How do I tell if the value of a variable is a symbol bound to a procedure in Scheme?

Question

I am familiar with Common Lisp and trying to learn some Scheme, so I have been trying to understand how I'd use Scheme for things I usually code in Common Lisp.

In Common Lisp there's fboundp, which tells me if a symbol (the value of a variable) is bound to a function. So, I would do this:

(let ((s (read)))
  (if (fboundp s)
      (apply (symbol-function s) args)
      (error ...)))

Is that possible in Scheme? I've been trying to find this in the R6RS spec but coudn't find anything similar.

Answer 1

This way?

1. check if it is a symbol
2. evaluate the symbol using EVAL to get its value
3. check if the result is a procedure with PROCEDURE?

Answer 2

In Scheme, functions are not tied to symbols like they are in Common Lisp. If you need to know, whether a value is actually a procedure, you can use the procedure? predicate:

(if (procedure? s) (do-something-with s) (do-something-else))

There is no direct way in portable Scheme to achieve, what your example code wants to do, as symbols in Scheme are simply kind of unified strings, lacking Common Lisp's value/function/plist slots.

You could try something like:

(define function-table (list `(car ,car) `(cdr ,cdr) `(cons ,cons) `(display ,display)))

(let* ((s (read))
       (f (cond ((assq s function-table) => cadr)
                (else (error "undefined function")))))
    (apply f args))

i.e., defining your own mapping of "good" functions. This would have the advantage, that you can limit the set of function to only "safe" ones, or whatsoever.