foo.hs

class BuildList r where
    type Build r 
    build' :: [Build r] -> Build r -> r 
 
instance BuildList [a] where
    type Build [a] = a
    build' l x = reverse $ x:l
 
instance BuildList r => BuildList (a-> r) where
     type Build (a -> r) = Build r
     build' l x = \ y -> build'(x:l)  y
 
-- --build :: forall r a. (BuildList a r) => a -> r
-- build :: (BuildList a r) => a -> r
-- build x = build' [] x

class Mutation m where
    type Ref m :: *         -> *
    newRef     :: a         -> m (Ref m a)
    readRef    :: (Ref m a) -> m a
    writeRef   :: (Ref m a) -> a -> m ()

instance Mutation IO where
    type Ref IO = IORef
    newRef   = newIORef
    readRef  = readIORef
    writeRef = writeIORef