What a Monad is not

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Revision as of 21:37, 23 November 2009 by Ertai (talk | contribs) (Fix the strictness part)
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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.

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 in a category of endofunctors. It's all very 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 what features of monads are specific to monads only and which stem from applicative functors are 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

It's a commonplace that monads are about ordering 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.

> Just 2 >> Just 3

Just 3

That's not commutative in the same intutive sense as "a + b === b + a".

Commutativity, wrt. Monads, means that

(x <- a; y <- b; return (x,y)) === (y <- b; x <- a; return (x,y))

So we need some better notion of "ordering" to get to the bottom of this.