Get variable value with a string in scheme

Question

How can we get variable value with a string in scheme language as we can achieve this in Common Lisp:

> (defvar s 3) 
> S
> (symbol-value (intern "S"))
> 3

I am accessing a parameter of parent function from the closure.

EDIT: I have found this solution, but I can't use eval because it evaluates at top level. Searching for alternatives.

(eval (string->symbol "s"))

EDIT 2: I have found that Common lisp code also try to find symbol in global space. So this question is basically for both Lisps(Common Lisp, Scheme).

You can't access a lexical variable in a lexical closure in Common Lisp via a string name. The DEFVAR example is about dynamic/special variables. — Rainer Joswig
– Rainer Joswig, Commented Nov 29, 2017 at 12:23
Maybe it is an X-Y-Problem? meta.stackexchange.com/questions/66377/what-is-the-xy-problem You could have a certain problem, accessing a variable by name could be an approach - which then does not work. But there might be a solution for your original problem? — Rainer Joswig
– Rainer Joswig, Commented Nov 29, 2017 at 12:32
Common Lisp does not have a global space. The default package for interning symbols is usually user (often called cl-user). user uses the package lisp. It is possible to create packages that do not use lisp and creating a package that does not use lisp is a common error when learning about Common Lisp packages. — ben rudgers
– ben rudgers, Commented Nov 30, 2017 at 1:36

Sylwester · Accepted Answer · 2017-11-29 12:54:06Z

5

Don't do that!

Variables are for when you know the variable at compile time. In that case it is never a string. You can still reason about strings in compile time but your code also needs to have a relation with the name for it to be interesting. When you use eval or other forms that evaluate structure and compile/run data in runtime you are probably not doing it right (but not always. I've in my 20 year career used eval intentionally in production code twice)

If you want to store values you use a data structure. An assoc would mimic a dynamic environment. You can also use a hash with a level indicator if the size is harmless.

answered Nov 29, 2017 at 12:54

Sylwester

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user5920214 · Accepted Answer · 2017-11-29 16:38:04Z

You can't do what you want to do and in fact it is a confused thing to want to do.

Here's why what you are trying to do is confused. Consider how a Lisp system for which it was possible to do what you wanted would work. In particular consider something like this:

(define (foo a name)
  (let ([b 10])
    (display (get-value name))
    (* a b)))

Where get-value is meant to be how you get the binding of whatever something is.

So, if I call (foo 10 "b") it should print 10 and return 100.

But wait: b is a compile-time constant in this code. Any compiler worth its salt is going to immediately turn this into

(define (foo a name)
  (display (get-value name))
  (* a 10))

And now there is no binding of b.

So there are two options here: what you want to work works and it is impossible to ever write a reasonable compiler for Scheme, or what you want to work doesn't work, and it is.

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