the little schemer - what is the difference between these two Scheme functions? -

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  1. (define self-add (let ((x 0))   (lambda ()     (set! x (+ x 1))     x)))

(self-add) => 1

(self-add) => 2

(self-add) => 3

(self-add) => 4

    2. 
 (define self-add1     (lambda ()       (let ((x 0))         (set! x (+ x 1))         x)))

(self-add1) => 1

(self-add1) => 1

(self-add1) => 1

please tell me how understand difference between above 2 functions? lot in advance! best regards.

the first function defines local variable x initial value of 0 , afterwards binds lambda special form name self-add - x "enclosed" lambda (that's why lambda behaves closure) , same invocations of self-add (you x "remembered" self-add), , each time gets called value of x incremented one.

the second function binds lambda procedure , afterwards defines local variable x inside lambda - here x redefined each time self-add1 gets called, , different invocations: x never "remembered" self-add1 , created anew every time procedure gets called, initialized 0 , incremented, returning value 1.


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