eval form supposed to evaluate a given form in a null lexical environment, I don't get what I expect

Question

Let\'s say I have a special var:

(defvar x 20)

then I do the following:

(let ((x 1)) (eval \'(+ x 1))

whi

Answer 1

in your example x is special which means it is bound in the dynamic environment

y is not special, so it is bound in the lexical environment

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so at the time of the first eval the environments could be represented like this:

dynamic environment:  { x : 1 } -> { x : 20, ...other global variables... } -> nil
lexical environment:  nil

the symbol x is special so eval looks up x in the current dynamic environment and finds x = 1

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assuming it was run in same lisp as the last example, the environment of your second eval looks like this:

dynamic environment: { x : 20,  ...other global variables... } -> nil
lexical environment: { y :  1 } -> nil

the symbol y is not special so eval looks up y in the null lexical environment -- not the current lexical environment -- and finds nothing.

this makes sense when you realize that lisp is usually compiled, and the lexical environment can be optimized down to simple mov instructions in some cases.