On Thu, 3 Jan 2008, Don Stewart wrote:
You can, with some caveats, use a single fusion system across data structures, and avoid the built in build/foldr system.
I'd start by installing the stream-fusion list library, from hackage, which gives you the list api, and a fusion mechanism.
To avoid the build in fusion system, you need to:
With 'build in' you mean the fusion system of Prelude?
* avoid list comprehensions * avoid .. (use Stream.enumFromTo instead)
No problem.
then you can write fusion rules for your structure in terms of streams, and they'll fuse with list operations as well.
But then in order to fuse your streams with my chunky sequences I have to know, how stream-fusion fuses streams, right? Anyway, I tried to wrap Prelude lists in a newtype and thus got GHC (still 6.4.1) to invoke my rules instead of the Prelude rules. But I encountered the following problem: I define something like nonFusable x y = fusable (aux x y) where fusion rules are defined for 'fusable', but not for 'nonFusable'. I hoped that 'nonFusable' will be inlined and then 'fusable' is fused with other expressions. This does not happen. If I state the function definition also as rule, then GHC fuses eagerly. Analogously I observed that usage of ($) and (.) blocks fusion, and when I add the rules "unfold-dollar" forall f x. f $ x = f x ; "unfold-dot" forall f g. f . g = \x -> f (g x) ; then fusion takes place as expected. Am I doing something wrong? Do I have to dig into phase control? I wouldn't like that, since it seems to be too fragile.