Question

I have an s-expression bound to a variable in Common Lisp:

(defvar x '(+ a 2)) 

Now I want to create a function that when called, evaluates the expression in the scope in which it was defined. I’ve tried this:

(let ((a 4))   (lambda () (eval x))) 

and

(let ((a 4))   (eval `(lambda () ,x))) 

But both of these create a problem: EVAL will evaluate the code at the top level, so I can’t capture variables contained in the expression. Note that I cannot put the LET form in the EVAL. Is there any solution?

EDIT: So if there is not solution to the EVAL problem, how else can it be done?

EDIT: There was a question about what exactly I am try to do. I am writing a compiler. I want to accept an s-expression with variables closed in the lexical environment where the expression is defined. It may indeed be better to write it as a macro.

Answer 1

You need to create code that has the necessary bindings. Wrap a LET around your code and bind every variable you want to make available in your code:

(defvar *x* '(+ a 2))  (let ((a 4))   (eval `(let ((a ,a))            ,*x*)))