Discussion on: Gang of Four Patterns in Kotlin

Great post, I just have one note: for what regards the Decorator example, you're using extension methods, which is a very desirable feature of Kotlin and other languages like C#. That's nice, but the point of the corresponding example in my Java 8 implementation was a bit different and probably more fundamental. The Decorator pattern as demonstrated in GoF is just a very convoluted way to achieve functions composition. Using functions composition, which is not a specific language feature, but a corner stone of functional programming, and then available in any language with a minimal functional support including Java 8, you can achieve the same result in a more elegant way. Conversely what you did is reimplementing the Decorator pattern literally and in its original form, only in a more concise way using Kotlin's better expressiveness. Both solutions are valid, but still I believe that the one using function composition is more functional-ish, general and possibly even easier to read and to implement.