#14661: Cannot derive (newtype I a b = I (F a -> F b) deriving newtype Category) for type family F -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.4.1-alpha1 Resolution: | Keywords: | DerivingStrategies, deriving, | TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by Iceland_jack): * type: bug => feature request Comment: Changed it to a feature request. This is actually a new deriving strategy. A second derivation for the same thing: I am able to mechanically write type constructors like this (clever / insider knowledge? I don't think so) and it compiles: {{{#!hs instance Category Ixed where id = Ixed id Ixed f . Ixed g = Ixed (f . g) }}} With this derivation your other example `newtype Ixed ix ix' = Ixed (Interp ix -> Maybe (Interp ix'))` does not compile. I am experimenting with categories over type families / data types and the above instance is boilerplate. This space may be too limited to be worth it, maybe someone else has useful examples of a different character. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14661#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler