Arc Forumnew | comments | leaders | submitlogin
1 point by bogomipz 6157 days ago | link | parent

I think the most interesting aspect of this discussion is that it shows a problem with destructuring a list you want to recurse over.

Traditionally, your function would be written like this;

  (def print-list (lst)
     (when lst 
        (prn (car lst)) 
        (print-list (cdr lst))))
This works as expected and is what lispers have done for half a century. The root of your problem is that the original cons is lost when the function destructures it to a and b, so you cannot do the above test.

Existing languages that support argument destructuring, AFAIK do so through pattern matching. In this case the problem does not exist because the matching is the branch.

Imagining Arc with pattern matching, your function would look something like this;

  (def print-list (nil)
     | print-list ((a . b))
       (prn a)
       (print-list b))
EDIT:

This can already be done with almkglor's pattern matching library;

  (require "lib/defpat.arc")

  (defpat print-list (nil) nil
                     ((a . b)) (do
          (prn a)
          (print-list b)))