gistfile1.txt

-- Applicative instance where we have NOT (ap = (<*>))
-- is this valid?

data ParFallible e a = Fail e | Success a

instance Functor (ParFallible e) where
    fmap _ (Fail e) = Fail e
    fmap f (Success a) = Succes (f a)
    
instance Monoid e => Applicative (ParFallible e) where
    pure = Success
    Fail e <*> Fail e' = Fail (e <> e')
    Fail e <*> Success _ = Fail e
    Success _ <*> Fail e = Fail e
    Success f <*> Success a = Success (f a)
    
instance Monoid e => Monad (ParFallible e) where
    return = Success
    Fail e >>= _ = Fail e
    Success a >>= f = f a