#12418: Make `MonadCont (ContT r m)` polykinded (r::k), (m::k -> Type) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Core Libraries | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): I want to make sure I understand this correctly, this brittleness applies to monad transformers and not the instances below `↓` since they are already kind polymorphic? {{{#!hs instance forall k (r :: k) (m :: k -> *). Functor (ContT r m) instance forall k (r :: k) (m :: k -> *). Applicative (ContT r m) instance forall k (r :: k) (m :: k -> *). Monad (ContT r m) }}} ---- I have no use for a kind-polymorphic instance (that I know of!), the original motivation was allowing {{{#!hs callCC :: ((a -> ContT r m b) -> ContT r m a) -> ContT r m a callCC @(ContT _ _) }}} to match the instance. {{{#!hs instance k ~ Type => MonadCont (ContT @k r m) }}} would allow `callCC @(ContT _ _)` but not `callCC :: ((a -> ContT r m b) -> ContT r m a) -> ContT r m a` it seems (I get `Couldn't match type ‘k1’ with ‘*’ arising from a use of ‘callCC’`). I don't understand your point about `MonadState` and `MonadReader`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12418#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler