Well, in DDC I believe the order is left to right. But you guys are right, many orders exist.
On the other hand, a language might offer primitives to convert pure-to-effectfull functions no, in which you indicate the order you want. e.g. preOrder map No? (anyway Oleg's reply seems to give a definite answer to this thread no? :-) On Thu, Aug 13, 2009 at 11:06 AM, Heinrich Apfelmus<apfel...@quantentunnel.de> wrote: > Russell O'Connor wrote: >> Peter Verswyvelen wrote: >> >>> I kind of agree with the DDC authors here; in Haskell as soon as a >>> function has a side effect, and you want to pass that function to a >>> pure higher order function, you're stuck, you need to pick the monadic >>> version of the higher order function, if it exists. So Haskell doesn't >>> really solve the modularity problem, you need two versions of each >>> higher order function really, >> >> Actually you need five versions: The pure version, the pre-order >> traversal, the post-order traversal, the in-order traversal, and the >> reverse in-order traversal. And that is just looking at syntax. If you >> care about your semantics you could potentially have more (or less). > > Exactly! There is no unique choice for the order of effects when lifting > a pure function to an effectful one. > > For instance, here two different versions of an effectful map : > > mapM f [] = return [] > mapM f (x:xs) = do > y <- f x > ys <- mapM f xs > return (y:ys) > > mapM2 f [] = return [] > mapM2 f (x:xs) = do > ys <- mapM2 f xs > y <- f x > return (y:ys) > > Which one will the DCC compiler chose, given > > map f [] = [] > map f (x:xs) = f x : map f xs > > ? Whenever I write a pure higher order function, I'd also have to > document the order of effects. > > > Regards, > apfelmus > > -- > http://apfelmus.nfshost.com > > _______________________________________________ > 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
