Re: "Subtract C, add Ada"

[kst@alsys.com (Keith Thompson)]

In <3f4mbe$rud@cronkite.seas.gwu.edu> dobrien@seas.gwu.edu (David O'Brien) writes:
> I have to wonder.  What if C defined the logical equals operator to be
> "=" and the assignment operator to be ":=" just like Ada, Algol, Pascal,
> Modula-x, etc?  Would this error still exist???
> 
> Quickly thinking about it, I feel that I write a lot more assignment
> statements than local statements, couldn't I be in the habit of typing
> ":=" and thus accidently type that when I meant logical comparison?
> Since that would still be a perfectly valid, conforming C program,
> the compiler is not required to give any warnings (just like current
> practice).  Thus, I would still have the same logic error in my program.
> Chances of happening???  Any bodies guess.  So is the problem really
> one of symbol choice, or program semantics?

I suspect this error would be rarer if C used ":=" for assignment.
The symbol "=" is used for equality in some languages (Pascal, standard
mathethematical notation, etc.) and assignment in others (Fortran,
C, etc.).  The symbol ":=" is used only for assignment.  I don't think
I've ever accidentally typed ":=" for a comparison in Ada.

Digression:
    I once heard about a relatively obscure language that used ":" as
    a catenation operator.  It also had C-style assignment operators.
    So, naturally, the statement "x := y;" assigned to x the result of
    catenating x and y.
end Digression;

Of course, if C used ":=" for assignment and "=" for comparison, the
opposite error would be likely to occur; people would commonly type
    x = y;
which, instead of assigning the value of y to x, would compare them
and discard the result.  This could be caught by a compiler warning;
an assignment in an expression context sometimes makes sense, but a
comparison in a statement context never does.  It could even be forbidden,
but I suppose that would violate the spirit of C.

Hmm.  How about ":=" and ".EQ."?  8-)}

Please note that this article is not an anti-C flame; it's a theoretical
discussion about programming language syntax design.  Yeah, there's
the ticket.

-- 
Keith Thompson (The_Other_Keith)  kst@thomsoft.com (kst@alsys.com still works)
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