Re: Compile time executed functions

[Brian Rogoff <bpr@shell5.ba.best.com>]

On Wed, 28 Mar 2001, Robert A Duff wrote:
> Ted Dennison<dennison@telepath.com> writes:
> 
> > After thinking about this some more, I'll go so far as to say that I don't think
> > there are *any* languages that supply this capability.
> 
> That's not correct.  The Bliss language has a macro facility that can
> use the full power of the language at compile time.  Lisp macros also
> allow the full power of Lisp at compile time.
> 
> As you point out, this means that you can have infinite loops at compile
> time.  In practise, that's not a big deal: you deal with infinite
> compile-time loops the same way as run-time ones.  If you think the
> thing is taking too long, you type control-C and investigate.  Or if
> you're doing a batch build, you set time-outs.  Or you could have the
> compiler count the number of iterations of each loop, and stop after
> "too many".  (After all, a very long loop is just as bad as an infinite
> one.)

That's just what C++ does with template instantiation, where the iteration
limit is set to 17 I think. Before that, you could express arbitrary
computations in the template language and do all sorts of interesting
things at compile time. This aspect of the language was unintentional I
think, but C++ programmers have been able to make good use of it. 

While I'm not generally fond of C++, this is actually one of those things
about the language I find very interesting. I'd like to see these ideas
explored in a future language that shed some of C++'s other baggage. 

Oh yeah, Mercury has a user defined iteration limit for resolving
instances of polymorphic recursion, since they opted to try to infer the 
types of polymorphically recursive functions and that problem is
undecidable. 

-- Brian