what environment does `eval` execute in? (scheme)

Question

I have the following myFunc function, the body of which is just an eval statement. Could anyone tell me why eval is NOT seeing a?

Or a broader question would be, where do the arguments to eval get evaluated?

(define myFunc
  (lambda (a)
    (eval 'a)))

(myFunc 1)       ; <<<< this causes undefined var error, WHY? 

; define a var named a in the global
(define a 2)

(myFunc 2)      ; <<<<< this returns 2, WHY?

Answer 1

You should not need to use eval until you are a few years into your Scheme career. By that I mean, many beginners incorrectly believe that eval is needed but it generally is not.

For your example, eval takes an optional second argument which is the environment. If you don't provide the second argument, then the default is (interaction-environment). So, your myFunc defines a local variable a (the argument to lambda) and returns the value for a in the interaction environment.

The first time you invoke (myFunc 1) there is no a in the interaction environment. Then you add a, bound to 2, so the next time you invoke (myFunc <anynumber>) you get a in the interaction environment, which is 2

Your definition for myFunc is not idiomatic. If you really want a function that returns the value of a defined variable use:

(define myFunc
  (lambda (ignore)
    a))

> (define a 2)
> (myFunc 1234567)
2
> (set! a 10)
> (myFunc -9876)
10