src/generr.lisp

   1 (in-package :maxima)
   2
   3 (defvar errset nil)
   4
   5 ;; Here is the desired behavior of errset:
   6 ;;
   7 ;; (let ((errset t)) (errset (+ 2 'a)))      ;==> signals error
   8 ;; (let ((errset nil)) (errset (+ 2 'a)))    ;==> nil
   9 ;; (let ((errset nil)) (errset (values)))    ;==> (nil)
  10 ;; (let ((errset nil)) (errset (+ 2 3)))     ;==> (5)
  11 ;; (let ((errset nil)) (errset (floor 4/3))) ;==> (1 1/3)
  12 ;;
  13 ;; Notice that there is one ambiguous case: a singleton list
  14 ;; containing only nil could mean either the form really did
  15 ;; return nil, or that the form returned no values at all.
  16 ;; The original "single value" errset had this ambiguous case,
  17 ;; and it was preserved explicitly in this "multiple value"
  18 ;; errset for backward-compatibility and to avoid the ambiguous
  19 ;; case where nil could be returned if an error occurred or if
  20 ;; the form returned no values.
  21
  22 (defmacro errset (&rest l)
  23   `(handler-case (or (multiple-value-list ,(car l)) (list nil))
  24     (error (e) (when errset (error e)))))
  25