[PRL] IOP and disjoint unions

[Richard Cobbe]

On Thu, Apr 28, 2005 at 03:45:45PM -0400, Carl Eastlund wrote:
> On 4/28/05, Dave Herman <dherman at ccs.neu.edu> wrote:
> > When you're writing OO code in an interface-oriented way, how do you
> > encode disjoint unions? The typical way to do it in class-based
> > languages is to encode the variants as subclasses of a base class.
> > 
> > But in IOP this would entail a separate interface for each variant
> > (blech). Worse, variants of disjoint unions in ML-like languages are
> > distinguished with dynamic tags, not static types; this would suggest a
> > dynamic interface check, which contradicts the intuition of interfaces
> > as being static.
> 
> What's wrong with a separate interface for each variant?  In ML, you
> have a separate type for each variant, and interfaces have a lot in
> common with types.  Further, what's wrong with using instanceof checks
> in place of ML's pattern matching or Scheme's constructor predicates?
> And who said anything about interfaces being purely static entities?

First, do you really have a separate type for each variant in ML?

Second, at least in our current thinking about Honu, interfaces/types
are not purely static entities any more than they are in Java.
The cast expression takes an interface name, rather than a class name,
and succeeds only if the object is an instance of a class that
implements the specified interface.

> interface AbsSyn {
> 
>   // Predicates report whether expression has the queried form.
>   Bool isAbs();
>   Bool isVar();
>   Bool isApp();
>   
>   // Selectors produce data from within the expression,
>   // or raise InvalidSelector if expression does not have expected form.
>   String varName();
>   String absVar();
>   AbsSyn absBody();
>   AbsSyn appProc();
>   AbsSyn appArg();
> }

That's essentially what Matthias envisions for Honu, in so far as I
understand it.  I haven't heard his justification for a single type as
opposed to four types.  (The language can express both, of course.)

Richard