[racket/en] Add more details about Racket (#2278)

* Add let* and letrec reference

* More elaboration on structs

* Add code about predefined car, cdr functions

* Mention explicit typing, int to real conversion

racket.html.markdown

@@ -114,18 +114,42 @@ some-var ; => 5
   "Alice"
   me) ; => "Bob"
 
+;; let* is like let, but allows you to use previous bindings in creating later bindings
+(let* ([x 1]
+       [y (+ x 1)])
+       (* x y))
+
+;; finally, letrec allows you to define recursive and mutually recursive functions
+(letrec ([is-even? (lambda (n)
+                       (or (zero? n)
+                           (is-odd? (sub1 n))))]
+           [is-odd? (lambda (n)
+                      (and (not (zero? n))
+                           (is-even? (sub1 n))))])
+    (is-odd? 11))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; 3. Structs and Collections
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ;; Structs
+; By default, structs are immutable
 (struct dog (name breed age))
 (define my-pet
   (dog "lassie" "collie" 5))
 my-pet ; => #<dog>
+; returns whether the variable was constructed with the dog constructor
 (dog? my-pet) ; => #t
+; accesses the name field of the variable constructed with the dog constructor
 (dog-name my-pet) ; => "lassie"
 
+; You can explicitly declare a struct to be mutable with the #:mutable option
+(struct rgba-color (red green blue alpha) #:mutable)
+(define burgundy
+   (rgba-color 144 0 32 1.0))
+(set-color-green! burgundy 10)
+(color-green burgundy) ; => 10
+
 ;;; Pairs (immutable)
 ;; `cons' constructs pairs, `car' and `cdr' extract the first
 ;; and second elements
@@ -143,6 +167,16 @@ my-pet ; => #<dog>
 ;; and a quote can also be used for a literal list value
 '(1 2 3) ; => '(1 2 3)
 
+;; Racket has predefined functions on top of car and cdr, to extract parts of a list
+(cadr (list 1 2 3)) ; => 2
+(car (cdr (list 1 2 3))) ; => 2
+
+(cddr (list 1 2 3)) ; => '(3)
+(cdr (cdr (list 1 2 3))) ; => '(3)
+
+(caddr (list 1 2 3)) ; => 3
+(car (cdr (cdr (list 1 2 3)))) ; => 3
+
 ;; Can still use `cons' to add an item to the beginning of a list
 (cons 4 '(1 2 3)) ; => '(4 1 2 3)