show maybet

maybet.hs

{-# LANGUAGE InstanceSigs #-}

module MaybeT where

import Control.Monad.Trans

newtype MaybeT m a = MaybeT { unMaybeT :: m (Maybe a) }

instance MonadTrans MaybeT where
  lift :: Monad m => m a -> MaybeT m a
  lift ma = MaybeT $ fmap Just ma

instance Functor m => Functor (MaybeT m) where
  fmap :: forall a b. (a -> b) -> MaybeT m a -> MaybeT m b
  fmap a2b (MaybeT ma) = MaybeT go
    where
      go :: m (Maybe b)
      go =
        let _ = ma :: m (Maybe a)
        in fmap inner ma

      inner :: Maybe a -> Maybe b
      inner maybeA = undefined

instance Applicative m => Applicative (MaybeT m) where
  pure :: a -> MaybeT m a
  pure a = MaybeT $ pure (Just a)

  (<*>) :: forall a b. MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b
  MaybeT ma2b <*> MaybeT ma = MaybeT go
    where
      go :: m (Maybe b)
      go =
        let _ = ma2b :: m (Maybe (a -> b))
            _ = ma :: m (Maybe a)
        in inner <$> ma2b <*> ma

      inner :: Maybe (a -> b) -> Maybe a -> Maybe b
      inner maybeA2b maybeA = undefined