#15852: Bad axiom produced for polykinded data family -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: TypeFamilies, | TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Oh dear. Yes, I //was// barking up the wrong tree—thank you for setting me straight. Now that I dig closer into `ppr_co_axiom_branch` (which is the real culprit), I now understand what went wrong. When implementing `etaExpandFamInstLHS`, I left [http://git.haskell.org/ghc.git/blob/d30352add1da67dd0346613853cd423c7becbaeb... this comment]: {{{#!hs -- @Note [Eta reduction for data families]@ in "FamInstEnv". Because the -- right-hand side is where the eta-reduced variables are obtained from, it -- is not returned from this function (as there is never a need to modify it). }}} In retrospect, I have no idea what I was smoking when I wrote this comment, because it's completely wrong. The right-hand side //is// eta- reduced, and that's what we're seeing in `-ddump-tc`. (Moreover, we aren't obtaining the eta-reduced variables from the RHS at all. They're actually being taken from the `tyConTyVars` of the representation `TyCon`.) This suggests a straightforward fix: just eta expand the RHS as well, and return that from `etaExpandFamInstLHS` (or really, it just should be called `etaExpandFamInst`). I'll give that a shot. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15852#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler