RE: [Caml-list] Modules and typing

["Gregory Morrisett" <jgm@CS.Cornell.EDU>]

>  + require that abstract types are pointer types, as in 
> Modula-3 (?) and, more
>    recently, Cyclone

Actually, Cyclone has two different kinds of abstract types:
One abstracts pointer types (really, types that are compatible
with void*) and another kind that abstracts any type.  The latter
kind can only be used under a pointer.  I think this corresponds more 
closely to what Modula-3 provides with it's notion of "ref"
types.  

There's another option that you didn't mention which is the approach
taken by Ada:  Have a notion of "private" types in interfaces, e.g.

  type t
  [private t = int]

The client is type-checked with t treated abstractly, but the 
compiler can then specialize the client knowing what the implementation
of t is.  Of course, by leaking this information into the interface,
you're effectively losing separate compilation in the sense that
if the implementation of t changes, then its interface must also
change and all clients must then be (potentially) re-compiled.  But
this is a simple option which avoids some of the complexity that
you run into if you try to use abstract kinds to classify types
according to implementation details that a compiler might need
(e.g., size, calling-convention, and alignment constraints.)

-Greg

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