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Comment by kirillzubovsky | Hacker News Reader

Thank you for the explanation, very interesting, and makes me want to dive more into lisp and see what's possible with this approach. Next thing you know, I am Clojure fanboy.

Can you help me understand, what is the purpose of (+ x 3) at the end of that let statement. Is that just the default return from the original function, using input given by the let function? Thanks!

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Again, if we say "let function" that is not accurate. let is an operator that creates a new nesting of the lexical scope in which it binds some variables, and then evaluates forms in that environment.

let implements something similar to what is achieved with a combination of special syntax in some other languages.

    Lisp                                 C

    (let ((x 1)                          { int x = 1;
          (y 2)                            int y = 2;
      (foo x y))                           foo(x, y); }

Both of these mean "create fresh instances of variables x and y, which are scoped only to this construct, initializing them with 1 and 2, and then call foo with x and y arguments."

Got it! Thanks.

The syntax of LET is

   let ({var | (var [init-form])}*)
     declaration*
     form*

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