Dear GHC devs,
I’m interested in stream fusion and would like to see what it takes to fix the
remaining issues, so that it can replace foldr/build fusion in base.
First of all I would like to know what exactly the challenges are that are
left. I believe one of the main remaining problems is the fusion of
‘concatMap’. Is that really the only thing?
Secondly, I would like to know what has already been tried. I know Sebastian
Graf has spent a lot of effort trying to get SpecConstr to work on lambda
arguments without success. I’ve read that Sebastian now considers the static
argument transformation more promising.
However, Duncan Coutts proposed in his thesis to make rewrite rules slightly
more powerful and use the rewrite rule:
concatMap (λx → Stream next (f x)) = concatMap' next f
Has that ever been tried? If so, what is the problem with this rewrite rule
approach? I can understand that the `f x` function application is usually in a
more reduced form, but it seems relatively easy to make the rewrite rule
matcher smart enough to see through beta-reductions like that.
So my main questions are:
1. Is the ‘concatMap’ problem really the only problem left on the way to using
stream fusion instead of foldr/build fusion in base?
2. Has the rewrite rule approach to solving the ‘concatMap’ problem ever been
tried?
Any other information about the current status of stream fusion is also much
appreciated.
Cheers,
Jaro Reinders
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