#10176: Invalid core generated with GHC 7.10 RC3 -------------------------------------+------------------------------------- Reporter: NeilMitchell | Owner: Type: bug | Status: new Priority: high | Milestone: 7.10.1 Component: Compiler | Version: 7.10.1-rc3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by nomeata): Ok, I think here is what happens, at least in my smaller test case: 1. Call Arity determines that `error1Arg` is always called with two arguments. Hence `CallArity=2`. This is correct. 2. The simplifier tries to eta-expand `error1Arg` to take two arguments. This fails in `mkEtaWW` in `CoreArity`, where we have this comment: {{{ -- This *can* legitmately happen: -- e.g. coerce Int (\x. x) Essentially the programmer is -- playing fast and loose with types (Happy does this a lot). -- So we simply decline to eta-expand. Otherwise we'd end up -- with an explicit lambda having a non-function type }}} 3. But still, the `Arity` field is updated. 4. Since it has `Arity=2`, `exprIsHNF (error1Arg @ _ ())` is true, and the simplifier does strange things. So maybe the solution is to ''not'' update `Arity` with the result from Call Arity, but instead let eta-expansion happen and update `Arity` with the manifest arity after eta expansion. I’ll try that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10176#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler