Path: utzoo!attcan!uunet!ogicse!uwm.edu!cs.utexas.edu!rice!uw-beaver!Teknowledge.COM!unix!hplabs!hpl-opus!hpccc!hpcc01!hp-ses!hpcuhb!hpcllla!hpclisp!hpclscu!shankar
From: shankar@hpclscu.HP.COM (Shankar Unni)
Newsgroups: comp.lang.c++
Subject: Re: default arguments
Message-ID: <58170008@hpclscu.HP.COM>
Date: 9 Mar 90 20:33:53 GMT
References: <820@xyzzy.UUCP>
Organization: Hewlett-Packard Calif. Language Lab
Lines: 38

> >      void bar(int x=i+j) {
> >      printf("%d\n", x);
> >      }
> >      main() {
> >      int i=20, j=30;
> >       bar();
> >       }
> >   ...I intend to propose this as a change in ANSI C++, and have already
> >   proposed it to AT&T in private correspondance...
> 
> Surely you can't be serious about this?  You are proposing dynamic
> scoping for C++.  C and C++ have always been lexically scoped, and the
> problems with dynamic scoping are so great that languages like Lisp,
> which used to be dynamically scoped, are moving away from it.
> 

Hello, "thunks"!! (Who called them that? Hoare?)

> - How do you decide which i and j should be used?  You have to scan up
> the stack at run-time -- a very expensive proposition.

Does anyone remember the famous "call-by-name" in Algol 6*? Just as an
example (at undergrad school), I tried calculating the Ackermann function
(I tried "ackermann(3,4)" for a decent data point) using both call-by-name
(the default!) and call-by-value, and got:

   Call by value : 6 seconds  (OK, so it was a slow machine :-/)
   Call by name : >1200 seconds (The job was killed by the sysadmin)

Mark, are you still serious about your proposal?

I suggest a re-reading of some elementary (and preferably slightly old)
principles-of-programming-languages text (Pratt, or the collection of
papers by Dahl, Dijkstra, and Hoare) for a description of call-by-name, and
what has to be done to implement it.
-----
Shankar Unni                                   E-Mail: shankar@hpda.hp.com
Hewlett-Packard California Language Lab.