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From: fjh@mundook.cs.mu.OZ.AU (Fergus Henderson)
Subject: Re: Uninitialized "out" parameters
Date: 1996/07/21
Message-ID: <4stagp$3vg@mulga.cs.mu.OZ.AU>#1/1
X-Deja-AN: 169207078
references: <31EEACDA.64880EEB@sage.inel.gov> <dewar.837740222@schonberg>
 <4sq614$kai@mulga.cs.mu.OZ.AU> <dewar.837899732@schonberg>
organization: Comp Sci, University of Melbourne
newsgroups: comp.lang.ada
Date: 1996-07-21T00:00:00+00:00
List-Id: <comp.lang.ada>


dewar@cs.nyu.edu (Robert Dewar) writes:

>Fergus said
>
>"No, certainly it is possible.  There are languages in which any code
>that might attempt to use an uninitialized variable is a compile-time
>error.  It would certainly be possible for an Ada compiler to give
>warnings in all cases where a variable might be used before it was
>initialized."
>
>Well it depends what you mean. If we read what you say literally, it is
>obviously incorrect, since it requires the halting problem to be solved:

No, if you read it literally, it's correct -- but you're not reading it
literally, you're reading into it more than what I wrote.

>If you don't mind getting bogus warnings, i.e. warnings that might be
>false, then your statement is trivially true,

Yes.  I never said anything about not getting some spurious warnings
(or spurious errors).

Presuming the spurious warnings/errors don't occur often, and so long
as there are easy work-arounds when they do occur, I don't think that
would be a major problem.  Certainly programmers have demonstrated
a willingness to accept that sort of thing in other aspect of
programming languages (e.g. type systems).

>    procedure x is
>        m,n : integer;
>    begin
>       (big chunk of code not referencing m,n)
>       m := n;
>    end;
>
>this code references the uninitialized variable n if and only if the big
>chunk of code halts. That's an easy proof.

Similarly, in a dynamically typed Ada-like language, the code

    procedure x is
        m : integer;
        n : some_other_type;
    begin
       (big chunk of code)
       m := n;
    end;

causes a run-time type error if and only if the big chunk of code halts.
Of course, Ada uses static type checking, rather than dynamic typing;
they report some "spurious" type errors such as for the example above.

There's no reason why Ada compilers couldn't do the analagous sort of
thing for uninitialized variables.

--
Fergus Henderson <fjh@cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh@128.250.37.3         |     -- the last words of T. S. Garp.