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Date: Thu, 12 Apr 2012 16:03:44 +0200
From: Alain Frisch <alain@frisch.fr>
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To: Gabriel Scherer <gabriel.scherer@gmail.com>
CC: Leo P White <lpw25@cam.ac.uk>, caml <caml-list@inria.fr>
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Subject: Re: [Caml-list] [ANN] ocamlopen 1.0.2

I'll let Leo answer all your questions.  Here are just my 2 cents:

On 04/12/2012 02:56 PM, Gabriel Scherer wrote:
> 3. What is the semantics of making a *constructor* private?

I guess the idea is to allow pattern matching on that constructor, but 
not using it in positive position (to construct a value).

> 4. When would you say that one should use polymorphic variants rather
> than your open datatypes? (I know how to argue in the other direction:
> unique constructors make for better error messages.)

I've wanted such open datatypes several times.  One example is a message 
bus across an application: some components can yield messages to be 
dispatched to all registered components.  Messages can hold data, and 
the set of possible messages (with the type of their associated data) is 
extensible (say, because components can be loaded dynamically, or just 
to make the application's architecture more modular).  It makes sense to 
use an extensible datatype to represent messages.  Components can react 
to messages with pattern-matching and two components can interact if 
their share a common constructor definition.  This is simpler than 
encoding open datatypes with a "universal" type (with 
injections/projections).  Of course, one can use the existing "exn" 
type, but then we don't distinguish between exceptions and messages at all.


Alain