#9496: Simplify primitives for short cut fusion -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: task | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.8.3 Keywords: fusion | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Other Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Currently, there appear to be two production primitives, `build` and `augment` (although I may have missed some). There are multiple consumption primitives (at least: `foldr`, `head`, `and`, `or`, `any`, and `all`). The rule sets for some producers seem to forget to handle `augment`, and the `augment/augment` rule is omitted as "true, but not, I think, useful". A number of other functions are instead rewritten into `foldr` forms, and then written back if they don't fuse. == What to do: == Personally, I'd be very tempted to start by saying `build g = augment g []` (or ''possibly'' even `build g = extend [] g []` if I can ever get that idea to work) and cut the problem in half. The main problem I see with this is if other people are importing `GHC.Exts` or `GHC.Base`, writing things with `build`, and expecting them to fuse. One way to deal with this, perhaps, is to hack a special rule into the RULES compiler to recognize `GHC.Base.build` on the LHS of RULES and replace it with the appropriate `augment` form, emitting a warning. Where to go after that: the question remains whether it's best in general to rewrite a form to `foldr` to make it fuse, or to fuse directly with `augment`. The answer presumably depends, at least in part, on whether there are additional `foldr`-based rules we may want to add that would take advantage of the effort put into the translation back from the `foldr` form. I would conjecture that most such rules we could want would go beyond what the RULES system actually can do. That said, if we can find a way to use `foldr` forms without the horrible pain of translating back from them, that would be a very good thing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9496 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler