Add details regarding abstract and virtual functions in traits (#207)

Author: Oni-giri

pages/book/contracts.mdx

@@ -30,7 +30,7 @@ Each contract can contain:
 * [Receiver functions](#receiver-functions)
 * [Internal functions](#internal-functions)
 
-Furthermore, contracts can inherit all the declarations from [traits](/book/types#traits) and override some of their default behaviours.
+Furthermore, contracts can inherit all the declarations and definitions from [traits](/book/types#traits) and override some of their default behaviours. If declared or defined in a trait, internal functions and constants can be marked as [virtual or abstract](/book/functions#virtual-and-abstract-functions) and overriden in contracts inheriting from the trait.
 
 ### Persistent state variables [#variables]
 

pages/book/functions.mdx

@@ -23,6 +23,31 @@ fun pow(a: Int, c: Int): Int {
 }
 ```
 
+## Virtual and abstract functions
+
+You can allow the contract inheriting a [traits](/book/types#traits) to modify an internal function, if it has the `virtual` keyword, using `override`. The function can be also marked as `abstract`, in which case the inheriting contract has to define its implementation:
+
+```tact
+trait FilterTrait with Ownable {
+    virtual fun filterMessage(): Bool { // virtual functions can be overridden by users of this trait
+        return sender() != self.owner;
+    }
+
+    abstract fun specialFilter(): Bool;
+}
+
+contract Filter with FilterTrait {
+    // the trait allows us to override the default behavior
+    override fun filterMessage(): Bool {
+        return true;
+    }
+
+    override fun specialFilter(): Bool {
+      return true;
+    }
+}
+````
+
 ## Extension function
 
 Extension functions allow you to implement extensions for any possible type.

pages/book/types.mdx

@@ -136,7 +136,9 @@ Read more about them on the dedicated page: [Contracts](/book/contracts).
 
 ### Traits
 
-Tact doesn't support classical class inheritance, but instead introduces the concept of _traits_, which can be viewed as abstract contracts (like abstract classes in popular object-oriented languages). They have the same structure as [contracts](#contracts), but can't [initialize persistent state variables](/book/contracts#init-function), while allowing to override some of their behaviors.
+Tact doesn't support classical class inheritance, but instead introduces the concept of _traits_, which can be viewed as abstract contracts (like abstract classes in popular object-oriented languages). They have the same structure as [contracts](#contracts), but can't [initialize persistent state variables](/book/contracts#init-function).
+
+A trait can also let the contract inheriting it to override the behavior of its [functions](/book/functions#virtual-and-abstract-functions) and the value of its [constants](/book/constants#virtual-and-abstract-constants).
 
 Example of a trait [`Ownable`](/language/libs/ownable#ownable) from [`@stdlib/ownable`](/language/libs/ownable):