version(ctfe)

[dsimcha]

== Quote from Don (nospam at nospam.com)'s article
> Denis Koroskin wrote:
> > On Mon, 19 Oct 2009 17:13:46 +0400, dsimcha <dsimcha at yahoo.com> wrote:
> >
> >> This discussion originated in the T[new] thread, but I think it
> >> deserves its own
> >> thread.
> >>
> >> == Quote from Denis Koroskin (2korden at gmail.com)'s article
> >>> An Array!(T) is really just a different name to a T[new]. You'll have
> >>> the
> >>> same problem explaining difference between Array!(T) and T[].
> >>> But you are also creating a nightmare for CTFE. Since you can't use
> >>> "a ~=
> >>> b;" anymore, you'll have to use "a = a ~ b;" which *always*
> >>> allocates. Not
> >>> only it is syntactically less pleasant, this way you render this
> >>> function
> >>> useless at run-time - who in the sane mind will use such an inefficient
> >>> stuff?
> >>
> >> Maybe what we need is a version(ctfe) statement.  Stuff inside such a
> >> block would
> >> be executed only if a function is being compile time evaluated.  When
> >> code is
> >> generated for runtime evaluation the else block would be used.  This
> >> would allow
> >> problems like this to be solved in a well-encapsulated way.  Example:
> >>
> >> uint[] findPrimes(uint maxPrime) {
> >>     version(ctfe) {
> >>         uint[] ret;
> >>     } else {
> >>         ArrayBuilder!uint ret;
> >>     }
> >>
> >>     foreach(i; 0..maxPrime) {
> >>         if(!isPrime(i)) {
> >>              continue;
> >>         }
> >>
> >>         version(ctfe) {
> >>             ret = ret ~ i;
> >>         } else {
> >>             ret ~= i;
> >>         }
> >>     }
> >> }
> >>
> >> Given that CTFE will likely never support everything that is supported
> >> at runtime,
> >> this will likely make it much more useful.
> >
> > It was suggested before and IIRC Walter said it is next to impossible to
> > implement.
> I had a bit of an attempt at it. version(ctfe) seems to be nearly
> impossible.
> But I'm almost certain I could get a magic bool __ctfe to work:
> if (__ctfe) {
>    ...      // only contains D code
> } else {
>     asm { .... }
> }
> It would only be accessible inside unsafe modules (which are the only
> place you'd need it).
> > What you suggest is almost the same as writing 2 different functions.
> > The killer feature of CTFE is that you write it just once and use both
> > at compile and run-time without modifications.
> Yes, but there are problems. A few low-level, high-speed functions can't
> be used in CTFE because they use asm or nasty unions.
> eg, std.math.poly()

So this magic bool would be like a regular if statement in that it would create a
new scope?  I guess if you could do it such that it worked with static if, you'd
be able to get it to work with version.