Macros and expand

Started by jsmall, September 24, 2004, 12:49:04 PM

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I'm trying to understand the proper use of

expand with macros.

So I define the macro "expand-let"

with the following semantics.

     (expand-let '(x y z) (x 1) (y 2) (z 3))

"expands" to the equivalent of

    (let ((x 1)(y 2)(z 3))

       (expand '(x y z) 'x 'y 'z))


    (1 2 3)

The following is my implementation of this semantics

  (define (keys alist)

    (map (fn (pair) (first pair)) alist))

  (define (bindings alist)

    (map (fn (pair) (cons (first pair) (eval (last pair)))) alist))

  (define-macro (expand-let expr)

    (let ((ks (keys (rest (args))))

          (bs (bindings (rest (args)))))

      (eval (expand '(let bs

        (apply expand (cons (eval expr) (quote ks))))

        'ks 'bs))))

Is the above definition of expand-let reasonable

or is it poorly constructed?

(I have a follow up example/question that builds on nested

macros if this is okay thus far.)



Nice idea the 'expand-let' and nicely implemented. This one follows a similar idea as yours, but collapses everythig into one function. Less readable but smaller:

(define-macro (expand-let )
  (eval (append (list 'let (rest (args)))
                (list (cons 'expand
                             (append (list (first (args)))
                             (map quote (map first (rest (args))))))))))

(expand-let '(x y z) (x 1) (y (+ 1 1)) (z 3)) => (1 2 3)