A newbie question about Arrows.

[Nicolas Oury]

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

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