Path: news.daimi.aau.dk!news.uni-c.dk!sunic!uunet!rosie.next.com!usenet
From: krab@next.com (Kresten Krab Thorup)
Newsgroups: comp.lang.beta
Subject: unsafe virtual patterns?
Date: 20 Sep 1994 21:56:58 GMT
Organization: NeXT, Inc.
Lines: 31
Message-ID: <35nlra$t2@rosie.next.com>
Reply-To: krab@next.com
NNTP-Posting-Host: akvavit.next.com

How can a Beta compiler know that the program below is incorrect, which I  
assume it is.  The problem I see is that it will try to execute the  
pattern xx in an instance of Foo.  It should be obvious that the compiler  
should protect the innocent from that kind of thing, or what?As far as I  
can see, the statement at {*} performs an implicit (unsafe) downcast from  
Foo to Bar; does that generate a runtime error?  Someone, please explain.

Kresten

(#
   Foo : (# do INNER #);

   A : (#
      O :< Foo;
      o: ^O
      enter o[]
   #);

   Bar : Foo (# xx :< (# do 'xx' -> putText #) #);

   B : A (#
      O ::< Bar
      do o.xx
   #);

   a : ^A

   do &B[] -> a;
      &Foo[] -> a {*}
 #)