#13353: foldr/nil rule not applied consistently -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I just had a project where it made a difference whether I add {{{ {-# RULES "foldr/nil" forall k n . GHC.Base.foldr k n [] = n #-} }}} to my file or not, despite this rule already being present in the library. I tried to minimize the problem and came up with this: {{{#!hs foo1 (f, fs) (x, xs) = (f x, map ($x) fs ++ map f xs) foo2 f fs x xs = (f x, map ($x) fs ++ map f xs) test1 x xs = foo1 (id, []) (x, xs) test2 x xs = foo2 id [] x xs test3 x xs = (id x, map ($x) [] ++ map id xs) }}} `test2` and `test3` nicely optimize the `map … [] ++` away, but `test` does not. (In this minimized example, adding the rule again locally does *not* help, but there is still something fishy.) Also, in all cases, `map id` remains, which should not be the case. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13353 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler