Capabilities ?

classes.hs

data V4 = V4 deriving Show

data V6 = V6 deriving Show

class V4Getter getter where getIPV4 :: getter -> IO V4

class V6Getter getter where getIPV6 :: getter -> IO V6

class (V4Getter getter, V6Getter getter) => V4V6Getter getter

data Svc1 = Svc1

instance V4Getter Svc1 where getIPV4 _ = return V4

data Svc2 = Svc2

instance V6Getter Svc2 where getIPV6 _ = return V6

data Svc3 = Svc3

instance V4Getter Svc3 where getIPV4 _ = return V4

instance V6Getter Svc3 where getIPV6 _ = return V6

instance V4V6Getter Svc3


f1 :: V4V6Getter getter => getter -> IO (V4, V6)
f1 getter = do
  v4 <- getIPV4 getter
  v6 <- getIPV6 getter
  return (v4, v6)

f2 :: Svc3 -> IO (V4, V6)
f2 getter = do
  v4 <- getIPV4 getter
  v6 <- getIPV6 getter
  return (v4, v6)

{-
-- error: Could not deduce (V6Getter getter)
g1 :: V4Getter getter => getter -> IO (V4, V6)
g1 getter = do
  v4 <- getIPV4 getter
  v6 <- getIPV6 getter
  return (v4, v6)
-}

{-
-- error: No instance for (V4Getter Svc2) arising from a use of ‘getIPV4’
g2 :: Svc2 -> IO (V4, V6)
g2 getter = do
  v4 <- getIPV4 getter
  v6 <- getIPV6 getter
  return (v4, v6)
-}