[Elixir] Implement new Concept Exercise: genserver

[angelikatyborska]

Getting started

Please please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read up on the following documents:

• The features of v3.
• Rationale for v3.
• What are concept exercise and how they are structured?

Please also watch the following video:

• The Anatomy of a Concept Exercise.

Goal

The student should understand the GenServer behaviour and know how to use it, writing a public module API and implementing the behaviour's callbacks.

Learning objectives

• Know about the GenServer behaviour and its callbacks.
• Know that the name stands for "generic server".
• Understand the difference between handle_call, handle_cast, and handle_info.
• Understand the possible return values of the callbacks.
• Know it's a good idea to have a catch-all handle_info to avoid an ever-growing process inbox.
• Know that GenServers usually expose an easy-to-use public API instead of expecting its users to call GenServer.call themselves.
• Know when not to use GenServer (if Agent is good enough, if there is no need to keep state)

Out of scope

• process linking, monitoring, :DOWN messages
• process registry, process names

Concepts

• genserver

Prerequisites

• atoms
• tuples
• processes
• pid
• agent (it will be easier to understand after having learned about agents)
• use (as in use GenServer)
• behaviours

Resources to refer to

• https://elixir-lang.org/getting-started/mix-otp/genserver.html
• https://hexdocs.pm/elixir/master/GenServer.html

Story

Look through existing stories (/reference/stories/) to find inspiration or an existing story that can work for this exercise. It might be a good idea to either duplicate or come up with a continuation of the stories used for the processes exercise (take-a-number machine) or the agent exercise (community garden). Ideally the story would force the student to use all three types of callbacks.

Analyzer

The analyzer should check that indeed a module using the GenServer behaviour was implemented, and not a generic receive loop or an Agent.

Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.

[angelikatyborska]

Migrated to the Elixir track repo: exercism/elixir#559