NoopMonad (https://jazzy.id.au/2015/04/01/noop-monad.html)

Noop.hs

module Noop (
  Noop(..),
  noop
) where

-- Groundbreaking work to do nothing safely
import Control.Applicative
import Control.Monad

newtype Noop a = Noop { runNoop :: a -> () }

noop :: Noop a
noop = Noop $ const ()

instance Eq (Noop a) where
  (==) _ _ = True

instance Ord (Noop a) where
  compare _ _ = EQ

instance Enum (Noop a) where
  fromEnum _ = 0
  toEnum _ = noop

instance Bounded (Noop a) where
  minBound = noop
  maxBound = noop

instance Read (Noop a) where
  readsPrec _ s = [(noop , "")]

instance Show (Noop a) where
  show _ = "<noop>"

instance Monoid (Noop a) where
  mempty = noop
  mappend _ _ = noop

instance Alternative Noop where
  empty = mempty
  (<|>) = mappend

instance MonadPlus Noop where
  mzero = mempty
  mplus = mappend

instance Functor Noop where
  fmap _ _ = noop
  {-# INLINE fmap #-}

instance Applicative Noop where
  pure _ = noop
  {-# INLINE pure #-}
  _ <*> _ = noop
  {-# INLINE (<*>) #-}

instance Monad Noop where
  return = pure
  {-# INLINE return #-}
  _ >>= _ = noop
  {-# INLINE (>>=) #-}