RE [Haskell-cafe] Monad Description For Imperative

ok ok at
Thu Aug 2 21:43:32 EDT 2007

I asked "How is IO a functor"?

On 3 Aug 2007, at 11:50 am, Dan Piponi wrote:
> IO is a fully paid up Monad in the categorical sense. The category is
> the category whose objects are types and whose arrows are functions
> between those types. IO is a functor. The object a maps to IO a. An
> arrow f::a->b maps to (>>= return . f)::IO a -> IO b and that can be
> used to make IO an instance of Functor. The natural transforms eta and
> mu are called return and join.

Please go over this again, but slowly this time.
You have convinced me, but I'd like to understand the details a little

I see that any type constructor TC :: * -> * is halfway to being a  
on this category of types.  It acts on the objects in the obvious way,
so the next step is to see about the arrows.

    If f :: a -> b then we want TC f :: TC a -> TC b

such that TC (f . g) = TC f . TC g and TC (id::a->a) = id :: TC a ->  
TC a

Now this is precisely the Haskell Functor class, so TC is the object  
and fmap is the arrow part.  You say that (>>= return . f) can be  
used to
make [a Monad] an instance of Functor.  Try it... by golly it's true.
I see:  fmap f = (>>= return . f).

So why *aren't* Monads already set up using the type class machinery
to always *be* Functors in Haskell?  Isn't it bound to confuse people
if monads are functors but Monads are not Functors?

This is especially puzzling because Maybe, [], and IO already *are*
Functors, but the way this is done makes it look accidental, not like
the fundamental property of Monads it apparently is.

(By the way, I note that the on-line documentation for Control.Monad  
 >>= as "Sequentially composes two actions...".)

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