haskell - Generalizing the functions of a key/value database for both on disk (IO) and pure cases? -

the basic functions of key value database store, fetch, , delete. i'm attempting write typeclass allows return types of these functions either pure or in io can write single engine works both in-memory data.map backed implementation , on-disk implementation.

the pure typeclass might this:

class purequeryable d   fetch  :: d -> key -> maybe value   insert :: d -> key -> value -> d   delete :: d -> key -> maybe d 

...whereas io typeclass might this:

class ioqueryable d   fetch  :: d -> key -> io (maybe value)   insert :: d -> key -> value -> io d   delete :: d -> key -> io (maybe d) 

i'm attempting combine these two, , suggested (vektorweg1) on #haskell wrote this:

class queryable d r   fetch  :: d -> key -> ???    insert :: d -> key -> value -> ???    delete :: d -> key -> ???  

i unsure of write in place of ??? work. ideas?

like carl, have doubts utility of well, here goes nothing (note i'm typing on phone, syntax associated types may wrong):

{-# language typefamilies #-}  class queryable (d :: * -> *)   type key   type m   fetch :: d v -> key -> m v   insert :: d v -> key -> v -> m v   delete :: d v -> key -> m () 

now can instantiate class using functor:

instance queryable iointdict   type key iointdict = int   type m = io   ... 

for pure stuff, use data.functor.identity functor, or maybe use identity type family (i'm not sure approach work out better).