[r6rs-discuss] Why are fx+ and fx* restricted to two arguments

[William D Clinger]

I am posting this as an individual member of the Scheme
community.  I am not speaking for the R6RS editors.

Concerning the restriction of fx+ and fx* to exactly two
arguments, Mikael Tillenius wrote:
> I still don't understand why. The programmer must make sure that no
> overflow happens (or be prepared to handle the overflow) independent of
> the number of arguments. Two arguments may overflow as well as ten.

That is a correct statement of the programmer's
responsibility, but consider the implementor's
responsibility to raise an exception if the
result is not expressible as a fixnum, while not
raising an exception if it is.

You may be thinking that (fx+ x y z) would be
equivalent to (fx+ (fx+ x y) z), but that is
not so.  For example,

    (let ((i (least-fixnum))
          (j (greatest-fixnum)))
      (list
       (fx+ i i i i i i i i i i j j j j j j j j j j)
       (fx* i i i i i i i i i 0 i i i i i i i i i)))

would not raise an exception but

    (let ((i (least-fixnum))
          (j (greatest-fixnum)))
      (fx+ i i i i i i i i i i j j j j j j j j j))

would.

Will