Languages / Elixir / Function.org

* Functions and Pattern Mathching

Behind the scenes, functions are patter-matching the arguments that they're called with.
Say we needed a function to accept a map but we're only interested in using a particular key.
We can pattern-match the argument on the presences of that key like this:

#+NAME: Greeter1
#+begin_src elixir
defmodule Greeter1 do
  def hello(%{name:  person_name}) do
    IO.puts "Hello, " <> person_name
  end
end
#+end_src

#+begin_src elixir
fred = %{name: "Fred", age: "95", favorite_color: "Taupe"}

Greeter1.hello(fred)
#+end_src

#+RESULTS:
: Hello, Fred
: :ok

What happens when we call the function with a map that doesn't contain the =:name= key?

#+begin_src elixir
Greeter1.hello(%{age: "95", favorite_color: "Taupe"})
#+end_src

#+RESULTS:
: ** (FunctionClauseError) no function clause matching in Greeter1.hello/1    
:     
:     The following arguments were given to Greeter1.hello/1:
:     
:         # 1
:         %{age: "95", favorite_color: "Taupe"}
:     
:     iex:2: Greeter1.hello/1
:     iex:2: (file)

The reason for this behavior is that Elixir pattern-matches the arguments that a function is called with against the arity the function is defined with.

In =Greeter1.hello/1=, the map we pass (=fred=) is evaluated against our argument (=%{name: person_name}=):

#+begin_src elixir
%{name: person_name} = ${name: "Fred", age: "95", favorite_color: "Taupe"}
#+end_src

It finds that there is a key that corresponds to =name= in the incoming map. We have a match!
And as a result of this successful match, the value of the =:name= key in the map on the right (i.e. the =fred= map) is bound to the variable on the left (=person_name=).

Now, what if we still wanted to assign Fred's name to =person_name= but we ALSO want to retain awareness of the entire person map?
Let's say we want to =IO.inspect(fred)= after we greet him.
At this point, because we only pattern-matched the =:name= key of our map, thus only binding the value of that key to a varaible, the function doen't have knowledge of the rest of Fred.

In order to retain in, we need to assign that entire map to its own variable for us to be able to use it.

Let's start a new function:
#+begin_src elixir
defmodule Greeter2 do
  def hello(%{name: person_name} = person) do
    IO.puts "Hello, " <> person_name
    IO.inspect person
  end
end
#+end_src

#+RESULTS:
: {:module, Greeter2,
:  <<70, 79, 82, 49, 0, 0, 6, 168, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 218,
:    0, 0, 0, 24, 15, 69, 108, 105, 120, 105, 114, 46, 71, 114, 101, 101, 116,
:    101, 114, 50, 8, 95, 95, 105, 110, 102, 111, ...>>, {:hello, 1}}

Remember that Elixir will pattern match the argument as it comes in.
Therefore in this case, eacm side will pattern match against the incoming argument and bind to whatever it matches with.
Let's take the right side first:

#+begin_src elixir
person = %{name: "Fred", age: "95", favorite_color: "Taupe"}
#+end_src

Now, =person= has been evaluated and bound to the entire fred-map.
We move on to the next pattern-match:

#+begin_src elixir
%{name: person_name} = %{name: "Fred", age: "95", favorite_color: "Taupe"}
#+end_src

Now this is the same as our original =Greeter1= function where we pattern matched the map and only retained Fred's name.
What we've achieved is two variables we can use instead of one:

1. =person=, referring to =%{name: "Fred", age: "95", favorite_color: "Taupe"}=
2. =person_name=, referring to ="Fred"=

So now when we call =Greeter2.hello/1=, we can use all of Fred's information:

#+begin_src elixir
Greeter2.hello(fred)

Greeter2.hello(%{name: "Fred"})
#+end_src

#+RESULTS:
: Hello, Fred
: %{age: "95", favorite_color: "Taupe", name: "Fred"}
: Hello, Fred
: %{name: "Fred"}

So we've seen that Elixir pattern-matches ata multiple depths because each argument matches tha incoming data independently,
leaving us with the variables to call them by inside our function.

*Summary*: Functions pattern-match the data passed in to each of its arguments independentry.
We can use this to bind values to separate variables within the function.
* Default arguments

If we want a default value for an argument we use the =argument \\ value= syntax:

#+begin_src elixir
defmodule Greeter do
  def hello(names, language_code \\ "en")

  def hello(names, language_code) when is_list(names) do
    names = Enum.join(names, ", ")

    hello(names, language_code)
  end

  def hello(name, language_code) when is_binary(name) do
    phrase(language_code) <> name
  end

  defp phrase("en"), do: "Hello, "
  defp phrase("es"), do: "Hola, "
end

Greeter.hello ["Sean", "Steve"]
Greeter.hello ["Sean", "Steve"], "es"
#+end_src

#+RESULTS:
: warning: redefining module Greeter (current version defined in memory)
:   iex:1: Greeter (module)
: 
: "Hola, Sean, Steve"