do iterator
Posted: Tue Mar 11, 2008 2:40 pm
For your enjoyment, here is a do macro like in common lisp (note, it is actually more like do*, since it uses letn, rather than let in its expansion):
This is something that I feel is really missing from newLISP. Except in specialized cases like dolist, iteration is more likely to be over multiple variables. The format of this is similar to for (without the step value), with multiple init/update expressions, then a list of the break-condition expression and the result expression.
Here is an example of factorial (from Ansi Common Lisp) using this do. It doesn't even have a body, because the update forms do all the work. I've included a comment where the body would be.
Code: Select all
(define-macro (do)
(letex ((iter-forms (args 0))
(break (args 1 0))
(result (args 1 1))
(body (cons begin (rest (rest (args))))))
(letex ((init-forms (map (fn (form) (0 2 form)) 'iter-forms))
(update-symbols (reverse (map first 'iter-forms)))
(update-values (reverse (map last 'iter-forms))))
(letn init-forms
(do-until break body
(map set 'update-symbols
(map eval 'update-values)))
result))))Here is an example of factorial (from Ansi Common Lisp) using this do. It doesn't even have a body, because the update forms do all the work. I've included a comment where the body would be.
Code: Select all
(define (factorial n)
(do ((j n (- j 1))
(f 1 (* j f)))
((= j 0) f)
; (println j ", " f)
))
(println (factorial 10))