#10447: DeriveFoldable rejects instances with constraints in last argument of data type -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #8678 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by RyanGlScott): I'm probably making an assumption about how robust GHC's equality constraint solver is, but I would think that it'd be capable enough to infer whether an argument's type is equal to the type parameter. If it were undecidable, then wouldn't this (legal) code have to be rejected? {{{#!hs data S a b where MkS :: (a ~ Int) => Int -> S a b deriving instance Foldable (S a) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10447#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler