In Haskell performing `and` and `or` for boolean functions

One simplification,

f_and = liftM2 (&&)
f_or  = liftM2 (||)

or

      = liftA2 (&&)         
      = liftA2 (||)

in the ((->) r) applicative functor.

Applicative version

Why? We have:

instance Applicative ((->) a) where
    (<*>) f g x = f x (g x)

liftA2 f a b = f <$> a <*> b

(<$>) = fmap

instance Functor ((->) r) where
    fmap = (.)

So:

  \f g -> liftA2 (&&) f g
= \f g -> (&&) <$> f <*> g          -- defn of liftA2
= \f g -> ((&&) . f) <*> g          -- defn of <$>
= \f g x -> (((&&) . f) x) (g x)    -- defn of <*> - (.) f g = \x -> f (g x)
= \f g x -> ((&&) (f x)) (g x)      -- defn of (.)
= \f g x -> (f x) && (g x)          -- infix (&&)

Monad version

Or for liftM2, we have:

instance Monad ((->) r) where
    return = const
    f >>= k = \ r -> k (f r) r

so:

  \f g -> liftM2 (&&) f g
= \f g -> do { x1 <- f; x2 <- g; return ((&&) x1 x2) }               -- defn of liftM2
= \f g -> f >>= \x1 -> g >>= \x2 -> return ((&&) x1 x2)              -- by do notation
= \f g -> (\r -> (\x1 -> g >>= \x2 -> return ((&&) x1 x2)) (f r) r)  -- defn of (>>=)
= \f g -> (\r -> (\x1 -> g >>= \x2 -> const ((&&) x1 x2)) (f r) r)   -- defn of return
= \f g -> (\r -> (\x1 ->
               (\r -> (\x2 -> const ((&&) x1 x2)) (g r) r)) (f r) r) -- defn of (>>=)
= \f g x -> (\r -> (\x2 -> const ((&&) (f x) x2)) (g r) r) x         -- beta reduce
= \f g x -> (\x2 -> const ((&&) (f x) x2)) (g x) x                   -- beta reduce
= \f g x -> const ((&&) (f x) (g x)) x                               -- beta reduce
= \f g x -> ((&&) (f x) (g x))                                       -- defn of const
= \f g x -> (f x) && (g x)                                           -- inline (&&)