mixing infix functions and the format4 types

[Anil Madhavapeddy <anil@recoil.org>]

Is there any way to mix infix operators with the Printf format6  
types?  I have a code printing module where this would be quite  
convenient syntactically, but the obvious attempt didn't work:

$ ocaml
         Objective Caml version 3.11.0
# open Printf;;
# let myfn m s = let xfn o = m ^ ": " ^ o in kprintf xfn s;;
val myfn : string -> ('a, unit, string, string) format4 -> 'a = <fun>
# myfn "foo" "bar";;
- : string = "foo: bar"
# myfn "foo" "%s %s" "bar1" "bar2";;
- : string = "foo: bar1 bar2"
# let (---) m s = myfn m s;;
val ( --- ) : string -> ('a, unit, string, string) format4 -> 'a = <fun>
# "foo" --- "bar";;;
- : string = "foo: bar"
# "foo" --- "%s %s" "bar1" "bar2";;
Error: This expression is not a function, it cannot be applied

Is there a trick to let this work, or do I need to stick to using non- 
infix functions for formatting?

thanks
Anil