Protocols

Protocols are a mechanism to achieve polymorphism in Elixir when you want behavior to vary depending on the data type. We are already familiar with one way of solving this type of problem: via pattern matching and guard clauses. Consider a simple utility module that would tell us the type of input variable:

defmodule Utility do
  def type(value) when is_binary(value), do: "string"
  def type(value) when is_integer(value), do: "integer"
  # ... other implementations ...
end

If the use of this module were confined to your own project, you would be able to keep defining new type/1 functions for each new data type. However, this code could be problematic if it were shared as a dependency by multiple apps because there would be no easy way to extend its functionality.

This is where protocols can help us: protocols allow us to extend the original behavior for as many data types as we need. That’s because dispatching on a protocol is available to any data type that has implemented the protocol and a protocol can be implemented by anyone, at any time.

Here’s how we could write the same Utility.type/1 functionality as a protocol:

defprotocol Utility do
  @spec type(t) :: String.t()
  def type(value)
end

defimpl Utility, for: BitString do
  def type(_value), do: "string"
end

defimpl Utility, for: Integer do
  def type(_value), do: "integer"
end

We define the protocol using defprotocol - its functions and specs may look similar to interfaces or abstract base classes in other languages. We can add as many implementations as we like using defimpl. The output is exactly the same as if we had a single module with multiple functions:

iex> Utility.type("foo")
"string"
iex> Utility.type(123)
"integer"

With protocols, however, we are no longer stuck having to continuously modify the same module to support more and more data types. For example, we could get the defimpl calls above and spread them over multiple files and Elixir will dispatch the execution to the appropriate implementation based on the data type. Functions defined in a protocol may have more than one input, but the dispatching will always be based on the data type of the first input.

One of the most common protocols you may encounter is the String.Chars protocol: implementing its to_string/1 function for your custom structs will tell the Elixir kernel how to represent them as strings. We will explore all built-in protocols later. For now, let’s implement our own.

Example

Now that you have seen an example of the type of problem protocols help solve and how they solve them, let’s look at a more in-depth example.

In Elixir, we have two idioms for checking how many items there are in a data structure: length and size. length means the information must be computed. For example, length(list) needs to traverse the whole list to calculate its length. On the other hand, tuple_size(tuple) and byte_size(binary) do not depend on the tuple and binary size as the size information is pre-computed in the data structure.

Even if we have type-specific functions for getting the size built into Elixir (such as tuple_size/1), we could implement a generic Size protocol that all data structures for which size is pre-computed would implement.

The protocol definition would look like this:

defprotocol Size do
  @doc "Calculates the size (and not the length!) of a data structure"
  def size(data)
end

The Size protocol expects a function called size that receives one argument (the data structure we want to know the size of) to be implemented. We can now implement this protocol for the data structures that would have a compliant implementation:

defimpl Size, for: BitString do
  def size(string), do: byte_size(string)
end

defimpl Size, for: Map do
  def size(map), do: map_size(map)
end

defimpl Size, for: Tuple do
  def size(tuple), do: tuple_size(tuple)
end

We didn’t implement the Size protocol for lists as there is no “size” information pre-computed for lists, and the length of a list has to be computed (with length/1).

Now with the protocol defined and implementations in hand, we can start using it:

iex> Size.size("foo")
3
iex> Size.size({:ok, "hello"})
2
iex> Size.size(%{label: "some label"})
1

Passing a data type that doesn’t implement the protocol raises an error:

iex> Size.size([1, 2, 3])
** (Protocol.UndefinedError) protocol Size not implemented for [1, 2, 3]

It’s possible to implement protocols for all Elixir data types:

• Atom
• BitString
• Float
• Function
• Integer
• List
• Map
• PID
• Port
• Reference
• Tuple

Protocols and structs

The power of Elixir’s extensibility comes when protocols and structs are used together.

In the previous chapter, we have learned that although structs are maps, they do not share protocol implementations with maps. For example, MapSets (sets based on maps) are implemented as structs. Let’s try to use the Size protocol with a MapSet:

iex> Size.size(%{})
0
iex> set = %MapSet{} = MapSet.new
#MapSet<[]>
iex> Size.size(set)
** (Protocol.UndefinedError) protocol Size not implemented for #MapSet<[]>

Instead of sharing protocol implementation with maps, structs require their own protocol implementation. Since a MapSet has its size precomputed and accessible through MapSet.size/1, we can define a Size implementation for it:

defimpl Size, for: MapSet do
  def size(set), do: MapSet.size(set)
end

If desired, you could come up with your own semantics for the size of your struct. Not only that, you could use structs to build more robust data types, like queues, and implement all relevant protocols, such as Enumerable and possibly Size, for this data type.

defmodule User do
  defstruct [:name, :age]
end

defimpl Size, for: User do
  def size(_user), do: 2
end

Implementing Any

Manually implementing protocols for all types can quickly become repetitive and tedious. In such cases, Elixir provides two options: we can explicitly derive the protocol implementation for our types or automatically implement the protocol for all types. In both cases, we need to implement the protocol for Any.

Deriving

Elixir allows us to derive a protocol implementation based on the Any implementation. Let’s first implement Any as follows:

defimpl Size, for: Any do
  def size(_), do: 0
end

The implementation above is arguably not a reasonable one. For example, it makes no sense to say that the size of a PID or an Integer is 0.

However, should we be fine with the implementation for Any, in order to use such implementation we would need to tell our struct to explicitly derive the Size protocol:

defmodule OtherUser do
  @derive [Size]
  defstruct [:name, :age]
end

When deriving, Elixir will implement the Size protocol for OtherUser based on the implementation provided for Any.

Fallback to Any

Another alternative to @derive is to explicitly tell the protocol to fallback to Any when an implementation cannot be found. This can be achieved by setting @fallback_to_any to true in the protocol definition:

defprotocol Size do
  @fallback_to_any true
  def size(data)
end

As we said in the previous section, the implementation of Size for Any is not one that can apply to any data type. That’s one of the reasons why @fallback_to_any is an opt-in behaviour. For the majority of protocols, raising an error when a protocol is not implemented is the proper behaviour. That said, assuming we have implemented Any as in the previous section:

defimpl Size, for: Any do
  def size(_), do: 0
end

Now all data types (including structs) that have not implemented the Size protocol will be considered to have a size of 0.

Which technique is best between deriving and falling back to any depends on the use case but, given Elixir developers prefer explicit over implicit, you may see many libraries pushing towards the @derive approach.

Built-in protocols

Elixir ships with some built-in protocols. In previous chapters, we have discussed the Enum module which provides many functions that work with any data structure that implements the Enumerable protocol:

iex> Enum.map [1, 2, 3], fn(x) -> x * 2 end
[2, 4, 6]
iex> Enum.reduce 1..3, 0, fn(x, acc) -> x + acc end
6

Another useful example is the String.Chars protocol, which specifies how to convert a data structure to its human representation as a string. It’s exposed via the to_string function:

iex> to_string :hello
"hello"

Notice that string interpolation in Elixir calls the to_string function:

iex> "age: #{25}"
"age: 25"

The snippet above only works because numbers implement the String.Chars protocol. Passing a tuple, for example, will lead to an error:

iex> tuple = {1, 2, 3}
{1, 2, 3}
iex> "tuple: #{tuple}"
** (Protocol.UndefinedError) protocol String.Chars not implemented for {1, 2, 3}

When there is a need to “print” a more complex data structure, one can use the inspect function, based on the Inspect protocol:

iex> "tuple: #{inspect tuple}"
"tuple: {1, 2, 3}"

The Inspect protocol is the protocol used to transform any data structure into a readable textual representation. This is what tools like IEx use to print results:

iex> {1, 2, 3}
{1, 2, 3}
iex> %User{}
%User{name: "john", age: 27}

Keep in mind that, by convention, whenever the inspected value starts with #, it is representing a data structure in non-valid Elixir syntax. This means the inspect protocol is not reversible as information may be lost along the way:

iex> inspect &(&1+2)
"#Function<6.71889879/1 in :erl_eval.expr/5>"

There are other protocols in Elixir but this covers the most common ones. You can learn more about protocols and implementations in the Protocol module.