Re: [Haskell-beginners] Search for docs on operator '<+>'.

On Wed, Nov 09, 2011 at 07:15:18PM +0100, Ertugrul Soeylemez wrote:

"Allen S. Rout" wrote:

...

Google seems to barf on '<+>'. I've found a haskell "list of operators", and it's not mentioned.

Could some kind soul point me at documentation for this thing?

This is the combination operator from the ArrowPlus type class for arrows, which can be combined in a monoidic fashion. It corresponds to (<|>) from the Alternative type class and in fact you can give a trivial Alternative instance for every arrow that is both ArrowPlus and ArrowZero. Reasonable laws would be:

zeroArrow <+> a = a <+> zeroArrow = a

But I don't think that's official.

Let me point out again, just so the OP is not confused: it completely depends where you saw the <+>. There is no "standard" meaning, although the ArrowPlus one that Ertugrul mentions is common. But you could have seen it in some module where the author defines <+> to mean something completely different. Until you tell us where you saw this <+> we cannot really help. In fact, Allen, I now recall seeing you on the xmonad mailing list -- perhaps you saw <+> in an xmonad config? In that case, indeed, it is NOT the ArrowPlus operator, but something defined by xmonad. In the context of xmonad, <+> is used for combining ManageHooks (actually, it can be used to combine any two things which have a type that is an instance of the 'Monoid' type class, but is most commonly used for ManageHooks). -Brent