Amit Garg wrote:
Hey all.
I am trying to declare a read-only state monad and a read-write state monad, so as to distinguish between methods on a data type that are read-only vs. read-write.
This is the best I could come up with:
newtype ST s a = ST ( s -> (s,a) ) -- read-only newtype SW s a = SW ( s -> (s,a) ) -- read-write
class ReadM m s a where readM :: m s s runM :: s -> m s a -> (a,s)
class WriteM m r s where updateM :: (s -> s) -> m s ()
instance ReadM ST s a where readM = ST (\s -> (s,s)) runM s (ST c) = c s -- Doesn't work
instance ReadM SW s a where readM = SW (\s -> (s,s)) runM s (SW c) = c s -- Doesn't work
And later on ... updateM s (SW c) = c s
Does that make sense? If not, how do I do it? If so, is there a simpler means of getting there? Thanks.
1. You don't need multiparameter type classes for this. "class ReadM m where ..." and likewise for instance declarations suffices. 2. The declaration for runM in ReadM has wrong order of elements in the result tuple, that causes your "-- Doesn't work" type problems. 3. The suggested definition for updateM doesn't make sense, not even with respect to types. HTH, Janis. -- Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de