#9495: Do What I Mean RULES for foldr2 look shady -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: highest | Milestone: 7.8.4 Component: | Version: 7.8.3 libraries/base | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: crash | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): To broaden this a bit, it is worth remembering that ''the main `foldr/build` rule itself is unsound in the presence of `seq`''. See, for example, the [http://www.haskell.org/haskellwiki/Correctness_of_short_cut_fusion#In_the_pr... Haskell wiki page]. This is not nice. The "right" solution is for `seq` to be an operation of a type class, something that was the case in Haskell originally, and then changed after ''extensive'' debate on the Haskell committee. So currently we are stuck in the unsatisfactory situation that certain optimisations, which have a generally very beneficial effect on performance, can change termination behaviour. Eta reduction/expansion is another. In that general context I don't have a strong opinion about the `foldr2` question. I'd consult the Core Libraries Committee. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9495#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler