Hello there,

I`m currently reading Programming Phoenix and struggle to understand some parts of the code.
2 Examples:

[web/models/user.ex]
def changeset(model, params \\ :empty) do
  model
  |> cast(params, ~w(name username), []) # WTF is ~w? Never explained
  |> validate_length(:username, @username_validation_length)
end

Why the ~w/1 function and this strange name? What does name/1 do?
When I look up the docs for cast/3 , I can just replace ~w/1 with [:name, :username] but I wan’t to know what’s the difference (besides eye cancer).

Another problem is the following code:

[web/controllers/auth.ex]
def call(conn, repo) do
  user_id = get_session(conn, :user_id)
  user = user_id && repo.get(Rumbl.User, user_id) # && should return boolean
  assign(conn, :current_user, user)
end

The docs are not really helpful in describing get_session/2:

get_session(conn, key)
Returns session value for the given key

I can assume that get_session/2 returns an int (but not because of the docs..).
I dont know what’s going on on the other lines of code, he is using this session id to get a user from the repo, but I thought the session id would be some kind of random value…
After that, I dont know whats going on with user (an int) and the returned struct…He is comparing them somehow…how is this working?
Thanks for taking time and reading this, I hope you can help me.