Julien, Thanks for your suggestion. I realize that I can paper over the 
problem in various ways, but I was hoping for a more type-theoretic 
approach to solving the problem of needing an implementation-specific 
but otherwise abstract type parameter in printf's signature. If I could 
declare 'b as a function of the particular implementation in question, 
that might solve it, but I'm not sure how to express this.

Warren


Julien Moutinho - julien.moutinho@gmail.com wrote:
> On Sun, Sep 23, 2007 at 07:11:21PM -0700, Warren Harris wrote:
>   
>> 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.
>> [...]
>> This type parameter propagates through numerous places in my code,
>> in some cases requiring other methods to become polymorphic.
>>     
>
> Consider using the class below instead of out_stream directly:
>
> class my'out_stream och buf =
>   object
>     val buffer = new out_stream_of_buffer buf
>     method buffer = buffer
>     val outchan = new out_stream_of_channel och
>     method outchan = outchan
>   end
>
> Or this one, depending on how you get your buf and och:
>
> class my'out_stream'opt
>   ?och ?buf () =
>   object (self)
>     val mutable buffer = None
>     method init_buffer buf =
>         buffer <- Some (new out_stream_of_buffer buf)
>     initializer
>         match buf with None -> ()
>         | Some buf -> self#init_buffer buf
>     method buffer =
>         match buffer with
>         | Some o -> o
>         | None -> failwith "MyStream.my'out_stream'opt#buf: no buffer provided"
>     
>     val mutable outchan = None
>     method init_outchan och =
>         outchan <- Some (new out_stream_of_channel och)
>     initializer
>         match och with None -> ()
>         | Some och -> self#init_outchan och
>     method outchan =
>         match outchan with
>         | Some o -> o
>         | None -> failwith "MyStream.my'out_stream'opt#och: no channel provided"
>   end
>
> HTH,
>   Julien.
>
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