From 41c38a0b3b17f076d3a4251b6cfd74f3fd366e97 Mon Sep 17 00:00:00 2001 From: Danny Yoo Date: Tue, 26 Jul 2011 14:58:50 -0400 Subject: [PATCH] eli's puzzle --- examples/eli-number-puzzle.rkt | 45 +++++++++++++++++++ excerpt.html | 82 ++++++++++++++++++++++++++++++++++ 2 files changed, 127 insertions(+) create mode 100644 examples/eli-number-puzzle.rkt create mode 100644 excerpt.html diff --git a/examples/eli-number-puzzle.rkt b/examples/eli-number-puzzle.rkt new file mode 100644 index 0000000..27ad805 --- /dev/null +++ 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)) \ No newline at end of file diff --git a/excerpt.html b/excerpt.html new file mode 100644 index 0000000..2f7b146 --- /dev/null +++ b/excerpt.html @@ -0,0 +1,82 @@ + + + + + + + +

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 lazy, + frtime, +and the +HTDP teaching +languages.

+ +

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.

+ +

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? + + +

+#lang planet dyoo/bf
+++++++[>++++++++++++<-]>.
+>++++++++++[>++++++++++<-]>+.
++++++++..+++.>++++[>+++++++++++<-]>.
+<+++[>----<-]>.<<<<<+++[>+++++<-]>.
+>>.+++.------.--------.>>+.
+
+ + +To put this in polite terms: what in the $@#! is this?

+ +

This +is brainf*ck. If +we enter this in DrRacket, it runs. If we +use raco on +it, we can create standalone executables.

+ +

What exactly is going on? All Racket programs start with +a #lang line, as we saw in the example above. +This #lang line is the hook we use to extend Racket toward +different programming languages. More specifically, the +planet dyoo/bf part of the #lang line names a +specific Racket module, which tells Racket how to do two things: + +

+ +Both these pieces are not too mysterious: they're the +front-end +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 +brainf*ck from scratch. +

+ + +

Toward that end, I've written a self-contained tutorial +at http://hashcollision.org/brainfudge +that shows the entire process, of how to write an implementation of +the brainf*ck language into Racket and how to deploy it on +PLaneT. I'd love to hear +any comments or suggestions about the tutorial. +

+ + + + + +