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lispcommon-lispeval

Unexpected eval result in CLISP

I run following code of CLISP, but the result looks strange to me.

(setq a 'b)
(setq b 'c)
(setq c 'd)
(setq d 8)
(eval a)
(eval c)
(eval (eval a))

The output of last three line is:

C

8

D

How do I understand the output?

How could last two line have different output?

Please help explain this, thank you so much!

Solution

  • Evaluate (eval c)

    1. get value of variable c -> symbol D
    2. call EVAL with symbol D -> number 8

    Evaluate (eval (eval a))

    1. get value of variable a -> symbol B
    2. call EVAL with symbol B -> symbol C
    3. call EVAL with symbol C -> symbol D

    Some basic evaluation rules for Lisp

    • a symbol evaluates to its value
    • a number evaluates to itself
    • a list (foo-function arg) evaluates first the argument and then calls the function foo-function with that evaluated argument
    • a list (quote something) returns something (whatever it is) as it is