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From: brnstnd@kramden.acf.nyu.edu (Dan Bernstein)
Newsgroups: comp.lang.misc
Subject: The search for heterogeneous lists is still on!
Message-ID: <167:Mar3121:32:0891@kramden.acf.nyu.edu>
Date: 31 Mar 91 21:32:08 GMT
Organization: IR
Lines: 22

So far, people have given lots of examples of lists of union types.
Message queues, for instance, are usually union types. The arguments to
printf are union types (though they're not typechecked---I think most
people consider this a flaw in the interface). Symbolic manipulation
routines deal with union types.

People have also given the example of ``list of object that understands
foo,'' where foo is an operation. In object-oriented languages, such
objects are basic types anyway, so these *still* aren't heterogeneous
lists.

Does anyone have an example of a heterogeneous list---something beyond
unions and callback functions---used in a real program?

I suspect not. After all, if a module is like printf and understands
all the possible types of objects in the list, it's dealing with a union
type. If a module does not know all the possible types, it's going to
see just a set of function pointers, and it won't even *want* to deal
with the object except through those function pointers. If there is a
middle case, how would you use it?

---Dan