racket - how to use the elements of a list as a procedure (function)

Question

I have a list that contains the "names" of some functions (for example '(+ - *))

I want to use that list to call the functions

(define list_of_names_of_functions '(+ - *))

(define sum (car list_of_names_of_functions))

(sum 2 3)

However, sum is a list, so can't be used as a procedure.

How should i do it?

Answer 1

As I mentioned in a comment, using eval is not the right answer to this: it 'works' but it opens the way to horrors.

If you have a symbol like + and you want to get its dynamic value then the Racket approach to doing this is to use namespaces. A namespace is really a machine which turns symbols into values in the same way that eval does, but that's all it does, unlike eval. Unfortunately I don't understand namespaces very well, but this will work:

(define-namespace-anchor nsa)
(define ns (namespace-anchor->namespace nsa))

(define (module-symbol-value s (in ns))
  (namespace-variable-value s #t #f in))

Then

(define function-names '(+ - / *))
(define sum (module-symbol-value (first function-names)))

And now

> (sum 1 2 3)
6
> (eq? sum +)
#t
>

The problem with this approach is that it's leaky: if you have something like this:

(module msv racket
  (provide module-symbol-value)

  (define-namespace-anchor nsa)
  (define ns (namespace-anchor->namespace nsa))

  (define secret "secret")
  
  (define (module-symbol-value s (in ns))
    (namespace-variable-value s #t #f in)))

(require 'msv)

(define oops (module-symbol-value 'secret))

Now oops is secret. A way around this is to use make-base-namespace to get a namespace which is equivalent to racket/base.