Question

I’m going through the Structure and Interpretation of Computer Programs MIT video lecture series, and I had a quick question about returning side-effects from functions.

In video 3A, the professor writes a quick equation for for-each similar to this:

(define (for-each p l)
   (if (null? l)
      "done"
      (p (car l)
         (for-each p (cdr l)))))

Is there a specific convention for returning a side-effect from a function in Scheme, or was the "done" an arbitrary choice?

Answer 1

That’s not really a side effect, but rather that every Scheme function must return something. The string "done" is something. Normally, when you call for-each you might do something like:

(for-each (lambda (x) (display x)) lst)

and ignore the return value from for-each. The author could have just as easily chosen any other value to return from the base case.