Difficulty understanding nested expressions in Lisp

Question

Here's a function that asks a number and returns the value if its type is indeed a number and else executes the function again:

(defun ask-number ()
  (format t "Please enter a number.~%")
  (let ((val (read)))
    (if (numberp val)
        val
        (ask-number))))

I understand that after the value is read, it is labelled as val and the whole ((val (read))) is an argument of let. What I don't understand is, why the if-statement is nested within let. I would have assumed that the program should be something like this:

(defun ask-number ()
  (format t "Please enter a number.~%")
  (let ((val (read))))
  (if (numberp val)
      val
      (ask-number)))

which leads to an error. I am not sure why this happens.

C. K. Young · Accepted Answer · 2014-10-03 14:17:52Z

4

The reason the if is inside the let is that the val you've created with the let is only valid within the let; once you exit the let, val doesn't exist any more.

let is syntactic sugar for creating and instantly calling a lambda expression, so your let expression is basically the same as:

((lambda (val)
   (if (numberp val)
       val
       (ask-number)))
 (read))

answered Oct 3, 2014 at 14:17

C. K. Young

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1 Comment

Joshua Taylor Over a year ago

As an analogy with a C program, you couldn't do void foo(int x) { x = 3; } printf("%d", x);" The variable is no longer in scope. let` introduces scope, just like a function definition does. You could do this, though: (let ((val nil)) (set! val (read)) (if (numberp val) ...)).

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