#9355: scanr does not participate in stream fusion -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.4 Component: | Version: 7.8.3 libraries/base | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Moderate (less Unknown/Multiple | than a day) Type of failure: Runtime | Blocked By: performance bug | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by dfeuer): OK, so it didn't turn out to be quite as trivial as I thought, but it looks like this does the trick: {{{#!haskell scanr :: forall a b . (a -> b -> b) -> b -> [a] -> [b] scanr f q0 ls = build scr where scr :: forall c . (b -> c -> c) -> c -> c scr c n = snd $ foldr go (q0, q0 `c` n) ls where go x (r,est) = let fxr = f x r in (fxr, fxr `c` est) }}} Fortunately, the tuples get unboxed as they should. Some extra rules may be needed for map, et al. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9355#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler