]> > Hi all. > I'm trying to learn how to programm lazily, but I'm kinda stuck. > I've a list, say let l = [[1;2;3];[4;5];[6;7;8]] and I want to > produce all possibile permutations (1,4,6) (1,4,7) (1,4,8) (1,5,6) > (1,5,7) ... > > it can easily be done with List.iter and a couple of recoursive steps, > but I'm trying to code it in a tail-recoursive style and using lazy > evaluation. Hence my problem is to write a function that gets the list > and gives me back one result (1,4,6) and a lazy structure that encode > the rest of the computation... I looked at lazy streams or lazy lists to > solve this problem, but I was unable to come up with any nice > solution... > > does anybody have any hints ? > > p Here's a nice solution, using Enum's from the Extlib ( http://ocaml-lib.sourceforge.net ). Purely lazy :-) open ExtList let rec enum_permut = function | [] -> Enum.empty() | l :: [] -> Enum.map (fun x -> [x]) (List.enum l) | l :: l2 -> let e = enum_permut l2 in Enum.concat ( Enum.map (fun x -> Enum.map (fun y -> x :: y) (Enum.clone e) ) (List.enum l) ) let print_list l = List.iter (fun i -> print_int i; print_string ",") l; print_newline() ;; let l = [[1;2;3];[4;5];[6;7;8]] in let e = enum_permut l in Enum.iter print_list e ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners