]> On Tue, 18 Nov 2003, Dustin Sallings wrote: > > I read something on the list about how a function may be tail > recursive, but not be compiled with tail call optimization. What kinds > of things might cause this? > > Specifically, I've got an ``iter_lines'' function I'd like to turn > into a ``fold_lines'' function that looks something like this (a few > different functions for different things): > > let rec fold_lines f init_value ch = > try > let v = f (input_line ch) init_value in > fold_lines f v ch > with End_of_file -> init_value; > ;; This function is not tail recursive. Basically, if the recursive call either a) is wrapped in a try block, or b) has it's return value modified in any way, the function isn't tail recursive. Your function violates clause a, the following function violates clause b: let append a b = match a with | [] -> b | h :: t -> h :: (append t b) ;; Since we're appending h to the return value of the recursive call, it isn't tail recursive. I recommend coding the your function like: let rec fold_lines f init_value ch = let line, eof = try (input_line ch), false with End_of_file -> "", true in if eof then init_value else fold_lines f (f line init_value) ch ;; Now that the recursive call is outside the try block, and you aren't modifying the return value, so all is good. > dustinti:~/prog/eprojects/snippets/ocaml/lib 586% wc -l numbers > 4769526 numbers > # Fileutils.fold_file_lines (fun x y -> y + 1) 0 "numbers";; > Stack overflow during evaluation (looping recursion?). Stack overflows are a classic sign of a function you thought was tail recursive not being tail recursive. -- "Usenet is like a herd of performing elephants with diarrhea -- massive, difficult to redirect, awe-inspiring, entertaining, and a source of mind-boggling amounts of excrement when you least expect it." - Gene Spafford 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