What is the purpose of Closures in Scheme/Racket?

Question

I am learning Scheme and just came across Closures. The following example provided, demonstrating the use of Closures:

(define (create-closure x)
    (lambda () x))

(define val (create-closure 10))

From what I understand, when the above code is evaluated, val will equal to 10. I realize this is just an example, but I don't understand just how a closure would be helpful. What are the advantages and what would a scenario where such a concept would be needed?

Answer 1

val is not 10 but a closure. If you call it like (val) it returns the value of x. x is a closure variable that still exists since it's still in use. A better example is this:

(define (larger-than-predicate n)
  (lambda (v) (> v n )))

(filter (larger-than-predicate 5) '(1 2 3 4 5 6 7 8 9 10))
; ==> (6 7 8 9 10)

So the predicate compares the argument with v which is a variable that still holds 5. In a dynamic bound lisp this is not possible to do because n will not exist when the comparison happens.

Lecical scoping was introduces in Algol and Scheme. JavaScript, PHP amd C# are all algol dialects and have inherited it from there. Scheme was the first lisp to get it and Common Lisp followed. It's actually the most common scoping.

Answer 2

From this example you can see that the closure allows the functions local environment to remain accessible after being called.

(define count
   (let ((next 0))
     (lambda ()
       (let ((v next))
         (set! next (+ next 1))
         v))))

(count)
(count) 
(count)

0..1..2