[fpc-pascal] New feature: IfThen() intrinsic

[Henry Vermaak]

On Tue, Feb 02, 2016 at 12:03:14PM +0100, Michael Van Canneyt wrote:
> On Tue, 2 Feb 2016, Henry Vermaak wrote:
> >When you call a function all the arguments get evaluated before the
> >function gets called.  iif will look exactly like a function call,
> >but all the arguments may not be evaluated (that's the point, right).
> >This is inconsistent and ambiguous, two things that pascal tries to
> >eschew.
> >
> >Even worse is that it's not a reserved symbol, so it can still be
> >overridden.  A call to iif in one unit may evaluate all its
> >arguments, while it may not in another unit.
> >
> >Pascal is not a language full of programmer traps, let's try not to
> >add any!
> 
> Well, see my reply about 'robust' code for an observation about these
> details.

There you talk about order of evaluation, which is irrelevant to what
I'm trying to say.  Maybe I'll try an example to explain what I'm on
about.

Say you have code like this:

IfThen(IsSomething, MungeA(A), MungeB(B));

Currently MungeA(A) and MungeB(B) are called, irrespective of the value
of IsSomething.  The order is irrelevant (I always assumes that it was
undefined, like in C) and I agree that it's wrong to rely on.

If you replace it with the new IfThen (or iif), only MungeA(A) _or_
MungeB(B) will be called.  This is exactly what you want from a
conditional operator.

The conditional operator shouldn't look like a function call, because it
doesn't act like a function call: it will not evaluate all its
arguments.  Again:  nothing to do with order, it's whether they actually
get evaluated in the first place.

Henry