Difference between revisions of "What a Monad is not"

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==Monads are not a good choice as topic for your first Haskell blog entry==
==Monads are not a good choice as topic for your first Haskell blog entry==
...just accept that they're [http://byorgey.wordpress.com/2009/01/12/abstraction-intuition-and-the-monad-tutorial-fallacy/ burritos], and wait until later.
...just accept that they're [http://blog.plover.com/prog/burritos.html burritos], and wait until later.
==Monads are not a language feature==
==Monads are not a language feature==

Revision as of 15:46, 21 December 2009


This page is currently an unprocessed braindump. Feel free to dump additional stuff or massage stuff into didactic pleasures.

Also, don't be surprised if you leave this page more confused than before. That just means that it has successfully destroyed your false assumptions, or that you've fallen for some horrible inside joke. Beware of Zygohistomorphic prepromorphisms. Go for warm and fuzzy, instead.

Monads are not a good choice as topic for your first Haskell blog entry

...just accept that they're burritos, and wait until later.

Monads are not a language feature

Really. They are defined in terms of Haskell, not Haskell in terms of them. Conversely,

Haskell doesn't need Monads

...well, apart from the Haskell standard defining the way IO is done in terms of Monads: It could be done differently and still work.

Monads are not impure

...In no way whatsoever. You don't even need flexible morals to claim it. To be more specific, it's IO that's impure. That makes the IO monad impure. But that's not a general property of monads - just IO. And even then, we can pretend that Haskell is a purely functional description language for imperative programs. But we didn't want to employ flexible morals, now did we?

Monads are not about state

While it is certainly possible to abstract away explicit state passing by using a Monad, that's not what a monad is.

(some elaboration needed)

Monads are not about strictness

Monad operations (bind and return) have to be lazy in fact, always! However other operations can be specific to each monad. For instance some are strict (like IO), and some are lazy (like []). Then there are some that come in multiple flavours, like State.

Monads are not values

This point might be driven home best by pointing out that instance Monad Foo where ... is not a data type, but a declaration of a typeclass instance. However, to elaborate:

Monads are not values in the same sense that addition and multiplication are not numbers: They capture a -- very specific -- relationship between values of a specific domain into a common abstraction. We're going to call these values monads manage mobits, somewhat like this:

type Mobit m a = Monad m => m a

The IO monad manages mobits representing side-effects ("IO actions").

The List monad manages mobits representing multiple values ("[a]")

The Reader monads manages mobits that are pure computations that use asks to propagate information instead of explicit arguments

...and while addition and multiplication are both monoids over the positive natural numbers, a monad is a monoid object in a category of endofunctors: return is the unit, and join is the binary operation. It couldn't be more simple.

Monads are not a replacement for applicative functors

Instead, every monad is an applicative functor (as well as a functor). It is considered good practice not to use >>= if all you need is <*>, or even fmap.

Not confusing which features of monads are specific to monads only and which stem from applicative functors is vitally important for a deeper understanding of monads. As an example, the applicative functor interface of parser libraries can parse context-free grammars (and look just like EBNF), while the monadic interface can parse context-sensitive grammars: Monads allow you to influence further processing by inspecting the result of your parse. To understand why, have a look at the type of >>=. To understand why applicative functors by themselves are sufficient to track the current parsing position, have a look at the uu-parsinglib tutorial.

The exact differences are elaborated in even greater detail in Brent Yorgey's excellent Typeclassopedia.

Monads are not about ordering

Monads are commonly used to order sequences of computations. But this is misleading. Just as you can use monads for state, or strictness, you can use them to order computations. But there are also commutative monads, like Maybe, that don't order anything. So ordering is not in any way essential to what a monad is.

I'll try to explain what's meant by ordering. Consider an expression like

   let x = a
       y = b
   in  f x y

That gives the same result as

   let y = b
       x = a
   in  f x y

It doesn't matter what order we write the two assignments. But for doing I/O we'd like ordering. Monads allow use to express

      x <- getChar
      y <- getChar
      return (x,y)

and have it be different from

      y <- getChar
      x <- getChar
      return (x,y)

The second example returns a pair of characters in the opposite order to which they were entered.

However, there are monads for which swapping the order of lines like this makes no difference. For example the Maybe monad.

So while it is correct to say that monads can be used to order operations, it would be wrong to say that monads are a mechanism for ordering operations.

This notion of commutativity is different from the familiar the one in algebra where a+b=b+a. So it's not about the fact that

 (Just 2 >> Just 3) == Just 3

(It took long to explain that, maybe someone can edit this down.)