Question

Is it possible to dereference a variable contained inside a list, to obtain its value? For example:

(define one 1)
(define two 2)

(define list '(one two))

(display (list-ref list 0))

Here list-ref references to one, and display shows one in letters. Could instead one dereference to the value contained by the homonym variable?

Answer 1

Eval can definitely solve this problem… but like most situations where eval is applicable, it’s a large and dangerous hammer.

Matthew Flatt’s blog post on this topic has become the go-to explanation:

http://blog.racket-lang.org/2011/10/on-eval-in-dynamic-languages-generally.html

Here’s how you might do it without eval in Racket. Stripping away the cruft, “dict-ref” can find a named
element (or elements) in an “association list”.

#lang racket

(define data
  '((one 1)
    (two 2)))


(define wanted-list '(two one))

;; evaluates to '((2) (1)):
(for/list ([wanted wanted-list])
  (dict-ref data wanted))