{-# LANGUAGE
  EmptyDataDecls,
  TypeFamilies
  #-}
module Types.Data.Maybe
    ( Maybe
    , Nothing
    , Just

    , IsNothing
    , IsJust

    , FromJust
    , FromMaybe
    )
    where

import Prelude ()
import Types.Data.Bool


data Nothing
data Just a

class    Maybe a
instance Maybe Nothing
instance Maybe (Just a)

type family   IsNothing m
type instance IsNothing Nothing  = True
type instance IsNothing (Just a) = False

type family   IsJust m
type instance IsJust Nothing  = False
type instance IsJust (Just a) = True

type family   FromJust m
type instance FromJust (Just a) = a

type family   FromMaybe a m
type instance FromMaybe a Nothing   = a
type instance FromMaybe a (Just a') = a'