good fusion (was Re: inits)

[Simon Marlow]

Josef Svenningsson wrote:
> On 4/10/06, Malcolm Wallace <Malcolm.Wallace at cs.york.ac.uk> wrote:
> 
>>Are the larger sizes of zipWith (zipWith3, zipWith4, ...) also good
>>producers?  I have some evidence from a recent application I have been
>>writing that they may not be.  At least, reading the -ddump-simpl
>>output, it does not seem clear-cut that a pipeline e.g.
>>    (concat . f . map g . zipWith3 h as bs) cs
>>is turned into a simple loop, although perhaps I am not reading it
>>right.  Is there any received wisdom on what should be expected here?
>>
> 
> The GHC documentation is rather clear at this point. See:
> http://www.haskell.org/ghc/docs/latest/html/users_guide/rewrite-rules.html#id3153312
> 
> The larger zipWiths are not listed as good producers, hence you cannot
> expect fusion to happen.

Thanks for the pointer, I wasn't aware of that :)  In fact, I recently 
added RULEs for deforesting take, so I need to add it to that list.

I don't see any good reason why the larger zipWiths cannot be good 
producers, it just needs somebody to write the RULEs and check that they 
work.  The tricky bit is usually making sure that the foldr/build 
versions (just build in this case) revert to the efficient variant in 
the event that fusion doesn't happen.

Cheers,
	Simon