output - SBCL 末尾的 'T' 是什么

Question

我是 SBCL 编程的新手，我运行了一个简单的加法程序:

    (defvar a)
    (defvar b)
    (defvar c)
    (defvar d)
    (write-line "Enter A:")
    (setf a (read))
    (write-line "Enter B:")
    (setf b (read))
    (format t "~D + ~D = ~D~%" a b (+ a b))

输出:

    * (load "lisp_calculator.lisp")
    Enter A:
    12
    Enter B:
    12
    A + B = 24
    T   <--------- Whats This
    *

那个“T”对我来说没有任何问题，但我只是好奇。

我正在考虑制作一个多线程程序，它将并行执行算术运算，所以我使用的是 SBCL 而不是 CLISP。我在 Kali Linux 2.0 上

Answer 1

在 Lisp 中，每个函数都会返回/计算一个值。 T这里是load的返回值，一般表示true。

来自 Common Lisp the Language :

Any data object other than nil is construed to be Boolean 'not false', that is, 'true'. The symbol t is conventionally used to mean 'true' when no other value is more appropriate. When a function is said to 'return false' or to 'be false' in some circumstance, this means that it returns nil. However, when a function is said to 'return true' or to 'be true'' in some circumstance, this means that it returns some value other than nil, but not necessarily t.