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From: Christopher Eltschka <celtschk@physik.tu-muenchen.de>
Subject: Re: Suggestion: "typeof" keyword
Date: 1999/08/14
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Approved: Fergus Henderson <fjh@cs.mu.oz.au>
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sirwillard@my-deja.com wrote:
> 
> In article <37AFAC37.6B7F4253@cs.utu.fi>,
>   Jaakko =?iso-8859-1?Q?J=E4rvi?= <jaakko.jarvi@cs.utu.fi> wrote:
> > Francis Glassborow wrote:
> > >
> > > with typeof you do not need declare, however declare would not
> provide
> > > all the utility of typeof, and is less clear in intent.
> >
> > I agree, that declare would not provide all the utility of typeof,
> > but I still think it would be useful (in addition to typeof).
> >
> > See my example:
> > > typeof(a*b+c*d+e/f) x = a*b+c*d+e/f;
> > vs.
> > > declare x = a*b+c*d+e/f;
> > I find the first line less clear in intent.
> 
> I don't.  Even if I agreed, however, I think the idiom to be poor.
> What you are doing is asking the compiler to determine the type of a
> complex expression and at the same time declare an instance of that
> type.  If I can't determine the type at the time I write such a line,
> I'd hardly want the compiler to do so for me.  There may be rare cases
> in which this is useful, but they are too rare to warrant yet another
> keyword when the typeof keyword gives the same identical functionality.

Think f.ex. of expression templates. There you can have
something like

void foo()
{
  Placeholder x;
  integrate(sin(x)/x, 1, 3);
}

Here sin(x)/x evaluates to a type like

Expression<Quotient<Sinus<Placeholder> >, Placeholder>

and integrate is a template on its first parameter.

Now, assume you want to compare the quality of two integration
routines on a complex expression, say sin(x)/x + tan(x)*exp(-3*x*x)

Now what is more clear (and less error prone):

double testinteg()
{
  Placeholder x;
  Expression<Sum<Quotient<Sinus<Placeholder>,Placeholder>,
                 Product<Tan<Placeholder>,
                         Exp<Times<Times<Constant<-3>,x>,x> > > > >
    expr = sin(x)/x + tan(x)*exp(-3*x*x);

  double result1 = integrate1(expr, 1, 3);
  double result2 = integrate2(expr, 1, 3);
  return (result1-result2)/result2;
}

or

double testinteg()
{
  Placeholder x;
  typeof(sin(x)/x + tan(x)*exp(-3*x*x))
    expr = sin(x)/x + tan(x)*exp(-3*x*x);
  // ...
}

or even

double testinteg()
{
  Placeholder x;
  declare expr = sin(x)/x + tan(x)*exp(-3*x*x);
  // ...
}

I guess if making more use of templates, more of those
situations would arise.

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