New Lesson: Protocols

[ybur-yug]

Add Protocols Lesson

This is a first run at a lesson on Protocols for the advanced section from what I started with and finished with @doomspork's feedback.

TODO

• Create issue for translation once reviewed/merged Translation Issue

[doomspork]

Some suggestion changes, thoughts?

[doomspork]

In this lesson we are going to look at Protocols, what they are, and how we use them in Elixir.

[doomspork]

So what are they? Protocols are a means of achieving polymorphism in Elixir. One pain of Erlang is extending an existing API for newly defined types. To avoid this in Elixir the function is dispatched dynamically based on the value's type. Elixir comes with a number of protocols built in, for example the String.Chars protocol is responsible for the to_string/1 function we've seen used previously. Let's take a closer look at to_string/1 with a quick example:

[doomspork]

As you can see we've called the function on multiple types and demonstrated that it work on them all. What if we call to_string/1 on tuples (or any type that hasn't implemented String.Char)? Let's see:

[doomspork]

As you can see we get a protocol error as there is no implementation for tuples. In the next section we'll implement the String.Chars protocol for tuples.

Implementing a protocol

We saw that to_string/1 has not yet been implemented for tuples so let's add it. To create an implementation we'll use defimpl with our protocol, and provide the :for option and our type. Let's take a look at how it might look:

defimpl String.Chars, for: Tuple do
  def to_string(tuple) do
    insides = 
      tuple
      |> Tuple.to_list
      |> Enum.map(&Kernel.to_string/1)
      |> Enum.join(", ")

    "{#{insides}}"
  end
end

If we copy this into IEx we should be now be able to call to_string/1 on a tuple without getting an error:

iex> to_string({3.14, "apple", :pie})
"{3.14, apple, pie}"

[doomspork]

How about we use an example that can be applied to a few different types? Something like:

We know how to implement a protocol but how we do we define a new one? For our example we'll implement to_atom/1, let's see how to do that with defprotocol:

defprotocol AsAtom do
  def to_atom(data)
end

defimpl AsAtom, for: Atom do
  def to_atom(atom), do: atom
end

defimpl AsAtom, for: BitString do
  defdelegate to_atom(string), to: String
end

defimpl AsAtom, for: List do
  defdelegate to_atom(list), to: List
end

Here we've defined our protocol and it's expected method, to_atom/1, along with implementations for a few types. Now that we have our protocol, let's put it to use in IEx:

iex> import AsAtom
AsAtom
iex> to_atom("string")
:string
iex> to_atom(:an_atom)
:an_atom
iex> to_atom([1, 2])
:"\x01\x02"

[doomspork]

@ybur-yug thoughts on the feedback?

[ybur-yug]

@doomspork let me read over this all after work today and give another pass :)

[ybur-yug]

Finished a cleanup of a first proposed draft with the whole lesson after initial start + feedback. Feel to comment/review etc, all.

[doomspork]

@ybur-yug looks good, one comment.

[doomspork]

This part I'm not sure I get: ", but we also want to be cautious with what features we spread between modules to avoid unintended results."

[ybur-yug]

If you implement stuff that just catches all and returns something (possible, but stupid) you are going to have a bad time and not get errors that should be being raised etc. I guess maybe its not worth mentioning. Makes sense without it still, too.

[doomspork]

@ybur-yug that isn't what that sentence conveys though, not to me at least.

[nscyclone]

Thanks, @ybur-yug!
I've proposed some improvements, looking forward to get your opinion on these.

[nscyclone]

String.Char → String.Chars

[nscyclone]

we do we → do we

[nscyclone]

If I am not mistaken Map's protocol implementation will not work for structs.
Thus probably it's worth to extend this example by adding a protocol implementation for Map and shortly explain how protocols work with structs.

[ybur-yug]

I'll have to look into this one. I'm going to give the lesson a second pass today at the airport/on my plane.

[doomspork]

Good idea @nscyclone 👍

[doomspork]

I think we're missing punctuation here.

[doomspork]

Looks good @ybur-yug! I think we're missing punctuation for one sentence but this is otherwise good-to-go 👍

[ybur-yug]

@doomspork incorporated + pushed