Mathias Kende wrote: > I need to write something like this : > > let f f i = if i = 0 then 1 else i * f (i - 1) > let rec g = f g > > Of course the compiler won't let me write it (even if the OCaml type > system is happy): > "This kind of expression is not allowed as right-hand side of `let rec'" In general, the best thing to do in this case is to switch to lazy evaluation:
# let f f i = if i = 0 then 1 else i * Lazy.force f (i-1);; val f : (int -> int) Lazy.t -> int -> int = <fun> # let rec g' = lazy (f g');; val g' : (int -> int) Lazy.t = <lazy> # let g = Lazy.force g';; val g : int -> int = <fun> # g 10;; - : int = 3628800 Lukasz Stafiniak wrote: > While we are at it, what is the best way to convert a "straight" list > into a cyclic list? > > i.e. convert > > let l = a::b::[] > > into > > let rec l = a::b::l > > (for arbitrary length lists). (The answer I recall from the archives > was using Obj.magic to mutate the [] in the original list). Obj.magic is not part of the OCaml language :-) Again, you can do that just fine using lazy lists instead of lists: type 'a lazylist = 'a lazylist_content Lazy.t and 'a lazylist_content = Nil | Cons of 'a * 'a lazylist Hope this helps, - Xavier Leroy _______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
