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From: franka@mmintl.UUCP
Newsgroups: net.lang
Subject: and if you put this in your language ...
Message-ID: <1187@mmintl.UUCP>
Date: Fri, 7-Mar-86 22:34:12 EST
Article-I.D.: mmintl.1187
Posted: Fri Mar  7 22:34:12 1986
Date-Received: Tue, 11-Mar-86 01:23:58 EST
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT
Lines: 87

There is a fairly common, relatively simple programming structure which
cannot be easily coded using standard control statements (not gotos) in
any language I know of.  In flow-chart form, the control structure is as
follows:

         |
       A:/\
     +---\/---+
    +-+       |
    |X|       |
    +-+       |
     |        |
   B:/\       |
  +--\/-------+
 +-+         +-+
 |Y|         |Z|
 +-+         +-+
  +-----+-----+
        |

One often winds up writing this in the form:

   if (A) then X;
   if (A && B) then Y; else Z;

However, this fails if executing X changes the value of A (or if A has side
effects); and if evaluating A is expensive, the additional evaluation may be
undesirable.  In languages where the order of execution is not specified for
the 'and' operator, the evaluation of (A && B) may fail if evaluation of B
is not valid unless A is true and/or X has been executed (which is fairly
common).

In C, one often sees something like:

   if (A &&  (X, B)) then Y; else Z;

However, this only works if X can be written in-line.

A solution in full generality requires defining a boolean variable, say L,
and writing:

   if (A) then {X; L = B;} else L = FALSE;
   if (L) then Y; else Z;

I am proposing that a new clause be added to the 'if' statement, which will
represent this function directly, and in a way I think is more readable than
any of these options.  This would be the 'and if' clause (or 'andif',
depending on the language).  It would be used as follows:

   if (A) then X; and if (B) then Y; else Z;

Naturally, one would be able to use multiple and ifs in a single if statement.

Having added an and if, it seems natural to add an or if, as well.  This
would be syntactically identical to the and if, and would represent the
following control structure:

         |
       A:/\
     +---\/---+
    +-+     B:/\
    |X|       \/-+
    +-+       |  |
     +---+----+  |
        +-+     +-+
        |Y|     |Z|
        +-+     +-+
         +---+---+
             |

This seems to me considerably less useful than the and if; I don't
particularly remember ever needing this structure.  There is also the
problem of mixing and ifs and or ifs; the effect of doing this is likely
to being confusing.  However, I don't see any real disadvantages to
including an or if clause in the language, at least if mixed ands and ors
are not allowed; the meaning is quite intuitive.

For languages which don't have and and or operators which specify the order
of evaluation, providing both and if and or if clauses deals with most cases
where such is needed.  In particular, if the statement labelled 'X' in each
example is null, the and if is just an &&, and the or if is an ||.

Has anyone seen constructs like these implemented anywhere?  Does anyone
else think they are a good idea?  Arguments both pro and con are welcomed.

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Multimate International    52 Oakland Ave North    E. Hartford, CT 06108