SBCL 意外陷入浮点数？

Question

考虑 Common Lisp 中的以下代码：

  (defun range (max &key (min 0) (step 1))
    (loop for n from min below max by step
          collect n))
  (reduce #'* (range 61 :min 1))

这给出了预期的 bignum 值 60!，即

8320987112741390144276341183223364380754172606361245952449277696409600000000000000

但是，下面的代码（不包括我能看到的任何浮点数强制转换）会生成一个浮点数答案：

  (defun fact (n)
    (if (= 0 n)
        1
        (* n (fact (- n 1)))))

  (fact 60)
  8.32098711274139e+81

问题是"why?"和"how can I write a straightforward, recursive fact in SBCL (Steel-Bank Common Lisp) that produces a bignum result?"

Answer 1

强制转换发生在 emacs 端。 ob-lisp.el 调用读取结果。尝试在 *scratch* 缓冲区上对其进行评估，以亲自查看

(read "8320987112741390144276341183223364380754172606361245952449277696409600000000000000")

SBCL surprise lapse into floating point?