Path: news.daimi.aau.dk!news.uni-c.dk!iesd.auc.dk!news
From: pnielsen@iesd.auc.dk (Peter Bak Nielsen)
Newsgroups: comp.lang.beta
Subject: Re: Templates
Date: 17 Oct 1995 15:39:22 GMT
Organization: Math & CS at Aalborg University
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Message-ID: <PNIELSEN.95Oct17163923@havlit.iesd.auc.dk>
References: <kolbjorn.aambo-1610951219380001@129.240.37.116>
	<EERNST.95Oct16232633@fraxinus.daimi.aau.dk>
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In-reply-to: eernst@fraxinus.daimi.aau.dk's message of 16 Oct 1995 22:26:33 GMT
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>>>>> "Erik" == Erik Ernst <eernst@fraxinus.daimi.aau.dk> writes:

    Erik> In article <kolbjorn.aambo-1610951219380001@129.240.37.116>
    Erik> kolbjorn.aambo@ub.uio.no (Kolbjørn Aambø) writes:
    Erik>     C++'s tool for creating tempates seems like a powerful
    Erik>     tool to generate better libraries.

    Erik>     [snip]

    Erik>     Have the originators of Beta experimented with the idea
    Erik>     of introducing templates to the language?

    Erik> Not in any way claiming to belong to that group of people ..

    Erik> My point in this article is: the problems solvable by using
    Erik> (some sort of) templates _are already solved_ in BETA today.
    Erik> The basic idea is to parameterize a pattern with one or more
    Erik> types (corresponding to the type arguments to templates) by
    Erik> using virtuals:


    Erik> list: (# element:< object; (* this is a "type parameter" *)
    Erik>          append:< (# elem: ^element; enter elem[] do ... #);
    Erik>           ...
    Erik>       #);

    Erik> personList:
    Erik>     list(# element::< person #); (* usage, i.e. "instantiation" *)

    Erik> This is the typical way to implement generic containers in
    Erik> BETA, and it has a number of advantages:

    Erik> - 'personList' is compile-time (not link-time) type-safe.

    Erik> - 'personList' is-a 'list', so you can have polymorphic
    Erik> access to instances of a "template" - this is not possible
    Erik> with templates, since a template is _nothing_ until you
    Erik> instantiate it with some concrete type arguments.  You
    Erik> cannot have arguments to routines of type "some_template",
    Erik> you cannot have references qualified by "some_template"
    Erik> etc.etc.  The template is simply not a full-fledged citizen,
    Erik> and must always be instantiated before it is useful.  In
    Erik> this way templates are not much more than textual macros.

        [...other advanteges removed...]

Not really related to the original question, but I was wondering how
this polymorphic access is typechecked?

That is, I have an instance of personList, and a reference to it with
the qualification List.

Something like (sorry if this is not correct Beta...):

     p: @personList;
     l: ^list;
     o: ^object;

     ....

     p[] -> l[];

Now, if I do something like

     o -> l.append;

then o must reference a person, thus requiring a run-time typecheck.

Worse, if I have something like

     u: @object;

     ...

     u -> l.append;

which is clearly a type-error. But this piece of code might be in a
precompiled library, compiled long before we thought about creating
personList. Thus we would still need a run-time typecheck.

How is this enforced in Beta? If run-time typechecks are inserted in
general in such situtations, doesn't this mean that most calls to
container classes must be run-time typechecked?


/Peter