Bringing structs and protocols together

Now that we have the %Folder{} struct defined, we can define its implementation for the Size protocol.

We'll first define the implementation for the %File.Stat{} struct, as we can then use this to implement the protocol for %Folder{}. Here's the implementation for %File.Stat{}:

$ cat examples/size_implementations_file_stat_and_folder.ex
defimpl Size, for: File.Stat do
  def size(file_stat), do: file_stat.size
end

# ...

With this in place, our implementation for our %Folder{} struct is as follows:

$ cat examples/size_implementations_file_stat_and_folder.ex
# ...

defimpl Size, for: Folder do
  def size(folder) do
    folder.files_info
    |> Enum.map(&Size.size(&1))
    |> Enum.sum()
  end
end

To find out the size of a folder, we sum the size of each file it contains. As such, this implementation iterates through our files_info list, using the Size implementation for %File.Stat{} to get the size of each file, summing all the sizes in the end. In the following snippet, we can see this implementation being used on the folder variable we just defined:

iex> Size.size(folder)
779

With this, we can see the full power of mixing structs and protocols, which lets us have polymorphic functions based on the data type of their arguments. We now have a common interface, Size.size(data), that allows us to find out the size of pretty much anything we want, provided that we implement the Size protocol for the data type we're interested in.