MaybeT.hs

-- | Extend a monad with the ability to terminate a computation without a value
module Mini.Transformers.MaybeT (
  -- * Type
  MaybeT (
    MaybeT
  ),

  -- * Runner
  runMaybeT,

  -- * Operations
  nothing,
  anticipate,
) where

import Control.Applicative (
  Alternative (
    empty,
    (<|>)
  ),
 )
import Control.Monad (
  ap,
  liftM,
  (>=>),
 )
import Mini.Transformers.Class (
  MonadTrans (
    lift
  ),
 )

{-
 - Type
 -}

-- | A terminable transformer with inner monad /m/, return /a/
newtype MaybeT m a = MaybeT
  { runMaybeT :: m (Maybe a)
  -- ^ Unwrap a 'MaybeT' computation
  }

instance (Monad m) => Functor (MaybeT m) where
  fmap = liftM

instance (Monad m) => Applicative (MaybeT m) where
  pure = MaybeT . pure . Just
  (<*>) = ap

instance (Monad m) => Alternative (MaybeT m) where
  empty = MaybeT $ pure Nothing
  m <|> n =
    MaybeT $
      runMaybeT m
        >>= maybe
          (runMaybeT n)
          (pure . Just)

instance (Monad m) => Monad (MaybeT m) where
  m >>= k =
    MaybeT $
      runMaybeT m
        >>= maybe
          (pure Nothing)
          (runMaybeT . k)

instance MonadTrans MaybeT where
  lift = MaybeT . fmap Just

{-
 - Operations
 -}

-- | Terminate the computation without a value
nothing :: (Monad m) => MaybeT m a
nothing = empty

-- | Run a computation and get its result
anticipate :: (Monad m) => MaybeT m a -> MaybeT m (Maybe a)
anticipate = lift . runMaybeT . (Just <$>) >=> maybe (pure Nothing) pure