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+#+title: Elegant Functional Programming Application Architecture
+#+description: An elegant and working code architecture
+#+keywords: blog static
+#+author: Yann Esposito
+#+email: yann@esposito.host
+#+date: [2021-09-26 Sun]
+#+lang: en
+#+options: auto-id:t
+#+startup: showeverything
+
+In this article I will expose you about how to architect your application
+using function programming paradigm.
+
+It is not tied to any programming language.
+The principles in this article could probably be used by most programming
+languages.
+* Pre
+:PROPERTIES:
+:CUSTOM_ID: pre
+:END:
+
+First note that many /functional programming languages/ are not functional.
+They all provide holes in order to trick the system to be faster.
+Also for internal use, some advanced feature might be provided.
+
+1. Type-classes ; this is not something I consider belongs to functional programming.
+2. Clojure Protocols, ~defmulti~  ; are not something I would consider belongs
+   to functional programming.
+
+And many other languages features.
+Let be radical functional programmer.
+
+Radical, means going back to the roots.
+
+So here we go.
+Functional programming is about manipulating functions with the meaning of
+*mathematical functions* not what most programming language call functions.
+A "function" in C, Clojure, Python, Java, Javascript is *not* a function in
+the functional programming sense.
+OTOH in Haskell it is /often/ a real function, but not always.
+
+So a mathematical function is something that takes a bunch of parameters
+and return a value (or not, we should allow partial functions unfortunately).
+And if you provide the same parameter twice the result will always be the same.
+Also, it should not have any side effect, for example, it should log, emit
+sounds, ... nothing.
+
+Whoa whoa... Give me a minute here. Why would we want to ... do nothing at
+all?
+Isn't writing an application about doing things?
+Yes, it is.
+
+Let just say, that, you generally, don't need more than plunging a
+pure function graph into a context where an interpreter will be able to use
+and this big pile of pure functions will be used to have side effect.
+A big ball of pure functions are reproducible, easy to test, easy to
+analyze and reason about.
+Easy to expose business logic out of technical detail to access data,
+etc...
+
+So I will provide a system as radical, and simple as possible that will
+have a lot of good properties that advanced functional programming methods
+(like Monad, Monads transformers, Free Monads, Effect systems) are trying
+to achieve.
+
+* The Architecture
+:PROPERTIES:
+:CUSTOM_ID: the-architecture
+:END:
+
+The main architecture is based on the /Service/ paradigm.
+A service will be a sub-application living at runtime, it will depends on
+other services, and have an internal state.
+
+So in the end your application should look like an acyclic graph of services.
+
+Every service has an init phase, then a living phase, then a stop phase.
+Every service declare a set of methods to be used and every service will
+keep an internal implicit state.
+
+Even if this look a lot like Object oriented programming.
+It is in fact a quite radical functional programming architecture.
+
+#+begin_src clojure
+(def Interface
+  (function-1 [arg-1 arg-2] "a function"))
+
+;; Main implementation of /my-service/
+(defservice my-service
+  Interface
+  [sub-service-1
+   sub-service-2
+   ,,,
+   sub-service-n]
+  (init [initial-ctx]
+        (let [service-state
+              (core/init {:sub-service-1 sub-service-1 ,,,})]
+          (into initial-ctx service-state)))
+
+  (function-1 [ctx arg-1 arg-2]
+              (core/function-1 ctx arg-1 arg-2)))
+
+;; A test implementation of /my-service/
+(defservice my-test-service
+  Interface
+  [sub-service-1
+   sub-service-2
+   ,,,
+   sub-service-n]
+  (init [initial-ctx]
+        (let [service-state
+              (test/init {:sub-service-1 sub-service-1 ,,,})]
+          (into initial-ctx service-state)))
+
+  (function-1 [ctx arg-1 arg-2]
+              (test/function-1 ctx arg-1 arg-2)))
+
+#+end_src