#14816: Missed Called Arity opportunity? -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nomeata): It works if you don't export `insertModifyingArr`. Then it gets inlined into `test`, and CallArity, when looking at the binding of `g_s9E` in your original snippet, sees all the uses, sees that they are called at most once, and is happy to eta-expand it! But without inlining `insertModifyingArr`, this is beyond the reach of Call Arity, because it is not a higher-order analysis. Now, why does the demand analyser (which is higher-order, i.e. knows how functions call their arguments) not fix this? Because the demand analyser does not see that `insertModifyingArr` calls its argument only once, because the call is in a recursive loop. Sebastian Graf (@sgraf812) has thoughts on combining the analyses to give us the best of both worlds, maybe he can comment. For now, does {{{ import GHC.Magic blink :: (a -> b) -> a -> (# b #) blink g = oneShot $ \a -> (# g a #) }}} do what you want? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14816#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler