eli's puzzle

2011-07-26 14:58:50 -04:00 · 2011-07-26 14:58:50 -04:00 · 41c38a0b3b
commit 41c38a0b3b
parent 5ebf716f69
2 changed files with 127 additions and 0 deletions
--- a/examples/eli-number-puzzle.rkt
+++ b/examples/eli-number-puzzle.rkt
@ -0,0 +1,45 @@
+#lang planet dyoo/whalesong
+
+;; Eli's puzzle
+;;
+;;    http://lists.racket-lang.org/users/archive/2011-July/046849.html
+
+(require (planet dyoo/whalesong/world))
+          
+(define-struct world (seq output))
+
+;; streak: (listof number) -> number
+(define (streak lst)
+  (let ([elt (car lst)])
+    (let loop ([lst lst])
+      (cond
+        [(null? lst) 0]
+        [(= (car lst)
+            elt)
+         (add1 (loop (cdr lst)))]
+        [else
+         0]))))
+            
+(define (my-drop lst n)
+  (cond
+    [(= n 0)
+     lst]
+    [else
+     (my-drop (cdr lst) (sub1 n))]))
+
+(define (tick w)
+  (let* ([streak-length (streak (world-seq w))]
+         [next-self-describing-chunk
+          (list streak-length (car (world-seq w)))])
+    (make-world (append (my-drop (world-seq w) streak-length) 
+                        next-self-describing-chunk)
+                (append (world-output w)
+                        (list streak-length
+                              (car (world-seq w)))))))
+
+(define (draw w)
+  (world-output w))
+
+(big-bang (make-world '(1) '())
+          (on-tick tick 1)
+          (to-draw draw))
--- a/excerpt.html
+++ b/excerpt.html
@ -0,0 +1,82 @@
+<!DOCTYPE html>
+<html>
+<head></head>
+
+<body>
+
+
+<p>If people say that Racket is just a Lisp, they are short-selling
+Racket a little.  It's more accurate to say that Racket is a language
+laboratory, because it supports many different languages,
+including <tt><a href="http://docs.racket-lang.org/lazy/index.html">lazy</a></tt>,
+ <tt><a href="http://docs.racket-lang.org/frtime/index.html">frtime</a></tt>,
+and the
+HTDP <a href="http://docs.racket-lang.org/htdp-langs/index.html">teaching</a>
+languages.</p>
+
+<p>However, these examples are all problematic: they lack the power to
+convince.  Skeptics may accept that Racket has a lot of Lisp dialects,
+but surely, they may add, there's a world of difference between a
+simple dialect of Lisp and a different programming language.  And even
+though each of these language examples use wildly different semantics,
+their differences are drowning in the homogenous sea of
+parentheses.</p>
+
+<p>In order to make the point that Racket is a language laboratory, we
+must show examples of Racket languages that look nothing like Lisp.
+Let's take a stab at the heart of the problem.  What would happen if
+we showed a Racket program like this?
+
+<code>
+<pre>
+#lang planet dyoo/bf
++++++[>++++++++++++<-]>.
+>++++++++++[>++++++++++<-]>+.
+++++++..+++.>++++[>+++++++++++<-]>.
+<+++[>----<-]>.<<<<<+++[>+++++<-]>.
+>>.+++.------.--------.>>+.
+</pre>
+</code>
+
+To put this in polite terms: what in the $@#! is this?</p>
+
+<p>This
+is <tt><a href="http://en.wikipedia.org/wiki/Brainfuck">brainf*ck</a></tt>.  If
+we enter this in DrRacket, it runs.  If we
+use <a href="http://docs.racket-lang.org/raco/index.html">raco</a> on
+it, we can create standalone executables.</p>
+
+<p>What exactly is going on?  All Racket programs start with
+a <tt>#lang</tt> line, as we saw in the example above.
+This <tt>#lang</tt> line is the hook we use to extend Racket toward
+different programming languages.  More specifically, the
+<tt>planet dyoo/bf</tt> part of the <tt>#lang</tt> line names a
+specific Racket module, which tells Racket how to do two things:
+
+<ul>
+    <li>how to parse the surface syntax into abstract syntax trees</li>
+    <li>how to attach semantics to each of the phrases of a language</li>
+</ul>
+
+Both these pieces are not too mysterious: they're the
+<a href="http://en.wikipedia.org/wiki/Compiler#The_structure_of_a_compiler">front-end</a>
+of a traditional compiler, and one of the distinguishing features of
+Racket is that, not only is its front-end programmable, but pleasingly
+so: it's an afternoon's worth of time to implement 
+<tt>brainf*ck</tt> from scratch.
+</p>
+
+
+<p>Toward that end, I've written a self-contained tutorial
+at <a href="http://hashcollision.org/brainfudge">http://hashcollision.org/brainfudge</a>
+that shows the entire process, of how to write an implementation of
+the <tt>brainf*ck</tt> language into Racket and how to deploy it on
+<a href="http://planet.racket-lang.org/">PLaneT</a>.  I'd love to hear
+any comments or suggestions about the tutorial.
+</p>
+
+
+
+</body>
+</html>
+