On Sun, Nov 30, 2008 at 10:25 AM, Larry Evans
Is there some version of haskell, maybe template haskell, that can do that, i.e. instead of:
cross::[[a]] -> [[a]]
have:
crossn::[a0]->[a1]->...->[an] -> [(a0,a1,...,an)]
Ah yes! This is straightforward usage of the list monad. I suggest applicative notation: import Control.Applicative (,,,) <$> xs0 <*> xs1 <*> xs2 <*> xs3 Or alternatively: import Control.Monad liftM4 (,,,) xs0 xs1 xs2 xs3 (I would have used liftA4, but it's not defined. The definition looks a lot like the first example :-) This notation seems a bit magical, but you can build what you want using a simple binary cross: cross :: [a] -> [b] -> [(a,b)] It's just kind of a pain (you build [(a,(b,(c,d)))] and then flatten out the tuples). The applicative notation is a neat little trick which does this work for you. If you're asking whether crossn, as a single function which handles arbitrarily many arguments, can be defined, the short answer is "no". I dare you to come up with a case in which such function adds more than cursory convenience. The long answer is "yes, but you don't want to". It involves mad typeclass hackery, and it doesn't buy you very much. Luke