Okay, that's the one thing why I dislike local and internal define. But should we really throw out the idea of nested x defs for this one flaw?
On Aug 21, 2010, at 11:39 AM, Shriram Krishnamurthi wrote: > On Sat, Aug 21, 2010 at 10:53 AM, Matthias Felleisen > <[email protected]> wrote: >> >> It's not about teaching, it's about freedom of expression. >> This is one of the least problems for programmers and I can't >> think of a bug I have seen that involved this issue. > > Radio Free Matthias is broadcasting from back his Scheme days. > > Perhaps I should clarify. > > Here's a simple Scheme program: > > ---------- > #lang r5rs > > (define (bar x) > (lambda () > (let ((x x)) > x))) > > (define f (bar 20)) > ---------- > > Matthias would tell you to "diagram" to make sure you understand where > the X's come from. If you're lazy you can even have DrRacket do it > for you, and everything is as expected. When you execute: > > (f) > --> 20 > > In other words, there is nothing surprising at all here, so you might > as well give programmers their freedoms. > > Sometimes, however, languages get confused. Here's the "same" program > "translated" into JavaScript (courtesy of Arjun Guha, from our ECOOP > 2010 talk): > > ---------- > function bar(x) { > return function() { > var x = x; > return x; > }; > } > var f = bar(200); > ---------- > > There's no Check Syntax for JavaScript, so you have to draw the arrows > by hand. You can't be misled by its purported similarities to Scheme > because the arrows are rather more weird, and > > f() > --> undefined > > Now let's go to HtDP: there is no "let", only "local". (And, keep in > mind that Racket is not Scheme.) > > ---------- > #lang racket > > (define (bar x) > (lambda () > (local ([define x x]) > x))) > > (define f (bar 20)) > ---------- > > Now we can again get DrRacket to draw our arrows, and they look rather > familiar. Sure enough: > > (f) > --> #<undefined> > > Or if you prefer the new direction we're going in: > > ---------- > #lang racket > > (define (bar x) > (lambda () > (define x x) > x)) > > (define f (bar 20)) > ---------- > > with the same arrows, and the same outcome: > > (f) > --> #<undefined> > > Obviously, Matthias can explain *what* happened in every case (and the > "what" is, I believe, a bit different in the JavaScript and the two > Racket cases). But you have to wonder, at what cost. > > Shriram _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
