[Gcl-devel] Re: [ 101603 ] numerical quirks

[Raymond Toy]

Camm Maguire wrote:
> Greetings!  I'm forwarding your note to the address@hidden list to
> solicit comments on what a compliant lisp system should do in the
> instances you note.  They all fall under the general rubric of fixed
> precision floating point arithmetic.  Is lisp supposed to be able to
> handle arbitrary precision floating point?

Not usually.

> Does any implementation do
> this?

Clisp allows long-floats with essentially arbitrary precision.  You just 
do  (setf (long-float-digits) 1024) and you have floats with 1024 bits 
of precision.

> I heard rumors once that clisp used gmp for floats too.

I don't think that's true.  Clisp has always used it's own 
implementation.  I think it's as good as gmp.

> From
> my understanding of earlier threads, a lisp implementation is not
> supposed to be able to recognize and return exact answers to special
> floating point problems, e.g. cos(pi/2), sqrt(-1), etc.

I don't think Lisp is forbidden from doing this.  See CLHS, section 
12.1.3.3.

Aloha,

Ray