I wish to say that a certain parametrized monad st works with a regular memory, but a subclass of my parametrized monad should impose an additional constraint on the type of memory. In code:
class Memory m where
...
class State st where
unit :: Memory m => a -> st m m a
bind :: (Memory m1, Memory m2, Memory m3) => st m1 m2 a -> (a -> st m2 m3 b) -> st m1 m3 b
class RMemory m where
...
class State st => RState st where
-- no operators
now my problem is that I wish to impose that whenever (RState st) is true, then inside the definition of (State st) Memory is replaced with RMemory; this would turn State into something that is parametric in the typeclass of its memory. Can this be done?
I suspect you can't do this directly. You can however cheat very effectively by adding a level of indirection. See John Hughes' Restricted Data Types in Haskell for how to do it: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.39.2816
This is the same technique used, eg, to obtain open recursion in syb-with-class.
I'm fairly certain that this should point you in the right direction.
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