#15519: Minor code refactoring leads to drastic performance degradation -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: 8.8.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: SpecConstr Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => SpecConstr Comment: OK, re comment:12, this is an old problem: see Trac #4448. The current (horrible) solution is to take control explicitly, like this {{{ import GHC.Type( SPEC(..) ) runTokenParser :: SPEC -> Grammar Char -> Text -> Result runTokenParser = \sp grammar stream -> case grammar of Tokens _ tst -> let head = Text.head stream in if tst head then Success (Text.tail stream) (Text.singleton head) else Fail Many (Tokens _ tst) -> let (!consumed, !rest) = Text.span tst stream in Success rest consumed X !grammar -> runTokenParser sp grammar stream }}} Notice that `SPEC` argument. It's not actually used, but it tells `SpecConstr` to specialise the call regardless of whether the function scrutinises the argument (see Trac #4448 for a simpler example). That indeed makes `test2` work nicely. I don't claim that it's nice. At all. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15519#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler