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From: brnstnd@kramden.acf.nyu.edu (Dan Bernstein)
Newsgroups: comp.lang.misc,comp.object
Subject: Re: CHALLENGE: heterogeneous collections
Message-ID: <26714:Mar2602:52:1891@kramden.acf.nyu.edu>
Date: 26 Mar 91 02:52:18 GMT
References: <jls.669591628@rutabaga> <1991Mar22.210725.29448@neon.S <1991Mar25.220525.11087@leland.Stanford.EDU>
Organization: IR
Lines: 24

In article <1991Mar25.220525.11087@leland.Stanford.EDU> hoelzle@leland.Stanford.EDU (urs hoelzle) writes:
> 2.  Here's why you want at least some form of heterogeneous collections:
> 	type T = "some type"
> 	type SubT = "subtype of T"
> 	procedure someProc(c: List[T])
>     I think that you'll agree with me that you'd like to use someProc with
>     a Collection[SubT], especially if someProc just iterates through the
>     list and sends 'foo' -- after all, every SubT "is a" T.  But as soon as
>     Lists are mutable (e.g. define insert(newElem: T)), the type system
>     won't let you do it because List[SubT] is *not* a subtype of List[T].

``Subtype? What's a subtype?'' says the C programmer.

To implement a generic list type in C, you use macros. Nowhere is there
an explicit ``insert'' operation that demands its own prototypes. So if
you do have an operation that works on multiple types, it will work
correctly through a list.

Maybe one ``object-oriented'' language or another works badly with
lists, but that's not related to static versus dynamic typing.

So: What use can anyone make of a heterogeneous list?

---Dan