]> Brian, The optimization you describe is sometimes known as "tail modulo cons", and is an example of "destination-passing style". In other words, the place to put the result (in this case, the address of the tail of a just-constructed cons cell) is passed on in a tail-recursive call. See "A Functional Representation of Data Structures with a Hole" by Minamide in POPL'98. http://www.score.is.tsukuba.ac.jp/~minamide/index.html Although Minimide formalizes the problem in the context of a typed intermediate language, it's probably quite easy to spot special cases quite far down the compiler pipeline. - Andrew. > -----Original Message----- > From: Brian Hurt [mailto:brian.hurt@qlogic.com] > Sent: Friday, January 24, 2003 12:48 AM > To: Ocaml Mailing List > Subject: [Caml-list] @, List.append, and tail recursion > > > > I hit a bug recently wiith @ and List.append. Since they're > recursive, > not tail-recursive, on long enough lists Ocaml thinks you've gone > infinitely recursive and aborts. The code: > > > let longlist len = > let rec longlist_int v c acc = > if (c == 0) then acc else longlist_int (v + 1) (c - > 1) (v :: acc) > in > longlist_int 0 len [] > ;; > > let x = longlist 65536 ;; > > List.append x [] ;; > > Exits with: > > Stack overflow during evaluation (looping recursion?). > > So does: > x @ [] ;; > > You can work around this like: > > let append' a b = > List.rev_append (List.rev a) b > ;; > > Since both rev_append and rev are tail recursive (looping) and not > recursive, this works. But some ad-hoc testing says that > this method is > about 50% slower than normal append for lists short enough > not to abort. > > Thinking about this, I realized that my code is doing stuff > like this all over the place. I'm basically doing sparse > vector/matrix stuff, handling > (effectively) (colno * value) list for vectors, and (rowno * > vector) list for matrix. And I may be hitting lists long > enough to trip the problem. > > Which means I'm currently doing a lot of recursion of the form: > > let rec foo x = > match x with > [] -> [] > | head :: tail -> (expr head) :: (foo tail) > ;; > > for various complexities. And it has occured to me that all of these > forms *should* be optimizable into loops. The general case > would work > something like this in C: > > struct list_t { > void * datum; > struct list_t * next_p; > } > > struct list_t * foo (struct list_t * x) { > struct list_t * retval = NULL; > struct list_t ** ptr_pp = &retval; > > while (x != NULL) { > struct list_t * temp = alloc(sizeof(struct list_t)); > *ptr_pp = temp; > temp->datum = expr(x->datum); > temp->next_p = NULL; /* be nice to the GC */ > ptr_pp = &(temp->next_p); > x = x->next_p; > } > return retval; > } > > If expr() returned a list, the only change necessary would be > to find the > end of the list before moving on, like: > > struct list_t * foo (struct list_t * x) { > struct list_t * retval = NULL; > struct list_t ** ptr_pp = &retval; > > while (x != NULL) { > *ptr_p = expr(x->datum); /* expr allocates the list */ > /* We assume the last element of the list expr() returned has > * NULL for next_p. > */ > while (*ptr_p != NULL) { > ptr_p = &((*ptr_p)->next_p); > } > x = x->next_p; > } > return retval; > } > > Rather than just looking at making @ an inline C function, I > think we (the > Ocaml community) should be looking at adding this more general > optimization in. > > So now we get to my two questions: > a) is anyone working on this/intending to work on this RSN? > b) if the answer to (a) is no, can anyone give me some > pointers on where > to start looking at code, so I can add it in? > > Brian > > > ------------------- > 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 ------------------- 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