Edward Z. Yang wrote:
However, explaining how/why sequence does this requires understanding the list monad, which you may or may not want to tackle at this point.
Interesting! According to Hoogle, the sequence function evaluates each monad and then collects the results. I know evaluating a list monad returns a list, but what do they mean by "collect"?
There is also sequence_ which does not collect the results, but throws them away. Prelude> sequence_ [ [1,2,3], [4,5,6] ] [(),(),(),(),(),(),(),(),()] (Also, concerning terminology, sequence evaluates each monadic action. You can't evaluate a monad, it's a type constructor, not a function or value.)
If you import 'Control.Applicative' you can even do exactly what you wanted, with pairs and all:
Prelude Control.Applicative> liftA2 (,) [1,2,3] [4,5,6] [(1,4),(1,5),(1,6),(2,4),(2,5),(2,6),(3,4),(3,5),(3,6)]
Here, liftA2 is applying the function (,) to every possible pair of values from the two lists.
I'm not even going to try to understand that now.
Also known as liftM2 (,) [1,2,3] [4,5,6] Which is the same as do x <- [1,2,3] y <- [4,5,6] return (x,y) Regards, apfelmus -- http://apfelmus.nfshost.com