question about polymorphic methods

[Warren Harris <warren@liveops.com>]

I have a simple output stream class that abstracts over out_channels and 
buffers. I would like to add a printf method that takes a format 
directive, and calls the appropriate Printf function. To do this, I 
needed to introduce a polymorphic method which abstracts over the first 
type parameter to the format type (the 'a parameter of printf):

class type ['b] out_stream =
object
  method print : string -> unit
  method printf : 'a . ('a, 'b, unit) format -> 'a
  method flush : unit
end

class out_stream_of_buffer buf =
object (self : Buffer.t #out_stream)
  method print str = Buffer.add_string buf str
  method printf : 'a . ('a, Buffer.t, unit) format -> 'a =
    fun fmt -> Printf.bprintf buf fmt
  method flush = ()
end

class out_stream_of_channel och =
object (self : out_channel #out_stream)
  method print str = output_string och str
  method printf : 'a . ('a, out_channel, unit) format -> 'a =
    fun fmt -> Printf.fprintf och fmt
  method flush = flush och
end

However, as you can see from this code, I also needed to abstract over 
the second parameter to format in the definition of the out_stream class 
type ('b is the type of the first argument of the Printf function). This 
type parameter propagates through numerous places in my code, in some 
cases requiring other methods to become polymorphic. This is 
unfortunate, since 'b should be completely hidden by the particular 
implementation of out_stream (Buffer.t in the case of 
out_stream_of_buffer, or out_channel in the case of out_stream_of_channel).

Is there some other way to implement this that I'm overlooking? It seems 
like 'b should be "monomorphic" ('_b) and determined uniquely whenever 
the printf method is called. However, if I eliminate the 'b class type 
parameter, I get the following error:

  .......... out_stream =
  object
    method print : string -> unit
    method printf : 'a . ('a, 'b, unit) format -> 'a
    method flush : unit
  end
Some type variables are unbound in this type:
  class type out_stream =
    object
      method flush : unit
      method print : string -> unit
      method printf : ('a, 'b, unit) format -> 'a
    end
The method printf has type 'a. ('a, 'b, unit) format -> 'a where 'b
is unbound

Any suggestions on a better way to do this would be appreciated.

BTW, all this code would be unnecessary if ocaml provided an 
output_channel_of_buffer primitive. :-)

Warren