bot.hs

{-
  The idea is to have a way to handle user settings
  in an IRC bot. Something like

  Set UserTimezone SomeTimeZone
  Unset UserTimezone

  Set UserDoB (1990, 1, 1)
  Unset UserDoB

  So I need two things:
  1) a way to specify the arguments for a setting key
  2) a way to execute these commands, in an arbitrary monad

  I'm trying something more advanced than a simple ADT like
  SettingCommand
    = SetTZ SomeTimeZone
    | UnsetTZ
    | SetDoB (Int, Int, Int)
    | UnsetDoB

  I managed 1), but I'm struggling with 2. Below are a few attemps
-}


-- type family to bind arguments to a command
type family SettingArg (key :: Type) :: Type

data UsrTZ = UsrTZ
data UsrDoB = UsrDoB
type instance SettingArg UsrTZ = Text
type instance SettingArg UsrDoB = (Int, Int, Int)


-- the class to actually do stuff
class HandleSettings m a where
  set :: a -> (SettingArg a) -> m ()
  unset :: a -> m ()

instance (MonadIO m) => HandleSettings m UsrTZ where
  set _ _ = pure ()
  unset _ = pure ()

instance (MonadIO m) => HandleSettings m UsrDoB where
  set _ _ = pure ()
  unset _ = pure ()


-- This works but I don't want to have the monad as a type parameter
-- The command parser should not be tied with these details
data UserSettingsCmd m where
  Set :: HandleSettings m a => a -> SettingArg a -> UserSettingsCmd m
  Unset :: HandleSettings m a => a -> UserSettingsCmd m