From c2ee39454809eaf7ba54704c041c6ac6165caa45 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kozio=C5=82ek?= <bjkuczynski@gmail.com>
Date: Fri, 21 Apr 2017 16:11:00 +0200
Subject: [PATCH] [PL] Update version of Collections lesson (#1059)

---
 pl/lessons/advanced/protocols.md | 115 +++++++++++++++++++++++++++++++
 pl/lessons/basics/collections.md |   2 +-
 2 files changed, 116 insertions(+), 1 deletion(-)
 create mode 100644 pl/lessons/advanced/protocols.md

diff --git a/pl/lessons/advanced/protocols.md b/pl/lessons/advanced/protocols.md
new file mode 100644
index 0000000000..f6373fd7de
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+---
+layout: page
+title: Protokoły
+category: advanced
+order: 12
+lang: pl
+---
+
+In this lesson we are going to look at Protocols, what they are, and how we use them in Elixir.
+
+{% include toc.html %}
+
+## What Are Protocols
+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:
+
+```elixir
+iex> to_string(5)
+"5"
+iex> to_string(12.4)
+"12.4"
+iex> to_string("foo")
+"foo"
+```
+
+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.Chars`)?
+Let's see:
+
+```elixir
+to_string({:foo})
+** (Protocol.UndefinedError) protocol String.Chars not implemented for {:foo}
+    (elixir) lib/string/chars.ex:3: String.Chars.impl_for!/1
+    (elixir) lib/string/chars.ex:17: String.Chars.to_string/1
+```
+
+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:
+
+```elixir
+defimpl String.Chars, for: Tuple do
+  def to_string(tuple) do
+    interior = 
+      tuple
+      |> Tuple.to_list
+      |> Enum.map(&Kernel.to_string/1)
+      |> Enum.join(", ")
+
+    "{#{interior}}"
+  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:
+
+```elixir
+iex> to_string({3.14, "apple", :pie})
+"{3.14, apple, pie}"
+```
+
+We know how to implement a protocol but how do we define a new one?
+For our example we'll implement `to_atom/1`.
+Let's see how to do that with `defprotocol`:
+
+```elixir
+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
+
+defimpl AsAtom, for: Map do
+  def to_atom(map), do: List.first(Map.keys(map))
+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:
+
+```elixir
+iex> import AsAtom
+AsAtom
+iex> to_atom("string")
+:string
+iex> to_atom(:an_atom)
+:an_atom
+iex> to_atom([1, 2])
+:"\x01\x02"
+iex> to_atom(%{foo: "bar"})
+:foo
+```
+
+It is worth nothing that although underneath structs are Maps, but they do not share protocol implementations with Maps. They are not enumerable, they cannot be accessed.
+
+As we can see, protocols are a powerful way to achieve polymorphism.
diff --git a/pl/lessons/basics/collections.md b/pl/lessons/basics/collections.md
index fa15659c78..859be292d3 100644
--- a/pl/lessons/basics/collections.md
+++ b/pl/lessons/basics/collections.md
@@ -1,5 +1,5 @@
 ---
-version: 0.9.0
+version: 1.0.0
 layout: page
 title: Kolekcje
 category: basics