#15969: Generic1 deriving should use more coercions -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.2 Resolution: | Keywords: Generics Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): I think this problem suggests that `Generic1` got composition the wrong way around. Let's take a slightly bigger example: {{{#!hs data Baz a = Baz (Maybe [Either Int a]) }}} We currently derive {{{#!hs Rep1 Baz = ... (Maybe :.: ([] :.: Rec1 (Either Int))) }}} Suppose we did it the other way: {{{#!hs Rep1 Baz = .... ((Rec1 Maybe :.: []) :.: Either Int) }}} then we'd have the `Rec1` "exposed" properly: {{{#!hs from1 (Baz q) = M1 (M1 (M1 (Comp1 (Comp1 (Rec1 q))))) }}} No fanciness required at all! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15969#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler