Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!mcnc!decvax!ima!haddock!karl
From: karl@haddock.ISC.COM (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: While talking about useful additions, how about this one??
Message-ID: <2249@haddock.ISC.COM>
Date: 13 Jan 88 23:20:24 GMT
References: <253@vsi1.UUCP>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 28
Summary: A simpler, yet more powerful, counterproposal

In article <253@vsi1.UUCP> steve@vsi1.UUCP (Steve Maurer) writes:
>While we are on the subject of useful additions to C, how about this
>one: pre-compiler switching operators '??' and '::', which ask for
>evaluation at compile time, instead of run time.

The proposed syntax is bad: "??" conflicts with trigraph notation (if they're
still in the standard), and "::" conflicts with C++ usage.

>[If one attempts to define a macro POW(X,Y) as
>Y == 0.5 ? sqrt(X) : Y == 0 ? 1 : Y == 1 ? X : Y == 2 ? X * X : pow(X,Y)
>,] the resulting code is horrendous.

If Y is a constant, any decent compiler will do the right thing.  You don't
need a new operator for that.  If I understand your proposal, the only real
enhancement is that "??" treats all non-constants as false.

I counterpropose a builtin "isconstant()", which tests whether its argument is
a compile-time constant.  Thus, "isconstant(Y) && Y == 2 ? X * X : pow(X,Y)".
This is a much smaller change than your proposal, and it's much easier to
squeeze in a new unary operator than a new ternary.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint
________
Note 1: I intentionally removed the redundant parens to make it easier to
concentrate on the issues at hand.
Note 2: Although "pow" is used in the example, a macro can't solve the pow()
problem.  However, C++ overloading and inlining, combined with isconstant(),
would do the trick.