How does one input positive and negative infinities into MIT Scheme?

Question

Section 4.7.2 of the MIT/GNU Scheme Reference Manual states that

The IEEE floating-point number specification supports three special ‘numbers’: positive infinity (+inf), negative infinity (-inf), and not-a-number (NaN).

These constants, in addition to being well-defined IEEE floating-point values, are also useful for range arithmetic. However, I’m unable to use them in my programs:

1 ]=> +inf

;Unbound variable: +inf

Generating these values isn’t easy, either: expressions which seem like they ought to evaluate to floating-point infinities simply don’t:

1 ]=> (flo:/ 1. 0.)

;Floating-point division by zero

How can I input or generate infinite floating-point constants in MIT Scheme?

Answer 1

tests/runtime/test-arith.scm suggests using flo:with-exceptions-untrapped:

;;; XXX The nonsense about IDENTITY-PROCEDURE here serves to fake
;;; out bogus constant-folding which needs to be fixed in SF (and
;;; probably LIAR too).

(define (zero)
  (identity-procedure 0.))

(define (nan)
  (flo:with-exceptions-untrapped (flo:exception:invalid-operation)
    (lambda ()
      (flo:/ (zero) (zero)))))

(define (inf+)
  (flo:with-exceptions-untrapped (flo:exception:divide-by-zero)
    (lambda ()
      (flo:/ +1. (zero)))))

(define (inf-)
  (flo:with-exceptions-untrapped (flo:exception:divide-by-zero)
    (lambda ()
      (flo:/ -1. (zero)))))

The results display as #[NaN], #[+inf], #[-inf] but cannot be input that way.