#7542: GHC doesn't optimize (strict) composition with id --------------------------------------------+------------------------------ Reporter: shachaf | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by nomeata): Ah, interesting. The problem is not `mkIdDot2` (that has Arity 1 allright), the problem is `unIdDot2`, which suddenly as artiy 2! {{{ T7542.unIdDot2 :: forall a_az8 b_az9. (a_az8 -> T7542.Id b_az9) -> a_az8 -> b_az9 [LclIdX, Arity=2, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=2, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [20 0] 50 0}] T7542.unIdDot2 = \ (@ a_aKU) (@ b_aKV) (f_aze :: a_aKU -> T7542.Id b_aKV) (eta_B1 :: a_aKU) -> case f_aze of f_Xzl { __DEFAULT -> T7542.unId @ b_aKV (f_Xzl eta_B1) } }}} (This is already after `InitialPhase [Gentle]`). One would expect `MkId` and `unId` to behave the same... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7542#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler