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
Hello Xiao,
From what I've read from this page (
http://hackage.haskell.org/trac/haskell-prime/wiki/NoDatatypeContexts ), you simply need to remove « Schedulable a » from 9th line.
It would there fore read like so :
data Schedulable a => RoundRobin = RoundRobin [a] [a]
Putting « Schedulable a) => » in your « add » and « next » functions should be sufficient.
Hope this helps,
Jon
(Sorry for the double post, but I think I did something wrong on the first one...)