[Haskell-beginners] The Missing Arrow Function Strikes Back

[Adrian May]

Well I just read this:

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

Adrian.



On 5 June 2013 21:58, Brent Yorgey <byorgey at seas.upenn.edu> wrote:

> 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:
> > 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" <es at ertes.de> wrote:
> >
> > > Adrian May <adrian.alexander.may at gmail.com> wrote:
> > >
> > > > I just banged up against this problem:
> > > >
> > > >
> > >
> http://haskell.1045720.n5.nabble.com/The-case-of-the-missing-Arrow-function-td3125388.html
> > > >
> > > > 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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