n ; _ -> i `c` go (i+1)
and
foldl f a0 = foldrI (\x k a -> k (f x a)) id (\k a -> k a) a0
we might get good code (but this is half-baked and written as I go).
It sounds a lot like the foldrW/buildW thing again, but maybe you can do better with it.
Shouldn’t this be on ghc-dev where others can join an, and people will find it in the archives later? I prefer to reserve private mail to, well, private matters :-)
If you like. David