在ocaml中打印列表

Question

I would write (for debugging) a printer for a list of ((string * string) * 'a)

I tried with:

(* print a list of ((string * string) * 'a)  *)
let rec print_list = function
[] -> Printf.printf "\n empty list \n %!"; ()
| ((s1,s2),a)::l -> Printf.printf "\n(s1,s2)= %s %s  \n %!" s1 s2; print_list l;;

but it doesn't print anything.

It should print at least empty list , right?

Answer 1

In many languages (C and Ocaml) you'll better put the \\n at end of printf format control string, or force the stdout to be flushed. Output to stdout is usually buffered (for performance reasons).

You should read the documentation of Ocaml before coding in Ocaml.

You may also want to call (from Pervasives ) print_newline ; read carefully documentation of Printf module ; you could end your printf format string with %!

Here is an example (copied from your edited question) with Ocaml 4.01:

% ocaml
    OCaml version 4.01.0

 # (* print a list of ((string * string) * 'a)  *)
 let rec print_list = function
   [] -> Printf.printf "\n empty list \n %!"; ()
   | ((s1,s2),a)::l -> Printf.printf "\n(s1,s2)= %s %s  \n %!" s1 s2; 
                       print_list l;;    

val print_list : ((string * string) * 'a) list -> unit = <fun>

# print_list [];;

empty list 
- : unit = ()
# 

As you can see, empty list gets printed. In the above code, % is a shell prompt, # is an ocaml toplevel prompt. Your function is copied verbatim. print_list [];; is input at toplevel.