Lab purpose

In this lab, we will implement a chat room. Elixir is an immutable language, which means that actions don't modify objects. Instead, we will have functions that transform our chat room data structure and return a new chat room, leaving the original unchanged.

The chat room data structure, expressed as an Elixir struct %Lab3.Chat{} in our code, has a single field :members. The :members field holds a map where the key is the user name and the value is the messages to that user, %{username => messages}. messages is a list of tuples {from, text} where from is the username of the sender and text is the contents of the message.

As you can see the chat room struct we are working with is nested with multiple levels of data structures. This is common in functional programming, and in this lab we will learn how to work with such data structures.

Lab instructions

The lab3 directory has a file lib/lab3.ex which contains an implementation of out chat room with some stubbed out functions. These are the functions you'll implement. For each new function, you will:

1. Run mix test. You will see the failures for each of your unimplemented features.

2. Implement the missing feature, and iterate on it until the test passes.

Below are the functions for the chat room we will implement in this lab.

• leave() will remove a member from the chatroom
• members() will return the names of the members in the chatroom, as a list of strings
• send_message() will add a new message to the message list for a user
• send_messages() will add a new message to the message list for all users

When joining, the function will raise if the user has already joined. The leave function should do the same if a user that hasn't joined yet tries to leave.

If you have more time, you can implement two other functions, which filter through messages in the chatroom.

• messages_from_user() and messages_to_user() will list all of the messages from or to a given user.

In this lab we will be using the Map and Enum modules, so take a quick look through the documentation for those modules before you get started so you know roughly what they provide. If you get stuck anywhere, try to go over the functions in those modules to see if there is a function that could help you solve your problem.

Links

Getting started guide: https://elixir-lang.org/getting-started/introduction.html

API docs: https://hexdocs.pm/elixir/

1. Maps https://elixir-lang.org/getting-started/maps-and-dicts.html

2. Map syntax https://hexdocs.pm/elixir/Kernel.SpecialForms.html#%25%7B%7D/1

3. Structs https://elixir-lang.org/getting-started/struct.html

4. raise/1 https://hexdocs.pm/elixir/Kernel.html#raise/1

5. assert_raise/1 https://hexdocs.pm/ex_unit/ExUnit.Assertions.html#assert_raise/2

6. Enum.flat_map/2 https://hexdocs.pm/elixir/Enum.html#flat_map/2

7. for comprehensions https://hexdocs.pm/elixir/Kernel.SpecialForms.html#for/1

8. Enum.into/3 https://hexdocs.pm/elixir/Enum.html#into/3

9. Map module https://hexdocs.pm/elixir/Map.html

Solution ( no peeking :) )

See solution.ex in the lab3 directory.