OCamlでも型安全なprintf

gistfile1.ml

let dummyVal () = failwith "dummy";;

class virtual ['a] dir (dummy : (unit -> 'a)) = object
  method virtual format_aux : string -> 'a
end;;

class d_end = object
 inherit [string] dir dummyVal
 method format_aux =
  fun out -> out
end;;

class ['a] d_lit (s : string) (d : 'a dir) = object
 inherit ['a] dir dummyVal
 method format_aux =
   fun out -> d#format_aux (out ^ s)
end;;

class ['a] d_int (d : 'a dir) = object
 inherit [int -> 'a] dir dummyVal
 method format_aux =
   fun out i -> d#format_aux (out ^ Printf.sprintf "%d" i)
end;;

class ['a] d_str (d : 'a dir) = object
 inherit [string -> 'a] dir dummyVal
 method format_aux =
   fun out s -> d#format_aux (out ^ s)
end;;

let l s d = new d_lit s d;;
let s d = new d_str d;;
let i d = new d_int d;;
let e () = new d_end;;
let format d = d#format_aux "";;

format (l "hello, " (s (l "!" (e ())))) "world";;