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copied!<p>This <code>let</code> expression:</p> <pre><code>(let ((x v1) (y v2)) e) </code></pre> <p>Is equivalent to the following <code>lambda</code> application, noticing that in here the variables can be evaluated in any order (no strict left-to-right order is enforced) and the definition of one variable can not reference the variables before it:</p> <pre><code>((lambda (x y) e) v1 v2) </code></pre> <p>On the other hand, this <code>let*</code> expression:</p> <pre><code>(let* ((x v1) (y v2) (z v3)) e) </code></pre> <p>Can be transformed into a series of nested <code>lambda</code>s, in a way that ensures that the variables are evaluated in the same order that was used to define them, and the ones defined first can be referenced in all subsequent definitions:</p> <pre><code>((lambda (x) ((lambda (y) ((lambda (z) e) v3)) v2)) v1) </code></pre> <p>Another example: this code will only work if we use the second transformation:</p> <pre><code>(let* ((x 1) (y (+ x 1))) (+ x y)) </code></pre> <p>As you can see, the definition of <code>y</code> references <code>x</code>, only in this way will it work:</p> <pre><code>((lambda (x) ((lambda (y) (+ x y)) (+ x 1))) 1) </code></pre> <p>Finally, here are two great online books for learning Scheme:</p> <ul> <li><a href="http://www.ccs.neu.edu/home/matthias/HtDP2e/" rel="nofollow">How to Design Programs</a></li> <li><a href="http://mitpress.mit.edu/sicp/full-text/book/book.html" rel="nofollow">Structure and Interpretation of Computer Programs</a></li> </ul>

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