Functor Applicative Monad.hs

module Optional where

data Optional a = Some a
                | None

instance Functor Optional where
    fmap f (Some x) = Some (f x)
    fmap _ None = None

instance Applicative Optional where
    pure x = Some x
    Some f <*> Some x = Some (f x)
    _ <*> _ = None

instance Monad Optional where
    return x = Some x
    Some x >>= f = f x
    _ >>= _ = None

data User = User { firstname :: String
                 , lastname  :: String
                 }

findUser :: Int -> Optional User
findUser 1 = Some User { firstname = "Christian"
                       , lastname = "Planck"
                       }
findUser _ = None

userFullName :: User -> String
userFullName user = firstname user ++ " " ++ lastname user

usingFmap :: Optional String
usingFmap =
    let
      user = findUser 1337 -- None ...
    in fmap userFullName user

getFirstname :: Optional String
getFirstname = Some "Christian"

getLastname :: Optional String
getLastname = Some "Planck"

usingApply :: Optional User
usingApply = User <$> getFirstname <*> getLastname

type Ranking = String

getRankings :: User -> Optional Ranking
getRankings _user = Some "Top score!!"

usingBind :: Optional Ranking
usingBind =
    let
      user = findUser 1 -- Some ...
    in user >>= getRankings