CODEWITHSUNDEEP
schemelispmit-scheme
JJTO
JJTO

Reputation: 897

How do I evaluate a symbol in MIT Scheme?

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?

Upvotes: 3

Views: 217

Answers (2)

Georg Fuss
Georg Fuss

Reputation: 315

The answer from Sylwester shows the right way to understand the problem. I will try to make it simpler.

(i) The problem arise with every function, not only with the function "null?". 

(define (square x) (* x x))
((car '(square square) 2) ---> The object square is not applicable. 
((car (list square square) 2) ---> 4

(ii) To reduct the problem to more simple expressions:

(car '(square)) ---> square
(car (list square)) --->  (#[compound-procedure 20 square])

(symbol? (car '(square)))        ---> #t
(procedure? (car (list square))) ---> #t

(iii) Before reading your question and sylwester's answer I thought that '(square) and (list square) are the same thing.

Upvotes: 0

Sylwester
Sylwester

Reputation: 48745

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))

Upvotes: 4

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