#8026: DatatypeContexts should be fixed, not deprecated -------------------------------------+------------------------------------- Reporter: gidyn | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.6.3 Component: Compiler | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): This proposal ''does'' propose a change to the type system, as it proposes a change to the set of well-typed programs. The same is true of partial type signatures. Type inference is an algorithm that implements a way to enforce the constraints of a type system. Your example is clear for that simple situation. But, Haskell's type system is not that simple! For example: {{{ data Eq a => HasEq a = MkHasEq a deriving Typeable data Show a => HasShow a = MkHasShow a a :: Maybe (HasEq a) -> Maybe (HasEq a) -> Bool a (Just x) (Just y) = x == y a _ _ = Nothing b :: Either (HasEq a) (HasShow a) -> String -- what constraints are used here??? b (Left x) = show (x == x) b (Right x) = show x c :: Proxy (HasEq a) -> () -- what constraints are used here??? c _ = () data Dynamic = forall a. Typeable a => MkDyn a d :: Dynamic -> Maybe Bool d (MkDyn y) | Just (MkHasEq z) <- cast y = Just (z == z) -- where does the Eq constraint come from?? d _ = Nothing }}} Your examples seem to address specific, easy cases. But, we need to consider all the wide range of compositions with other Haskell features! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8026#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler