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

Answer 2

No, the monad laws are not enforced by the language. But if you don't adhere to them, your code may not necessarily behave as you'd expect in some situations. And it would certainly be confusing to users of your code.