#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 ekmett): `Foldable` is safe in the presence of any constraint at all on `a`. It doesn't ever produce one, just consumes them. All occurrences of `a` in the `Foldable` dictionary are in negative position, and it can safely ignore any constraints on `a` you put in place. `Functor` and `Traversable` need to keep the check, though. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10447#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler