Lists are non-deterministic, but the function taken by liftM2 does not necessarily generate all possible outcomes. In the case of (+) it does, not in the case of (-): liftM2 (-) [0,1] [2,3] => [0-1,0-2,1-2,1-3] => [-2,-3,-1,-2] if all possible cases were generated between the two lists we have to include also: [2-0,2-1,3-0,3-1]
-deech On Fri, Jul 23, 2010 at 3:44 PM, Alex Stangl <a...@stangl.us> wrote: > On Fri, Jul 23, 2010 at 11:43:08AM -0700, michael rice wrote: >> What does it mean to "promote a function to a monad?" >> >> It would seem that the monad values must understand the function that's >> being promoted, like Ints understand (+). >> >> Prelude Control.Monad> liftM2 (+) (Just 1) (Just 1) >> Just 2 >> >> But how does one add [0,1] and [0,2] to get [0,2,1,3]? > > It depends upon the semantics of the particular monad. List monads > represent nondeterminism. So, for example, [0,1] represents a 0 or > 1, and [0,2] represents a 0 or 2. > > When you add 0 or 1 to 0 or 2, your possible answers are [0,2,1,3]. > > Alex > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
