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Abstract classes: mark a class abstract to prevent direct instantiation; use abstract def to require subclasses to implement a method abstract class Shape { 3 def area -> Float } class Square < Shape { attr length: Float abstract def area -> Float { self.length * self.length } } print Square.new(length: 4.0).area

Structural interfaces: declare an interface with required method signatures; any class implementing those methods satisfies the interface, no explicit declaration needed: interface Drawable { 3 def draw -> String } class Badge { def draw -> String { "drawing badge" } } def render(item: Drawable) -> String { item.draw } render (Badge.new)

match expression: pattern match on literals, ranges, and multiple values per arm; use _ as a wildcard and if guards for conditional arms: grade = match score { [i 90..100 = TAT 80..89 = J TE "Sat", "Sun" => { "Weekend" } n if n> 50 => { "Pass" } _ EE GN }

Modules and mixins: define reusable method groups with module and mix them into classes with include. module Greetable { def greet -> String { "hello from " + self.name } } class Person { include(Greetable) def name -> String { "Alice" } } Person.new. greet # "hello from Alice"