Question

I’m creating a program that allow a user introduce a lambda function only writing it in the interface writing box like this:

(lambda (p q) (and (not p) q))

This is the result “(lambda (p q) (and (not p) q))” because scheme convert it to a string instead of a procedure that can be evaluated.

How can i convert this string into a procedure?

Answer 1

In Scheme, the eval function only operates on lists of symbols, so this would work fine:

(eval '(+ 1 2))

But this would give an error:

(eval "(+ 1 2)")

To convert a string from the UI into a list of symbols (AKA an S-expression), you’ll need to call the read function. Regularly, read only operates on “ports” (file stream), so you’ll need to convert your string into an input port before it’s usable with read:

(define (read-string text)
  (read
    (open-input-string text)))

All that’s left is to evaluate the string, and call the lambda that you get as a result:

(define (call-string text-lambda . args)
  (apply
    (eval (read-string text-lambda))
    args))

Here’s how you can now calculate the number 3:

(call-string "(lambda (a b) (+ a b))" 1 2)

You find the APIs for read and open-input-string at the R5RS and SRFI websites.