[Haskell-beginners] Illegal datatype context when modelling round robin scheduler

6 Dec 2012

      Hi, I'm trying to model a round robin scheduler in Haskell.
...
class Schedulable s where
  isFinal :: s -> Bool
class Scheduler s where
  add :: (Schedulable a) => a -> s -> s
  next :: (Schedulable a) => s -> (a, s)
  empty :: s -> Bool
data Schedulable a => RoundRobin = RoundRobin [a] [a]
instance Scheduler RoundRobin where
  add p (RoundRobin ps qs) = RoundRobin (ps ++ [p]) qs
next (RoundRobin []     qs) = next (RoundRobin qs [])
  next (RoundRobin (p:ps) qs) = (p, RoundRobin ps (qs ++ [p]))
empty (RoundRobin [] _) = True
  empty _                 = False
However, GHC complains that
...
main.hs:9:6:
    Illegal datatype context (use -XDatatypeContexts): Schedulable a =>
After some searches I realized that datatype contexts are deprecated.
So how can I model the scheduler without using datatype contexts?

Thanks.

--
Regards,
Xiao Jia