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Re: Optional types

   Date: 07 Dec 2001 12:26:37 -0500
   From: Eric Kidd <eric.kidd@pobox.com>

   Dylan's approach might be summarized as:

   "A vector which happens to contain nothing but integers is not the same
   thing as a vector-of-integers."

   The first has the type '<vector>', the second, 'limited(<vector>, of:

Another approach that one might take here would be to draw a greater
distinction between "class" and "type".  (When I was reviewing drafts
of the Java Language Spec, this was something I talked about a lot;
the released spec has a section entitled "Variables Have Types,
Objects Have Classes" that talks about this.)

What you might do is say that the first is of class '<vector>', and
the second is of class 'limited(<vector>, of: <integer>)'.  But types
need not correspond in a one-to-one-onto way with classes (as in the
Curl example I mentioned recently).

So you could have a type which means "the value of this variable must
be an object that obeys the defined protocol (contract) of a vector,
and furthermore every element of the vector must be an integer".

Obviously there are tradeoffs here; the more your type language can
express things beyond "the value of this variable is an object of
class X", the more you need a whole syntax and semantics of types that
goes beyond the syntax and semantics of classes.  I guess you could
think of such a type language as being a "little language" (in the
sense that Olin uses the phrase), a special-purpose declarative
language for writing certain assertions that turn out to be frequently