#13966: Skip-less stream fusion -------------------------------------+------------------------------------- Reporter: jmspiewak | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc3 Resolution: | Keywords: JoinPoints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by mpickering): Here's how I am understanding this post as I have also been thinking about this recently. 1. In the Join Points PLDI paper there is an example in section 5 where join points allows `find` defined in terms of `any` to fuse. The author wants(/expects(?)) this technique to also fuse together normal functional pipelines as well as this simple examples. 2. The introduction of type classes seems irrelevant to the first point. GHC has quite a few optimisations which it uses to eliminate type classes and you can play lots of tricks around this to cause fusion like this to happen. If you look at the code is structured, in `sum` where the specific instance of `next3` is resolved to `Filter3 Int (EnumFromTo Int)` which means that all the consumers nicely line up with each other and hence fuse. The control flow of this program is very different to the first one. 3. A program with a polymorphic return type parametrised by a type class is very similar to a program written in CPS. To see this similarity, when writing a program in CPS in order for execution to continue you must provide a function which says what to do next. When using type classes, you must instead provide the *type* of the result which in then is elaborated to a function or dictionary of functions which explain how to proceed. Thus structuring your program in this way usually allows this kind of fusion to happen. 4. Ah-ha! but purpose of join points is to allow the compiler to optimise code written in direct style as well as code written in continuation passing style so can we do better in this case? The answer to which I don't yet know and I think is an open research problem. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13966#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler