Re: [Haskell-beginners] The Missing Arrow Function Strikes Back

Ah. Well, you can implement split and unsplit just as they are written on that page. You can even make split nicer by implementing it as split = id &&& id which avoids the use of 'arr'. However, I don't really see much practical point to 'split' (though it is nice theoretically). Usually, when you split something you follow it up by applying more arrows to both components of the tuple. But in that case you might as well just write (f &&& g) in the first place, instead of split >>> (f *** g). -Brent On Wed, Jun 05, 2013 at 10:46:10PM +0800, Adrian May wrote:

Well I just read this:

http://www.haskell.org/haskellwiki/Arrow_tutorial

Adrian.

On 5 June 2013 21:58, Brent Yorgey wrote:

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What are the types of 'split' and 'unsplit'? It is hard to guess what you want just from their names.

-Brent

On Wed, Jun 05, 2013 at 01:02:40PM +0800, Adrian May wrote:

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Thanks Ertugrul. In the meantime I noticed that split and unsplit are also missing. Is there a similar replacement for them?

Adrian. On 5 Jun 2013 12:57, "Ertugrul Söylemez" wrote:

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Adrian May wrote:

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I just banged up against this problem:

http://haskell.1045720.n5.nabble.com/The-case-of-the-missing-Arrow-function-...

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Was liftA2 (not the applicative one) a bad idea, or is there another way to do it, or what?

That liftA2 (let me call it liftA2') likely has this type signature:

liftA2' :: (Arrow cat) => (b -> c -> d) -> cat a b -> cat a c -> cat a d

Does this sound familiar? You can write this function in terms of the arrow combinators:

liftA2' f c d = arr (uncurry f) . (c &&& d)

However, if your arrow is also a family of applicative functors (i.e. pretty much always),

instance Applicative (MyArrow a)

then it's probably a bad idea, because you really want to use the cleaner liftA2 instead:

liftA2 :: (Applicative f) => (a -> b -> c) -> f a -> f b -> f c

Greets, Ertugrul

-- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.

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