MessageFunDeps.hs

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}

-- works too

class Message params msg | msg -> params, params -> msg where
  create :: params -> msg

data FooMessage = FooMessage { from, to :: Int } deriving (Show)

data FooMessageParams = FooMessageParams { _from, _to :: Maybe Int }

instance Message FooMessageParams FooMessage where
  create (FooMessageParams from to) = FooMessage (fromMaybe 0 from) (fromMaybe 0 to)

showMessage :: (Message params msg, Show msg) => msg -> String
showMessage = show

fooStr :: String
fooStr = showMessage $ create $ FooMessageParams Nothing Nothing

MessageTypeFamily.hs

{-# LANGUAGE TypeFamilies #-}

class Message a where
  type Params a :: *
  create :: Params a -> a

data FooMessage = FooMessage { from, to :: Int } deriving (Show)

data FooMessageParams = FooMessageParams { _from, _to :: Maybe Int }

instance Message FooMessage where
  type Params FooMessage = FooMessageParams
  create (FooMessageParams from to) = FooMessage (fromMaybe 0 from) (fromMaybe 0 to)

showMeesage :: (Message msg, Show msg) => msg -> String
showMeesage = show

fooStr :: String
fooStr = showMeesage $ create $ FooMessageParams Nothing Nothing

{-
    Couldn't match expected type ‘Params r0’
                with actual type ‘FooMessageParams’
    The type variable ‘r0’ is ambiguous
    In the second argument of ‘($)’, namely
      ‘FooMessageParams Nothing Nothing’
    In the second argument of ‘($)’, namely
      ‘create $ FooMessageParams Nothing Nothing’
-}

MessageTypeFamilyWork.hs

{-# LANGUAGE TypeFamilies #-}

class Message a where
  data Params a :: *
  create :: Params a -> a

data FooMessage = FooMessage { from, to :: Int } deriving (Show)

data FooMessageParams = FooMessageParams { _from, _to :: Maybe Int }

instance Message FooMessage where
  data Params FooMessage = FP FooMessageParams
  create (FP (FooMessageParams from to)) = FooMessage (fromMaybe 0 from) (fromMaybe 0 to)

showMeesage :: (Message msg, Show msg) => msg -> String
showMeesage = show

fooStr :: String
fooStr = showMeesage $ create $ FP $ FooMessageParams Nothing Nothing