Alain Frisch writes:
 > As I want to do it often, it would be useful to have a function :
 > 
 > gen_printf: (string -> ()) -> ('a, Buffer.t, unit) format -> 'a
 > [or for performance (string -> ()) -> (char -> ()) -> ('a, ... ]
 > 
 > taking as its first argument a 'printing function'

You can do this with Format (and of course you get pretty-printing as
well)---see Format.formatter_of_out_channel, etc.  I don't know what
the performance difference, if any, between Printf and Format is.