complete objects section

content/objects.md

@@ -3,39 +3,64 @@ title: Objects
 ---
 # Objects
 
-In Nimrod, objects are like structs from C family languages and define a grouping of fields.
+In Nimrod, objects are like structs from C family languages and define a grouping of fields. They are by default traced by the garbage collector, so there is no need to explicitly free them when allocated.
 
 ``` nimrod
 type
   Animal* = object
     name*, genus*, species*: string
-    age: int # None of your business!
+    age: int # Not exported
   PAnimal* = ref Animal
 
 proc sleep*(a: var Animal) =
   a.age += 1
 
-# create a new Animal, not allocated on the heap
+# create a new Animal, mutable and not on the heap
 var carl = Animal(name : "Carl",
                   genus : "Lama",
                   species : "glama",
                   age : 12)
 
+# create a new Animal, immutable and not on the heap
+let joe = Animal(name : "Joe",
+                 genus : "Homo",
+                 species : "sapiens",
+                 age : 23)
+
+when false:
+  # Will not compile, Joe's value is immutable
+  joe.age = 57
+
 # Allocate an Animal on the heap
-# Note that the pointer is immutable, not the pointee
 let mittens: ref Animal = new(Animal)
+# Fill out all the fields, the start out as 0's
 mittens.name = "Mittens"
 mittens.genus = "Panthera"
-mittens.species = "Leo"
+mittens.species = "leo"
 mittens.age = 6
 
-# This also goes on the heap since PAnimal is a
+# This also goes on the heap since PAnimal is a reference type.
-# reference type
+# Initialization is much nicer this way
 let spot = PAnimal(name: "Spot",
                    genus: "Canis",
-                   species: "Lupus",
+                   species: "lupus",
                    age: 1)
 
 # Same as before, only pointer value is unchangeable
+# Pointee is modifiable
 spot.age = 2
+
+# Carl is copied, not a reference to Steve
+var steve = carl
+steve.name = "Steve"
+# Carl's name is still "Carl"
+assert carl.name == "Carl"
+assert steve.name == "Steve"
+
+# Value of the pointer is copied
+let lucy = spot
+lucy.name = "Lucy"
+# Spot's name is now "Lucy"!
+assert spot.name == "Lucy"
+assert lucy.name == "Lucy"
 ```