Merge pull request #1 from elibarzilay/master

Lots of improvements

1 files changed, 301 insertions, 171 deletions

diff --git a/racket.html.markdown b/racket.html.markdown
index 50c8c523..8486fb35 100644
--- a/racket.html.markdown
+++ b/racket.html.markdown
@@ -7,7 +7,7 @@ contributors:
 
 ---
 
-Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family. 
+Racket is a general purpose, multi-paradigm programming language in the Lisp/Scheme family.
 
 Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3rac25) or th3rac25 [at] [google's email service]
 
@@ -17,18 +17,19 @@ Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3r
 
 ;;; Comments
 
-; Single line comments start with a semicolon
+;; Single line comments start with a semicolon
 
 #| Block comments
    can span multiple lines and...
     #|
-       they can be nested !
-    |#   
+       they can be nested!
+    |#
 |#
 
-; S-expression comments discard the following expression
-#; "this expression will be discarded" "2nd expression" ; => "2nd expression"
- 
+;; S-expression comments discard the following expression,
+;; useful to comment expressions when debugging
+#; (this expression is discarded)
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 1. Primitive Datatypes and Operators
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -40,15 +41,15 @@ Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3r
 #x111                  ; hexadecimal => 273
 3.14                   ; reals
 6.02e+23
-1/2                    ; rationals   
-1+2i                   ; complex numbers   
+1/2                    ; rationals
+1+2i                   ; complex numbers
 
-; Function application is written (f x y z ...) 
-; where f is a function and x, y, z, ... are operands
-; If you want to create a literal list of data, use ' to stop it from
-; being evaluated
+;; Function application is written (f x y z ...)
+;; where f is a function and x, y, z, ... are operands
+;; If you want to create a literal list of data, use ' to stop it from
+;; being evaluated
 '(+ 1 2) ; => (+ 1 2)
-; Now, some arithmetic operations
+;; Now, some arithmetic operations
 (+ 1 1)  ; => 2
 (- 8 1)  ; => 7
 (* 10 2) ; => 20
@@ -60,189 +61,242 @@ Feedback is appreciated! You can reach me at [@th3rac25](http://twitter.com/th3r
 (exact->inexact 1/3) ; => 0.3333333333333333
 (+ 1+2i  2-3i) ; => 3-1i
 
-;;; Booleans 
-#t ; for true  
+;;; Booleans
+#t ; for true
 #f ; for false -- any value other than #f is true
 (not #t) ; => #f
 (and 0 #f (error "doesn't get here")) ; => #f
 (or #f 0 (error "doesn't get here"))  ; => 0
 
-;;; Characters 
+;;; Characters
 #\A ; => #\A
-#\λ ; => #\λ 
+#\λ ; => #\λ
 #\u03BB ; => #\λ
 
 ;;; Strings are fixed-length array of characters.
 "Hello, world!"
-"Benjamin \"Bugsy\" Siegel" ; backslash is an escaping character
-"λx:(μα.α→α).xx" ; any Unicode character can appear in a string constant
+"Benjamin \"Bugsy\" Siegel"   ; backslash is an escaping character
+"Foo\tbar\41\x21\u0021\a\r\n" ; includes C escapes, Unicode
+"λx:(μα.α→α).xx"              ; can include Unicode characters
 
-; Strings can be added too!
+;; Strings can be added too!
 (string-append "Hello " "world!") ; => "Hello world!"
 
-; A string can be treated like a list of characters
+;; A string can be treated like a list of characters
 (string-ref "Apple" 0) ; => #\A
 
-; format can be used to format strings:
+;; format can be used to format strings:
 (format "~a can be ~a" "strings" "formatted")
 
-; Printing is pretty easy
+;; Printing is pretty easy
 (printf "I'm Racket. Nice to meet you!\n")
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 2. Variables
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-; You can create a variable using define
-; a variable name can use any character except: ()[]{}",'`;#|\
+;; You can create a variable using define
+;; a variable name can use any character except: ()[]{}",'`;#|\
 (define some-var 5)
 some-var ; => 5
 
-; You can also use unicode characters
+;; You can also use unicode characters
 (define ⊆ subset?)
-(⊆ (set 3 2) (set 1 2 3)); => #t
+(⊆ (set 3 2) (set 1 2 3)) ; => #t
 
-; Accessing a previously unassigned variable is an exception
-;x ; => x: undefined ...
+;; Accessing a previously unassigned variable is an exception
+; x ; => x: undefined ...
 
-; Local binding: me is bound to "Bob" only within (let ...)
+;; Local binding: `me' is bound to "Bob" only within the (let ...)
 (let ([me "Bob"])
-    "Alice"
-    me) ; => "Bob"
+  "Alice"
+  me) ; => "Bob"
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 3. Structs and Collections
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; Structs
+;; Structs
 (struct dog (name breed age))
-(define my-pet 
+(define my-pet
   (dog "lassie" "collie" 5))
 my-pet ; => #<dog>
 (dog? my-pet) ; => #t
 (dog-name my-pet) ; => "lassie"
 
 ;;; Pairs (immutable)
-; "cons" constructs pairs, "car" and "cdr" extract the first
-; and second elements
+;; `cons' constructs pairs, `car' and `cdr' extract the first
+;; and second elements
 (cons 1 2) ; => '(1 . 2)
 (car (cons 1 2)) ; => 1
 (cdr (cons 1 2)) ; => 2
 
 ;;; Lists
 
-; Lists are linked-list data structures
+;; Lists are linked-list data structures, made of `cons' pairs and end
+;; with a `null' (or '()) to mark the end of the list
+(cons 1 (cons 2 (cons 3 null))) ; => '(1 2 3)
+;; `list' is a convenience variadic constructor for lists
 (list 1 2 3) ; => '(1 2 3)
+;; and a quote can also be used for a literal list value
+'(1 2 3) ; => '(1 2 3)
+
+;; Can still use `cons' to add an item to the beginning of a list
+(cons 4 '(1 2 3)) ; => '(4 1 2 3)
 
-; Use "cons" to add an item to the beginning of a list
-(cons 4 '(1 2 3)) ; => (4 1 2 3)
+;; Use `append' to add lists together
+(append '(1 2) '(3 4)) ; => '(1 2 3 4)
 
-; Use "append" to add lists together
-(append '(1 2) '(3 4)) ; => (1 2 3 4)
+;; Lists are a very basic type, so there is a *lot* of functionality for
+;; them, a few examples:
+(map add1 '(1 2 3))          ; => '(2 3 4)
+(map + '(1 2 3) '(10 20 30)) ; => '(11 22 33)
+(filter even? '(1 2 3 4))    ; => '(2 4)
+(count even? '(1 2 3 4))     ; => 2
+(take '(1 2 3 4) 2)          ; => '(1 2)
+(drop '(1 2 3 4) 2)          ; => '(3 4)
 
 ;;; Vectors
 
-; Vectors are fixed-length arrays
+;; Vectors are fixed-length arrays
 #(1 2 3) ; => '#(1 2 3)
 
-; Use "vector-append" to add vectors together
+;; Use `vector-append' to add vectors together
 (vector-append #(1 2 3) #(4 5 6)) ; => #(1 2 3 4 5 6)
 
 ;;; Sets
 
-; create a set from a list
+;; Create a set from a list
 (list->set '(1 2 3 1 2 3 3 2 1 3 2 1)) ; => (set 1 2 3)
 
-; Add a member with "set-add"
-(set-add (set 1 2 3) 4); => (set 1 2 3 4)
+;; Add a member with `set-add'
+;; (Functional: returns the extended set rather than mutate the input)
+(set-add (set 1 2 3) 4) ; => (set 1 2 3 4)
 
-; Remove one with "set-remove"
+;; Remove one with `set-remove'
 (set-remove (set 1 2 3) 1) ; => (set 2 3)
 
-; Test for existence with "set-member?"
+;; Test for existence with `set-member?'
 (set-member? (set 1 2 3) 1) ; => #t
 (set-member? (set 1 2 3) 4) ; => #f
 
 ;;; Hashes
 
-; Create an immutable hash table (There are also mutables ones)
+;; Create an immutable hash table (mutable example below)
 (define m (hash 'a 1 'b 2 'c 3))
 
-; Retrieve a value
+;; Retrieve a value
 (hash-ref m 'a) ; => 1
 
-; Retrieving a non-present value is an exception
+;; Retrieving a non-present value is an exception
 ; (hash-ref m 'd) => no value found
 
-; You can provide a default value for missing keys
+;; You can provide a default value for missing keys
 (hash-ref m 'd 0) ; => 0
 
-; Use "hash-set" to extend a hash table
-(define m2 (hash-set m 'd 4)) 
+;; Use `hash-set' to extend an immutable hash table
+;; (Returns the extended hash instdead of mutating it)
+(define m2 (hash-set m 'd 4))
 m2 ; => '#hash((b . 2) (a . 1) (d . 4) (c . 3))
 
-; Remember, these hashes are immutable!
-m ; => '#hash((b . 2) (a . 1) (c . 3))
+;; Remember, these hashes are immutable!
+m ; => '#hash((b . 2) (a . 1) (c . 3))  <-- no `d'
 
-; Use "hash-remove" to remove keys
+;; Use `hash-remove' to remove keys (functional too)
 (hash-remove m 'a) ; => '#hash((b . 2) (c . 3))
 
+;; Create an empty mutable hash table and manipulate it
+(define m3 (make-hash))
+(hash-set! m3 'a 1)
+(hash-set! m3 'b 2)
+(hash-set! m3 'c 3)
+(hash-ref m3 'a)   ; => 1
+(hash-ref m3 'd 0) ; => 0
+(hash-remove! m3 'a)
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 3. Functions
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; Use lambda to create new functions. 
-; A function always returns its last statement.
+;; Use `lambda' to create functions.
+;; A function always returns the value of its last expression
 (lambda () "Hello World") ; => #<procedure>
+;; Can also use a unicode `λ'
+(λ () "Hellow World")     ; => same function
 
-; (You need extra parens to call it)
+;; Use parens to call all functions, including a lambda expression
 ((lambda () "Hello World")) ; => "Hello World"
+((λ () "Hello World"))      ; => "Hello World"
 
-; Assign a function to a var
+;; Assign a function to a var
 (define hello-world (lambda () "Hello World"))
 (hello-world) ; => "Hello World"
 
-; You can shorten this to:
+;; You can shorten this using the function definition syntatcic sugae:
 (define (hello-world2) "Hello World")
 
-; The () is the list of arguments for the function.
-(define hello 
+;; The () in the above is the list of arguments for the function
+(define hello
   (lambda (name)
     (string-append "Hello " name)))
 (hello "Steve") ; => "Hello Steve"
+;; ... or equivalently, using a sugared definition:
+(define (hello2 name)
+  (string-append "Hello " name))
 
-; You can have multi-variadic functions, too
-(define hello2
-  (case-lambda 
+;; You can have multi-variadic functions too, using `case-lambda'
+(define hello3
+  (case-lambda
     [() "Hello World"]
     [(name) (string-append "Hello " name)]))
-(hello2 "Jake") ; => "Hello Jake"
-(hello2) ; => "Hello World"
+(hello3 "Jake") ; => "Hello Jake"
+(hello3) ; => "Hello World"
+;; ... or specify optional arguments with a default value expression
+(define (hello4 [name "World"])
+  (string-append "Hello " name))
 
-; Functions can pack extra arguments up in a list
+;; Functions can pack extra arguments up in a list
 (define (count-args . args)
   (format "You passed ~a args: ~a" (length args) args))
 (count-args 1 2 3) ; => "You passed 3 args: (1 2 3)"
+;; ... or with the unsugared `lambda' form:
+(define count-args2
+  (lambda args
+    (format "You passed ~a args: ~a" (length args) args)))
 
-; You can mix regular and packed arguments
+;; You can mix regular and packed arguments
 (define (hello-count name . args)
   (format "Hello ~a, you passed ~a extra args" name (length args)))
 (hello-count "Finn" 1 2 3)
 ; => "Hello Finn, you passed 3 extra args"
+;; ... unsugared:
+(define hello-count2
+  (lambda (name . args)
+    (format "Hello ~a, you passed ~a extra args" name (length args))))
+
+;; And with keywords
+(define (hello-k #:name [name "World"] #:greeting [g "Hello"] . args)
+  (format "~a ~a, ~a extra args" g name (length args)))
+(hello-k)                 ; => "Hello World, 0 extra args"
+(hello-k 1 2 3)           ; => "Hello World, 3 extra args"
+(hello-k #:greeting "Hi") ; => "Hi World, 0 extra args"
+(hello-k #:name "Finn" #:greeting "Hey") ; => "Hey Finn, 0 extra args"
+(hello-k 1 2 3 #:greeting "Hi" #:name "Finn" 4 5 6)
+                                         ; => "Hi Finn, 6 extra args"
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 4. Equality
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; for numbers use "="
+;; for numbers use `='
 (= 3 3.0) ; => #t
 (= 2 1) ; => #f
 
-; for object identity use "eq?"
+;; for object identity use `eq?'
 (eq? 3 3) ; => #t
 (eq? 3 3.0) ; => #f
 (eq? (list 3) (list 3)) ; => #f
 
-; for collections use "equal?"
+;; for collections use `equal?'
 (equal? (list 'a 'b) (list 'a 'b)) ; => #t
 (equal? (list 'a 'b) (list 'b 'a)) ; => #f
 
@@ -254,20 +308,20 @@ m ; => '#hash((b . 2) (a . 1) (c . 3))
 
 (if #t               ; test expression
     "this is true"   ; then expression
-    "this is false"  ; else expression
-    ) ; =>  "this is true"
+    "this is false") ; else expression
+; => "this is true"
 
-; In conditionals, all non-#f values are treated as true
-(member "Groucho" '("Harpo" "Groucho" "Zeppo")) ; => '("Groucho" "Zeppo")
-(if (member "Groucho" '("Harpo" "Groucho" "Zeppo"))
-      'yep
-      'nope) ; => 'yep
+;; In conditionals, all non-#f values are treated as true
+(member 'Groucho '(Harpo Groucho Zeppo)) ; => '(Groucho Zeppo)
+(if (member 'Groucho '(Harpo Groucho Zeppo))
+    'yep
+    'nope)
+; => 'yep
 
-; "cond" chains a series of tests to select a result
-(cond
-   [(> 2 2) (error "wrong!")]
-   [(< 2 2) (error "wrong again!")]
-   [else 'ok]) ; => 'ok
+;; `cond' chains a series of tests to select a result
+(cond [(> 2 2) (error "wrong!")]
+      [(< 2 2) (error "wrong again!")]
+      [else 'ok]) ; => 'ok
 
 ;;; Pattern Matching
 
@@ -277,45 +331,36 @@ m ; => '#hash((b . 2) (a . 1) (c . 3))
     [(list 0 _) 'fizz]
     [(list _ 0) 'buzz]
     [_          #f]))
-  
+
 (fizzbuzz? 15) ; => 'fizzbuzz
-(fizzbuzz? 37) ; => #f 
+(fizzbuzz? 37) ; => #f
 
 ;;; Loops
 
-; looping can be done through recursion
+;; Looping can be done through (tail-) recursion
 (define (loop i)
   (when (< i 10)
-    (printf "i:~a~n" i)
-    (loop (add1 i)))) 
-
-(loop 5) ; => i:5 i:6 ... 
+    (printf "i=~a\n" i)
+    (loop (add1 i))))
+(loop 5) ; => i=5, i=6, ...
 
-; similarly, with a named let
+;; Similarly, with a named let
 (let loop ((i 0))
   (when (< i 10)
-    (printf "i:~a~n" i)
-    (loop (add1 i)))) ; => i:0 i:1 ...
+    (printf "i=~a\n" i)
+    (loop (add1 i)))) ; => i=0, i=1, ...
 
-;;; Comprehensions
+;; See below how to add a new `loop' form, but Racket already has a very
+;; flexible `for' form for loops:
+(for ([i 10])
+  (printf "i=~a\n" i)) ; => i=0, i=1, ...
+(for ([i (in-range 5 10)])
+  (printf "i=~a\n" i)) ; => i=5, i=6, ...
 
-(for/list ([i '(1 2 3)])
-    (add1 i)) ; => '(2 3 4)
+;;; Iteration Over Other Sequences
+;; `for' allows iteration over many other kinds of sequences:
+;; lists, vectors, strings, sets, hash tables, etc...
 
-(for/list ([i '(1 2 3)] #:when (even? i))
-    i) ; => '(2)
-
-(for/hash ([i '(1 2 3)])
-    (values i (number->string i))) ; => '#hash((1 . "1") (2 . "2") (3 . "3"))
-
-; To combine iteration results, use "for/fold"
-(for/fold ([sum 0]) ([i '(1 2 3 4)])
-  (+ sum i)) ; => 10
-
-;;; Sequences
-
-; "for" allows iteration over sequences: 
-; lists, vectors, strings, sets, hash tables, etc... 
 (for ([i (in-list '(l i s t))])
   (displayln i))
 
@@ -329,91 +374,170 @@ m ; => '#hash((b . 2) (a . 1) (c . 3))
   (displayln i))
 
 (for ([(k v) (in-hash (hash 'a 1 'b 2 'c 3 ))])
-  (printf "key:~a value:~a ~n" k v))
+  (printf "key:~a value:~a\n" k v))
+
+;;; More Complex Iterations
+
+;; Parallel scan of multiple sequences (stops on shortest)
+(for ([i 10] [j '(x y z)]) (printf "~a:~a\n" i j))
+; => 0:x 1:y 2:z
+
+;; Nested loops
+(for* ([i 2] [j '(x y z)]) (printf "~a:~a\n" i j))
+; => 0:x, 0:y, 0:z, 1:x, 1:y, 1:z
+
+;; Conditions
+(for ([i 1000]
+      #:when (> i 5)
+      #:unless (odd? i)
+      #:break (> i 10))
+  (printf "i=~a\n" i))
+; => i=6, i=8, i=10
+
+;;; Comprehensions
+;; Very similar to `for' loops -- just collect the results
+
+(for/list ([i '(1 2 3)])
+  (add1 i)) ; => '(2 3 4)
+
+(for/list ([i '(1 2 3)] #:when (even? i))
+  i) ; => '(2)
+
+(for/list ([i 10] [j '(x y z)])
+  (list i j)) ; => '((0 x) (1 y) (2 z))
+
+(for/list ([i 1000] #:when (> i 5) #:unless (odd? i) #:break (> i 10))
+  i) ; => '(6 8 10)
+
+(for/hash ([i '(1 2 3)])
+  (values i (number->string i)))
+; => '#hash((1 . "1") (2 . "2") (3 . "3"))
+
+;; There are many kinds of other built-in ways to collect loop values:
+(for/sum ([i 10]) (* i i)) ; => 285
+(for/product ([i (in-range 1 11)]) (* i i)) ; => 13168189440000
+(for/and ([i 10] [j (in-range 10 20)]) (< i j)) ; => #t
+(for/or ([i 10] [j (in-range 0 20 2)]) (= i j)) ; => #t
+;; And to use any arbitrary combination, use `for/fold'
+(for/fold ([sum 0]) ([i '(1 2 3 4)]) (+ sum i)) ; => 10
+;; (This can often replace common imperative loops)
 
 ;;; Exceptions
 
-; To catch an exception, use the "with-handlers" form
-; To throw an exception use "raise"
-(with-handlers 
-    ([(lambda (v) (equal? v "infinity"))   
-      (lambda (exn) +inf.0)])
-  (raise "infinity"))
+;; To catch exceptions, use the `with-handlers' form
+(with-handlers ([exn:fail? (lambda (exn) 999)])
+  (+ 1 "2")) ; => 999
+(with-handlers ([exn:break? (lambda (exn) "no time")])
+  (sleep 3)
+  "phew") ; => "phew", but if you break it => "no time"
+
+;; Use `raise' to throw exceptions or any other value
+(with-handlers ([number?    ; catch numeric values raised
+                 identity]) ; return them as plain values
+  (+ 1 (raise 2))) ; => 2
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 6. Mutation
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; Use set! to assign a new value to an existing variable
+;; Use `set!' to assign a new value to an existing variable
 (define n 5)
-(set! n 6) 
+(set! n (add1 n))
 n ; => 6
 
-; Many Racket datatypes can be immutable or mutable 
-; (Pairs, Lists, Strings, Vectors, Hash Tables, etc...)
+;; Use boxes for explicitly mutable values (similar to pointers or
+;; references in other languages)
+(define n* (box 5))
+(set-box! n* (add1 (unbox n*)))
+(unbox n*) ; => 6
 
-; Use "vector" to create a mutable vector
+;; Many Racket datatypes are immutable (pairs, lists, etc), some come in
+;; both mutable and immutable flavors (strings, vectors, hash tables,
+;; etc...)
+
+;; Use `vector' or `make-vector' to create mutable vectors
 (define vec (vector 2 2 3 4))
-; Use vector-set! to update a slot
+(define wall (make-vector 100 'bottle-of-beer))
+;; Use vector-set! to update a slot
 (vector-set! vec 0 1)
+(vector-set! wall 99 'down)
 vec ; => #(1 2 3 4)
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 7. Modules
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; Modules let you organize code into multiple files and reusable libraries
+;; Modules let you organize code into multiple files and reusable
+;; libraries; here we use sub-modules, nested in the whole module that
+;; this text makes (starting from the "#lang" line)
+
+(module cake racket/base ; define a `cake' module based on racket/base
 
-(module cake racket/base ; define a new module 'cake' based on racket/base
-  
   (provide print-cake) ; function exported by the module
-  
+
   (define (print-cake n)
     (show "   ~a   " n #\.)
     (show " .-~a-. " n #\|)
     (show " | ~a | " n #\space)
     (show "---~a---" n #\-))
-  
-  (define (show fmt n ch) ;; internal function       
+
+  (define (show fmt n ch) ; internal function
     (printf fmt (make-string n ch))
     (newline)))
 
-; Use "require" to import all functions from the module
-(require 'cake) 
-(print-cake 3)  
-;(show "~a" 1 #\A) ; => error, "show" was not exported
+;; Use `require' to get all `provide'd names from a module
+(require 'cake) ; the ' is for a local submodule
+(print-cake 3)
+; (show "~a" 1 #\A) ; => error, `show' was not exported
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 8. Classes and Objects
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; Create a class fish%
+;; Create a class fish% (-% is idomatic for class bindings)
 (define fish%
-  (class object% 
+  (class object%
     (init size) ; initialization argument
-    (super-new) ; superclass initialization 
-    ; Field
-    (define current-size size) 
-    ; Public methods
-    (define/public (get-size)  current-size)
-    (define/public (grow amt) (set! current-size (+ amt current-size)))
-    (define/public (eat other-fish) (grow (send other-fish get-size)))))
-
-; Create an instance of fish%
-(define charlie 
+    (super-new) ; superclass initialization
+    ;; Field
+    (define current-size size)
+    ;; Public methods
+    (define/public (get-size)
+      current-size)
+    (define/public (grow amt)
+      (set! current-size (+ amt current-size)))
+    (define/public (eat other-fish)
+      (grow (send other-fish get-size)))))
+
+;; Create an instance of fish%
+(define charlie
   (new fish% [size 10]))
 
-; Use "send" to call an object's methods
+;; Use `send' to call an object's methods
+(send charlie get-size) ; => 10
 (send charlie grow 6)
 (send charlie get-size) ; => 16
- 
+
+;; `fish%' is a plain "first class" value, which can get us mixins
+(define (add-color c%)
+  (class c%
+    (init color)
+    (super-new)
+    (define my-color color)
+    (define/public (get-color) my-color)))
+(define colored-fish% (add-color fish%))
+(define charlie2 (new colored-fish% [size 10] [color 'red]))
+(send charlie2 get-color)
+;; or, with no names:
+(send (new (add-color fish%) [size 10] [color 'red]) get-color)
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 9. Macros
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; Macros let you extend the syntax of the language
+;; Macros let you extend the syntax of the language
 
-; Let's add a while loop
+;; Let's add a while loop
 (define-syntax-rule (while condition body ...)
   (let loop ()
     (when condition
@@ -425,48 +549,54 @@ vec ; => #(1 2 3 4)
     (displayln i)
     (set! i (add1 i))))
 
-; Macros are hygienic, you cannot clobber existing variables!   
-(define-syntax-rule (swap x y)
-  (begin
-    (define tmp x) 
+;; Macros are hygienic, you cannot clobber existing variables!
+(define-syntax-rule (swap! x y) ; -! is idomatic for mutation
+  (let ([tmp x])
     (set! x y)
     (set! y tmp)))
 
-(define tmp 1) 
+(define tmp 1)
 (define a 2)
 (define b 3)
-(swap a b)
-(printf "tmp = ~a; a = ~a; b = ~a~n" tmp a b) ; tmp is unaffected by swap
+(swap! a b)
+(printf "tmp = ~a; a = ~a; b = ~a\n" tmp a b) ; tmp is unaffected
+
+;; But the are still code transformations, for example:
+(define-syntax-rule (bad-while condition body ...)
+  (when condition
+    body ...
+    (bad-while condition body ...)))
+;; this macro is broken: it generates infinite code, if you try to use
+;; it, the compiler will get in an infinite loop
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 10. Contracts
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-; Contracts impose constraints on values exported from modules
+;; Contracts impose constraints on values exported from modules
 
 (module bank-account racket
   (provide (contract-out
-            [deposit (-> positive? any)] ; amount will always be a positive number
+            [deposit (-> positive? any)] ; amounts are always positive
             [balance (-> positive?)]))
-  
+
   (define amount 0)
   (define (deposit a) (set! amount (+ amount a)))
   (define (balance) amount)
-)
+  )
 
 (require 'bank-account)
 (deposit 5)
 
 (balance) ; => 5
 
-; Any client that attempt to deposit a non-positive amount, will be blamed
-; (deposit -5) ; => deposit: contract violation 
-; expected: positive?
-; given: -5 
-; more details....
+;; Clients that attempt to deposit a non-positive amount are blamed
+;; (deposit -5) ; => deposit: contract violation
+;; expected: positive?
+;; given: -5
+;; more details....
 ```
 
 ## Further Reading
 
-Still up for more? Try [Quick: An Introduction to Racket with Pictures](http://docs.racket-lang.org/quick/)
-
+Still up for more? Try [Getting Started with Racket](http://docs.racket-lang.org/getting-started/)