Explanation of Monad laws

Question

From a gentle introduction to Haskell, there are the following monad laws. Can anyone intuitively explain what they mean?

return a >>= k             = k a
         


        

Answer 1

Your descriptions seem pretty good. Generally people speak of three monad laws, which you have as 1, 2, and 4. Your third law is slightly different, and I'll get to that later.

For the three monad laws, I find it much easier to get an intuitive understanding of what they mean when they're re-written using Kleisli composition:

-- defined in Control.Monad
(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c
mf >=> n = \x -> mf x >>= n

Now the laws can be written as:

1) return >=> mf = mf                  -- left identity
2) mf >=> return = mf                  -- right identity
4) (f >=> g) >=> h = f >=> (g >=> h)   -- associativity

1) Left Identity Law - returning a value doesn't change the value and doesn't do anything in the monad.

2) Right Identity Law - returning a value doesn't change the value and doesn't do anything in the monad.

4) Associativity - monadic composition is associative (I like KennyTM's answer for this)

The two identity laws basically say the same thing, but they're both necessary because return should have identity behavior on both sides of the bind operator.

Now for the third law. This law essentially says that both the Functor instance and your Monad instance behave the same way when lifting a function into the monad, and that neither does anything monadic. If I'm not mistaken, it's the case that when a monad obeys the other three laws and the Functor instance obeys the functor laws, then this statement will always be true.

A lot of this comes from the Haskell Wiki. The Typeclassopedia is a good reference too.

Answer 2

No disagreements with the other answers, but it might help to think of the monad laws as actually describing two sets of properties. As John says, the third law you mention is slightly different, but here's how the others can be split apart:

Functions that you bind to a monad compose just like regular functions.

As in John's answer, what's called a Kleisli arrow for a monad is a function with type a -> m b. Think of return as id and (<=<) as (.), and the monad laws are the translations of these:

1. id . f is equivalent to f
2. f . id is equivalent to f
3. (f . g) . h is equivalent to f . (g . h)

Sequences of monadic effects append like lists.

For the most part, you can think of the extra monadic structure as a sequence of extra behaviors associated with a monadic value; e.g. Maybe being "give up" for Nothing and "keep going" for Just. Combining two monadic actions then essentially concatenates the sequences of behaviors they held.

In this sense, return is again an identity--the null action, akin to an empty list of behaviors--and (>=>) is concatenation. So, the monad laws are translations of these:

1. [] ++ xs is equivalent to xs
2. xs ++ [] is equivalent to xs
3. (xs ++ ys) ++ zs is equivalent to xs ++ (ys ++ zs)

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These three laws describe a ridiculously common pattern, which Haskell unfortunately can't quite express in full generality. If you're interested, Control.Category gives a generalization of "things that look like function composition", while Data.Monoid generalizes the latter case where no type parameters are involved.

Answer 3

The first three laws say that "return" only wraps a value and does nothing else. So you can eliminate "return" calls without changing the semantics.

The last law is associativity for bind. It means that you take something like:

do
   x <- foo
   bar x
   z <- baz

and turn it into

do
   do
      x <- foo
      bar x
   z <- baz

without changing the meaning. Of course you wouldn't do exactly this, but you might want to put the inner "do" clause in an "if" statement and want it to mean the same when the "if" is true.

Sometimes monads don't exactly follow these laws, particularly when some kind of bottom value occurs. That's OK as long as its documented and is "morally correct" (i.e. the laws are followed for non-bottom values, or the results are considered equivalent in some other way).

Answer 4

In terms of do notation, rule 4 means we can add an extra do block to group a sequence of monadic operations.

    do                          do
                                  y <- do
      x <- m                             x <- m
      y <- k x          <=>              k x
      h y                         h y

This allows functions that return a monadic value to work properly.