A newbie question about Arrows.

6 Jan 2003

      Hello,

I read some of the material  about Arrows on www.haskell.org/arrows and 
I have some questions :

* Why is made the choice to use (,) as Cartesian in first?

Can't we write something like :

class Cartesian p where
     pair :: a -> b -> (p a b)
     projLeft :: (p a b) -> a
     projRight :: (p a b) -> b

class Cartesian pair => Arrow ar where
	arr ::( ar pair a b) -> (ar pair a b)
	(>>>) :: (ar pair a b) -> (ar pair b c) -> (ar pair a c)
	first :: (ar pair a b) -> (ar (pair a d) (pair b d))

(The same could be said for ArrowChoice)

I think this could be more powerful and could allow a smarter 
management of inputs and outputs of arrows than tuples.

I think for example to event-driven arrows : we could make a pair of 
inputs without mixing the "event happened" information.

* Why is made the choice of first as being a base function?

We have

first f = f *** (arr id)

second f = (arr id) *** f

So they could be define from (***).

Moreover,  the definition of f *** with first and second creates an 
order in the use of the f and g (first f >>> second g) whereas it seems 
that (***) is a "parallel"  operator.

In an event-driven arrow we have to give the same events to f and g 
even if they are independent.

Best regards,

Nicolas Oury