Dear Kakadu, Gabriel, Edouard,

thanks a lot for the valuable hints! It is not so obvious from the manual. For those curious (like me) how the types of such functions might look like, here are a few examples:

(* 'a is a function from the arguments of the first format (prefix) to 'c.
   'c is a function from the arguments of the second format (fmt) to 'd.
   'd is a function from the arguments of the third format (postfix) to unit.
   So 'a is a function from arguments required by all formats to unit.
   And 'c is a function from arguments required by the last 2 formats and to unit. *)
let bracketprintf
   (prefix : ('a, out_channel, unit, unit, 'b, 'c) format6)
   (suffix : ('d, out_channel, unit, 'e, unit, unit) format6)
   (oc     : out_channel)
   (fmt    : ('c, out_channel, unit, 'b, 'e, 'd) format6)
: 'a
= Printf.fprintf oc (prefix ^^ fmt ^^ suffix);;

(* 'a is a function from the arguments of <fmt> followed by two int arguments for the supplied suffix to unit.
   The two string arguments for the internal prefix are supplied by the string -> string -> 'a *)
let myprintf1 (oc : out_channel) (fmt : ('a, out_channel, unit, unit, unit, int -> int-> unit) format6) : string -> string -> 'a =
  bracketprintf "Prefix1 %s %s\n" "Suffix1 %d %d\n" oc fmt
;;

(* As above, but with the ^^ directly in the function *)
let myprintf2 (oc : out_channel) (fmt : ('a, out_channel, unit, unit, unit, int -> int-> unit) format6) : string -> string -> 'a =
  Printf.fprintf oc ("Prefix2 %s %s\n" ^^ fmt ^^ "Suffix2 %d %d\n")
;;

(* As above, but the two string parameters for the prefix are supplied internally *)
let myprintf3 (oc : out_channel) (fmt : ('a, out_channel, unit, unit, unit, int -> int-> unit) format6) : 'a =
  Printf.fprintf oc ("Prefix3 %s %s\n" ^^ fmt ^^ "Suffix3 %d %d\n") "abc" "def"
;;

(* If parameters for the suffix shall be applied internally, kfprintf needs to be used instead of format concatenation *)
let myprintf4 (oc : out_channel) (fmt : ('a, out_channel, unit, unit, unit, unit) format6) : 'a =
  Printf.kfprintf (fun oc -> Printf.fprintf oc "Suffix4 %d %d\n" 5 6) oc ("Prefix4 %s %s\n" ^^ fmt) "abc" "def"
;;

let () =
  let test = open_out "Test.txt" in
  myprintf1 test "%d %d\n" "abc" "def" 3 4 5 6;
  myprintf2 test "%d %d\n" "abc" "def" 3 4 5 6;
  myprintf3 test "%d %d\n" 3 4 5 6;
  myprintf4 test "%d %d\n" 3 4;

Best regards,

Michael

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