#9043: Add missing type class instances for data types in GHC.Generics -------------------------------------+------------------------------------ Reporter: ocharles | Owner: ocharles Type: feature request | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by ekmett): Makes a lot of sense to me. The missing `Functor`/`Foldable`/`Traversable` instances for these have bitten me before and the instances are entirely determined by their laws / convention. There are a few other instances that make sense uniformly: e.g. `U1`, `Par1`, `Rec1`, and `(:*:)`, and `M1` can also be made instances of `Applicative`, `Monad`, `MonadZip` in a completely unambiguous way determined by the laws just by lifting them from their parts. `(:.:)` admits an unambiguous `Applicative`, but not the others. It might also make sense to look into making the types in there instances of `Generic` / `Generic1` for bootstrapping purposes, and they are currently missing `Typeable` and `Data` instances as well. The instances probably have to live out mostly where the typeclasses come from rather than in the module itself to avoid cycles, but otherwise I don't see any headaches, and they are entirely determined by the definitions of the classes involved, so there is no bikeshedding opportunity. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9043#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler