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From: new@ee.udel.edu (Darren New)
Newsgroups: comp.object
Subject: Re: A Hard Problem for Static Type Systems
Message-ID: <51785@nigel.ee.udel.edu>
Date: 24 Apr 91 20:22:24 GMT
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In article <2392@optima.cs.arizona.edu> gudeman@cs.arizona.edu (David Gudeman) writes:
>OK, I give up.  What is the difference between dynamic typing and
>dynamic binding?

How about an example?  Hermes gives a declaration (polymorph) and two
operations  (wrap and unwrap) for dealing with it.  You can take a
typed value and wrap it into a polymorph. Later, you can take that
polymorph and unwrap it into a typed value (with appropriate runtime
checks).  However, there is nothing else you can do with the polymorph
(except ask its type and such) while it is wrapped: specifically, you
cannot operate on the wrapped typed value.  This feature allows
building of hetrogeneous lists, resource managers, dictionaries, and so
on. However, it seems to be a pain to build `objects' in Hermes because
each capability for sending a message must be unwrapped before use if
it is passed in a polymorph and must be predeclared of the right type
if it is not a polymorph.  Either each "object" will have a dictionary
of polymorphs which must be unwrapped or will have a separate
declaration for all possible combinations of messages handled (clearly
unfeasible).

C++ has static typing (in that it is illegal to perform an operation on
something that the declared type does not support), but it is possible
that the same call will invoke different methods for different values
of arguments. This I call dynamic binding.

Smalltalk mixes this up because the type of a value *always* determines
what method a message will be bound to, and thus people tend to confuse
these two properties.

-- 
--- Darren New --- Grad Student --- CIS --- Univ. of Delaware ---
----- Network Protocols, Graphics, Programming Languages, FDTs -----
+=+ Nails work better than screws, when both are driven with hammers +=+