deft/journal/2021-03-14--13-00-04Z--service_pattern.org

#+TITLE: service-pattern
#+Author: Yann Esposito
#+Date: [2021-03-14]

- tags :: programming
- source ::

The question about code structure and organization is one of the most
prolific one.
The problem are always the same.

Here I will talk about one possible solution in this huge design space.
First of all, I will focus on a functional programming pattern.
But I think the lessons could be extended to any generic programming
language.

Before explaining the pattern I would like to take the time to provide a
few distinctions between different programming language patterns.
Quite often, one fundamental question very important when choosing a
pattern for your code is about find the the correct level of the pattern.

There are a tower of patterns and meta-patterns.
For example in imperative programming not using =goto= statement was
considered as a programming pattern.
Once that idea was accepted there were work done on /Object Oriented
Programming/.
And OOP was considered as a programming language pattern.
But OOP while already providing quite a constraint on your code
architecture was enough not sufficient.
OOP alone leave a lot of room in the design space.
Thus we've seen numerous "OOP Design Pattern".
That used the underlying OOP paradigm as a base and constructed
abstractions over it.

Even with all those Design Pattern it was up to the programmer to decide
which one applies or not.
Quite often there is not a single path easy to detect correct design
pattern.
Mainly the very hard part in programming is choosing the right abstraction.

There are other code structures to choose from.
In functional programming there are FRP.
Here also there are stories about how design pattern once choosen make a
natural evolution toward meta-design-patterns.
Mainly design pattern that rely on a lower level design pattern.

If you take the story behind Elm Architecture you can see it.
At first there were FRP.
Elm removed the behaviour from FRP to only deal with events to simplify the
model.
But with FRP the author clearly though it was a good-enough design pattern.
But the design space was a bit too big.
So it was difficult to take the right decision.
So a natural meta-pattern appeared.
It is [[https://guide.elm-lang.org/architecture/][/Elm Architecture/]].
So while Elm imposed so structure of your program using static types to
prevent common coding mistakes and enforce a specific code structure.
Elm did not constrain the file organisation, the number of buffers to
send/receive events, the way they should talk/listen between each other.

So Elm Architecture is a non enforced meta structure for your code application.
Unlike the underlying layer of architecture.