Enumerable

The Enumerable protocol is the basis of all the functions in the Enum module. Any type implementing it can be used as a collection argument to Enum functions.

We’re going to implement Enumerable for our Midi structure, so we’ll need to wrap the implementation in something like this:

defimpl Enumerable, for: Midi do 
  # ...
end

Major functions

The protocol is defined in terms of four functions: count, member?, reduce, and slice, as shown below:

defprotocol Enumerable do
  def count(collection)
  def member?(collection, value) 
  def reduce(collection, acc, fun) 
  def slice(collection)
end

The count function returns the number of elements in the collection. The member? function is truthy if the collection contains value. The reduce function applies the given function to successive values in the collection and the accumulator; the value it reduces becomes the next accumulator. Finally, slice is used to create a subset of a collection. Perhaps surprisingly, all the Enum functions can be defined in terms of these four.

However, it isn’t that simple. Maybe we’re using Enum.find to find a value in a large collection. Once we’ve found it, we want to halt the iteration because continuing is pointless. Similarly, we may want to suspend an iteration and resume ...