[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)
 

standard-ml.html.markdown

@@ -24,6 +24,12 @@ val phone_no = 5551337
 val pi = 3.14159
 val negative_number = ~15  (* Yeah, unary minus uses the 'tilde' symbol *)
 
+(* Optionally, you can explicitly declare types. This is not necessary as
+   ML will automatically figure out the types of your values. *)
+val diameter = 7926 : int
+val e = 2.718 : real
+val name = "Bobby" : string
+
 (* And just as importantly, functions: *)
 fun is_large(x : int) = if x > 37 then true else false
 
@@ -31,6 +37,8 @@ fun is_large(x : int) = if x > 37 then true else false
 val tau = 2.0 * pi         (* You can multiply two reals *)
 val twice_rent = 2 * rent  (* You can multiply two ints *)
 (* val meh = 1.25 * 10 *)  (* But you can't multiply an int and a real *)
+val yeh = 1.25 * (Real.fromInt 10) (* ...unless you explicitly convert
+                                      one or the other *)
 
 (* +, - and * are overloaded so they work for both int and real. *)
 (* The same cannot be said for division which has separate operators: *)