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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Spurious diagnostic messages
Date: Wed, 27 May 2015 13:46:34 -0700
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glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
> Keith Thompson <kst-u@mib.org> wrote:
>> Morris Dovey <mrdovey@iedu.com> writes:
>> [...]
>>> One that I've wondered about from time to time is
>
>>>    switch (x)
>>>    {  case { expression }:
>>>          /* case code */
>>>       /* other cases */
>>>    }
>
> (snip)
>>> which causes the named function to be invoked to produce the
>>> TRUE/FALSE result.
>  
>> What's the point of specifying x in `switch (x)`?  
>
> I presume x should be true (1).

That seems unlikely.  Given the proposed syntax:

    switch (x) {
        case { expr1 }: ...;
        case { expr2 }: ...;
        case { expr3 }: ...;
        default:        ...;
    }

I presume it would branch to the first case whose expression is true.

What I don't understand from Morris's description is the relationship
between x and the expressions.  If x itself is expected to be true,
then there seems little point in specifying it.  (I'm probably
making too big a deal over a minor oversight.)

You could have an enhanced form of switch statement where the switch
expression is somehow substituted into each of the case expressions.
For example:

    int sign(int x) {
        switch (x) {
            case <  0: return -1;
            case == 0: return  0;
            case >  0: return +1;
        }
    }

But defining the syntax and how it's evaluated would be non-trivial, and
I'm not convinced it's that much better than an equivalent if/else chain
that refers to x explicitly in each expression.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"