How do I evaluate a symbol in MIT Scheme?

Question

I have a following in Scheme:

((car '(null? null?)) ())

which should evaluate to #t but I'm getting an error:

the object null? is not applicable

I tried some of the solutions in other SO questions but none of them seems to work.

How do I evaluate a symbol?

Answer 1

It should not evaluate to #t. You are mixing symbols and variables. The second you quote something it's the code representation that becomes data.

'(null? null?)
; ==> (null? null?)

That is a list with two symbols. They have noting to do with:

null? 
; ==> #<procedure:null?> (implementation dependent visualization)

When you evaluate the variable null? you get the closure object. If you want to make a assoc of primitives you need to use list or cons to not quote the variables or you need to use quasiquote-syntax:

(define *primitives* 
  `((null? . ,null?)
    (car . ,car) 
    (cdr . ,cdr)))

This is just syntax sugar for using list and cons. When you evaluate this you notice right side isn't symbols:

*primitives*
; ==> ((null? . #<procedure:null?>)
;      (car . #<procedure:car>)
;      (cdr . #<procedure:cdr>))

Again. The visualization of a procedure differs. Here is how you can use it:

(define prim 'car)
(let ((found (assq prim *primitives*)))
  (if found
      ((cdr found) '(1 2 3))
      'signal-error))