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    <p>Welcome to the wonderful world of closures! This is a textbook example of how closures in Scheme work.</p> <p>So <code>make-counter</code> returns a function that has 3 variables that it captures from it's enclosing environment: <code>n</code>, <code>old</code>, <code>new</code>. In this case, the starting environment looks something like</p> <pre><code>_name_|_value_ n | 3 old | 0 new | 1 </code></pre> <p>On each invocation, it increments <code>old</code> and <code>new</code> and wraps them around if they're greater than <code>n</code>. Because it's using <code>set!</code>, this incrementing is mutating the variables in the lambda's environment, but since these variables are captured from the surrounding environment, they are changed for all future calls as well.</p> <p>That's why you get different returns even with the same input.</p> <p>If this seems like witchcraft, you can think of it like objects in more common languages:</p> <p>Eg Python:</p> <pre><code>class Foo(): def __init__(self, n, m): self.n = n self.m = m def count(self): self.n += 1 if self.n == self.m: self.n = 1 return self.n-1 f = Foo(0, 2) f.count() # 1 f.count() # 0 </code></pre> <p>This is the same basic idea except here we're being a bit more explicit about where the environment is coming from, <code>self</code>. In Scheme, we mimic this with the lambda capturing the surrounding variables.</p> <p>For more, check out <a href="http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-19.html#%_chap_3" rel="nofollow">SICP</a></p>
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    1. POUnderstanding objects with local state -- Scheme
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    1. COthe main thing i dont get is , doesnt old and new have to be greater than 3 for the incrementing set to work?
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    2. CO@GoZoner Yes, but this isn't really relevant for this explanation...
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    3. CO@user2036503 yes, but they're being incremented, `(set! new (+ 1 new))`
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