Path: news.cs.au.dk!krab
From: Kresten Krab Thorup <krab@california.cs.au.dk>
Newsgroups: comp.lang.beta
Subject: Re: Is Beta a statically typed language?
Date: 09 Feb 1999 17:48:46 +0100
Organization: DAIMI, Computer Science Dept. of Aarhus Univ.
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Jean-Louis Leroy <jll@skynet.be> writes:

> Unfortunately the current Beta collections suffer from the same problem:
> 
> (#
>    Person: (#  #);
>    people: @Set (# element:: Person #);
>    backdoor: ^Set
> do
>    &Person[]->people.insert;
>    people[]->backdoor[]; (* no warning *)
>    &integerObject[]->backdoor.insert (* trouble *)
> #)
> 
> The problem here is that there exists an automatic, silent conversion from a 
> Set of Persons to a Set of Objects.

You're right that there is a silent conversion, but is not the
problem.  The problem is the insertion of an ^IntegerObject into Set.

    &IntegerObject[] -> backdoor.insert 

The compiler flags this as problematic (it warns) because "backdoor"
may refer to a subtype of Set which has a stronger requirement on the
element type, (and that is indeed the case here).

Our new paper "Unifying Genericity" to appear at ECOOP'99 discusses
these issues at further detail.

	http://www.cs.au.dk/~krab/ug3.{pdf,dvi,ps.gz}   

Herein we suggest a type system similar to BETA's which flags these
situations as errors rather than warnings.  (Plus we add some
structural subtyping for virtual types, making them more useful as
parameterized classes).  

The "Virtual Types for Java" paper mentioned earlier is at 

	http://www.cs.au.dk/~krab/virtuals.{dvi,ps}

-- Kresten

       Kresten Krab Thorup, M.Sc.,  "I like my eggs ploded"
       Dept. of Computer Science, University of Aarhus
       Container #210, Åbogade 34, DK-8200 Aarhus N
       Ph: +45 8942 5665; Mobile: +45 2343 4626; Fax: +45 8942 5624