[Caml-list] Passing printf format strings to functions

[Erik de Castro Lopo <ocaml-erikd@mega-nerd.com>]

Hi all,

I've got a little problem which I can't seem to get to the
bottom of. The following code snippet won't compile:


    let rec print_string_pairs (fmt:string) =
        function
            [] -> print_endline ""
        |    a :: b ->
                Printf.printf fmt (fst a) (snd a) ;
                print_string_pairs fmt b
        ;;

    let fmt = Printf.sprintf "      %%%ds  ==  %%s\n" 20 ;;

    print_endline fmt ;;

    let pairs = [ ("a", "b") ; ("c", "d") ; ("e", "f") ] ;;

    print_string_pairs fmt pairs ;;

and it gives me an error (line 5 is the Printf.printf line):

    File "fmt.ml", line 5, characters 18-21:
    This expression has type string but is here used with type
      ('a -> 'b -> 'c, out_channel, unit) format =
        ('a -> 'b -> 'c, out_channel, unit, unit) format4

If I replace "fmt" on that line with

    Printf.printf "%s     %s\n" (fst a) (snd a) ;

there is no problem. I think this is something to do with type
inferencing of format strings but I can't get to the bottom
of it.

Any suggestions?

TIA,
Erik
-- 
+-----------------------------------------------------------+
  Erik de Castro Lopo  nospam@mega-nerd.com (Yes it's valid)
+-----------------------------------------------------------+
"Using Java as a general purpose application development language
is like going big game hunting armed with Nerf weapons."
-- Author Unknown

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