#9725: Constraint deduction failure -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 (Type checker) | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: GHC | Related Tickets: rejects valid program | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): But you don't have to decompose the given equality to get the wanted. We have {{{ [G] e1 :: ent k1 ~ M b [G] e2 :: Kn (M b) [W] e3 :: Kn (ent k1) }}} Solve with {{{ e3 := e2 |> Kn (sym e1) }}} The problem is that GHC tries to decompose the given, fails utterly, and then has no way forward. So, I'd say this shows a completeness bug in the solver. It's one we may not want to address, as I agree my kind equality stuff makes this straightforward and I don't see an easy way of solving this problem. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9725#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler