An attempt at foldr/build fusion for zip

Olaf Chitil
Mon, 23 Apr 2001 14:57:50 +0100

Hi Matt,

> I think I may have found a way to get zip & friends to fuse with *both*
> of their input lists.
> ...
> I have no idea
> what kind of code this would actually end up creating.

However, that is the important point. The goal of deforestation/fusion
is to optimise a program. Removing data structures is not the final
goal. In principal you can replace all algebraic data types by higher
order functions (at least with the second order types that ghc allows).
You just don't gain anything by doing it.

I'm sorry that I don't have the time to look into your definition in
detail. But basically you replace the intermediate list by higher order
functions. Your foldr2_both does about the same amount of work as
foldr2. Fusion of foldr2 with both arguments should give an expression
without any recursively defined function (i.e. any foldr variant).

Btw: The real benefit of deforestation does not come from saving the
time for constructing and destructing the intermediate list. The real
benefit comes from moving the code for the construction of an element
next to the code for destructing the element which usally enables many
further optimisations.

A solution to the zip fusion problem is presented by John Launchbury,
Sava Krstic, and Tim Sauerwein in:
I haven't yet looked into it in detail. Some problems with this approach
are mentioned in the paper and I suppose they are the reason why the
approach is not used in ghc.

> \begin{code}
> newtype BuildZip a b = BZ ((a -> (BuildZip a b) -> b) -> b)
> bz :: (forall b. (a->b->b)->b->b) -> b -> BuildZip a b
> bz f n = f (\x xs -> BZ (\c -> c x xs)) (BZ (\_ -> n))
> {-# INLINE bz #-}
> foldr2_both :: (a->b->c->c) -> BuildZip a c -> BuildZip b c -> c
> foldr2_both k (BZ xs) (BZ ys) =
>        xs (\x xs' ->
>        ys (\y ys' ->
>            k x y (foldr2_both k xs' ys')
>        ) )
> {-# RULES
> "foldr2/both"   forall k n (f::forall z.(a->z->z)->z->z)
>                            (g::forall z.(b->z->z)->z->z) .
>                   foldr2 k n (build f) (build g) =
>                     foldr2_both k (bz f n) (bz g n)
>  #-}
> \end{code}


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