The Collectable Protocol

Introduction

The Enumerable protocol lets us iterate over the elements in a type. Given a collection, we can get the elements. The Collectable protocol is in some sense the opposite. It allows us to build a collection by inserting elements into it.

Not all collections are collectable. Ranges, for example, can’t have new entries added to them.

Examples

The Collectable API is pretty low-level, so we’ll typically access it via Enum.into and when we’re using comprehensions (which we cover in the next lesson). For example, we can inject the elements of a range into an empty list using the following:

iex> Enum.into 1..5, [] 
[1, 2, 3, 4, 5]

If the list isn’t empty, the new elements are tacked onto the end:

iex> Enum.into 1..5, [ 100, 101 ] 
[100, 101, 1, 2, 3, 4, 5]

Output streams are collectable, so the following code lazily copies standard input (we’ll have to enter some input) to standard output:

iex> Enum.into IO.stream(:stdio, :line), IO.stream(:stdio, :line)

Use the terminal below for practice: