Erik de Castro Lopo writes:
 > 
 > 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
 >         ;;

A format is not of type "string" but of type "('a,'b,'c) format". Here
is how you can write such a function:

======================================================================
let rec print_string_pairs (fmt:(string->string->'a,'b,'c) format) =
  ...
======================================================================

But the type of fmt can be inferred, thus you can simply write:

======================================================================
let rec print_string_pairs fmt =
  ...
======================================================================

Note that this second version is now polymorphic : it applies to any
format of type ('a -> 'b -> 'c, out_channel, unit) format and a list
of type ('a * 'b) list:

======================================================================
# print_string_pairs "%s->%s\n" [ ("a", "b") ; ("c", "d") ; ("e", "f") ];;
a->b
c->d
e->f

# print_string_pairs "%d->%d\n" [ 1,2; 3,4; 5,6 ];;
1->2
3->4
5->6
======================================================================

 >     let fmt = Printf.sprintf "      %%%ds  ==  %%s\n" 20 ;;
 > 
 >     print_endline fmt ;;
 > 
 >     let pairs = [ ("a", "b") ; ("c", "d") ; ("e", "f") ] ;;
 > 
 >     print_string_pairs fmt pairs ;;

There  is a additional  difficuly here,  because you  want to  build a
format string  dynamically. With the code  above, fmt will  be of type
string, and then cannot  be passed to print_string_pairs. As suggested
by Michael,  you can use format_of_string  to convert a  string into a
format:

======================================================================
# let fmt = format_of_string "%20s == %s\n";;
val fmt : (string -> string -> '_a, '_b, '_c, '_a) format4 = <abstr>
# print_string_pairs fmt [ ("a", "b") ; ("c", "d") ; ("e", "f") ];;
                   a == b
                   c == d
                   e == f
======================================================================

But format_of_string  only applies  to a _constant_  string, not  to a
string built from an evaluation:

======================================================================
# let fmt = format_of_string (Printf.sprintf "      %%%ds  ==  %%s\n" 20) ;;
Characters 27-71:
  let fmt = format_of_string (Printf.sprintf "      %%%ds  ==  %%s\n" 20) ;;
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This expression has type string but is here used with type
  ('a, 'b, 'c, 'd) format4
======================================================================

Indeed, there is  no static way to check that  the resulting format is
indeed of  type (string->string->'a,'b,'c) format. 

Hope this helps,
-- 
Jean-Christophe

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