Are monad laws enforced in Haskell?

Question

From the Haskell wiki:

Monads can be viewed as a standard programming interface to various data or control structures, which is captured by the Monad cl

Answer 1

You are responsible for enforcing that a Monad instance obeys the monad laws. Here's a simple example that doesn't.

Even though its type is compatible with the Monad methods, counting the number of times the bind operator has been used isn't a Monad because it violates the law m >>= return = m

{-# Language DeriveFunctor #-}

import Control.Monad

data Count a = Count Int a
    deriving (Functor, Show)

instance Applicative Count where
    pure = return
    (<*>) = ap

instance Monad Count where
    return = Count 0
    (Count c0 a) >>= k = 
        case k a of
            Count c1 b -> Count (c0 + c1 + 1) b