Python Internals: Adding a new statement to Python

The Scheme version is not worthy of a blog post:

  (define-syntax until
    (syntax-rules ()
      ((until pred expr ...)
       (let loop ()
         (if (not pred)
	     (begin
	       (begin expr ...)
	       (loop)))))))

In dialects with WHEN syntax (mzcheme, Kawa, probably many others) it's even more concise:

  (define-syntax until
    (syntax-rules ()
      ((until pred expr ...)
       (let loop ()
	 (when (not pred)
	       (begin expr ...)
	       (loop))))))

I will never, ever do this. But I still love this kind of article, that gives a deeper insight into how things work.

General observation about until. I think it works better "linguistically" as a REPEAT/UNTIL (ala BASIC) and not as a negative while loop.

So block of code always gets run at least once. For eg. in Perl:

    my $x;
    do { 
        print "guess the number? ";
        $x = readline;
    } until $x == 10;

Disclaimer: Can't recall using until since I last played with BASIC. And that was many years ago :)

Great breakdown.

I tried something simpler once, trying to map str.format() to a binary operator in Py3k.

http://github.com/jcsalterego/py3k-atsign

Needless to say it didn't pass the python-ideas (or was that python-dev?) mailing list trial, mostly because it produced ambiguities that plagued the % operator, which prompted this new str method in the first place.

Can anybody write a macro statement this way?

Something like:

macro square(x): return exec("#{x}*#{x}")

might be useful.

Edit: can anybody help me how about the comments markup. I tried searching around but couldn't find anything.

"There should be one-- and preferably only one --obvious way to do it." 'while' is it, 'until' is not.

The author even notes that 'until cond' is functionally identical to 'while not condition'. A fun exercise for the author, if a bit silly.