Observable.hs

{-# LANGUAGE TypeFamilies, FlexibleContexts, ExistentialQuantification, ScopedTypeVariables #-}

type Token = String

class Observable ob where
    type EventType ob
    addObserver :: ob -> (EventType ob -> IO ()) -> IO Token
    removeObserver :: ob -> Token -> IO ()

class Observable ob => ObservableData ob where
    readData :: ob -> IO (EventType ob)

data AnyObservableData t = forall o. (ObservableData o, EventType o ~ t) => AnyObservableData o

instance Observable (AnyObservableData t) where
    type EventType (AnyObservableData t) = t
    addObserver (AnyObservableData o) = addObserver o
    removeObserver (AnyObservableData o) = removeObserver o

instance ObservableData (AnyObservableData t) where
    readData (AnyObservableData o) = readData o

test :: (ObservableData ob, Show (EventType ob)) => ob -> IO ()
test ob = do
    d <- readData ob
    putStrLn $ show d

main = do
    return ()