TIL_15_June_2017 - Polymorphism with Protocols in Elixir

[vinhnglx]

Polymorphism is the ability in programming to present the same interface for differing underlying forms -- (source: djm's blog)

From the Elixir in Action book, polymorphism is a runtime decision about which code to execute, based on the nature of the input data. In Elixir, the basic way of doing this is by using the language feature called Protocols.

iex > [1,2,3,4,5] |> Enum.map(fn(x) -> x* 2 end)
[2,4,6,8,10]

iex > %{first_name: "Vincent", last_name: "Nguyen"} |> Enum.map(fn({k, v}) -> IO.puts v end)
Vincent
Nguyen

From the example above, we can see the input data of Enum.map/2 can be a list, a map, range etc. So, how does the Enum.map/2 know how to walk each structure?. It doesn't.

The code in Enum.map/2 is generic and relies on a contract. This contract called a Protocol must be implemented for each data type you wish to use with Enum functions.

So, the protocol's concept is quite similar to abstract interfaces from OOP languages.

[vinhnglx]

A protocol is a module in which you declare functions without implementing them.

In the example above, Enum is a protocol - here is the source

• defprotocol Enumerable -> Definition of the protocol
• def all? -> Declaration of protocol functions.

If the protocol isn't implemented for the given data types, an error will raised

iex > [1,2,3,4,5] |> Enum.map(fn(x) -> x* 2 end)
[2,4,6,8,10]

iex> Vincent |> Enum.map(fn(x) -> x* 2 end)
** (Protocol.UndefinedError) protocol Enumerable not implemented for Vincent

[vinhnglx]

How to implement a protocol for a specific type

We start the implementation by calling the defimpl macro, then you specify which protocol to implement and the corresponding data type.

[vinhnglx]

Built-in protocols in Elixir

• Enumerable
  • Enumerable protocol used by Enum and Stream modules.
• Collectable
  • A protocol to traverse data structures. The Enum.into/2 function uses this protocol to insert an enumerable into a collection:
• Inspect
  • The Inspect protocol is responsible for converting any Elixir data structure into an algebra document. This document is then formatted, either in pretty printing format or a regular one.
• List.Chars
  • The List.Chars protocol is responsible for converting a structure to a list (only if applicable)
• String.Chars
  • The String.Chars protocol is responsible for converting a structure to a binary (only if applicable)

Detail can read from Elixir documentation.