The Enumerable protocol lets you iterate over the elements in a type—given a collection, you can get the elements. Collectable is in some sense the opposite—it allows you to build a collection by inserting elements into it.
Not all collections are collectable. Ranges, for example, cannot have new entries added to them.
The collectable API is pretty low-level, so you’ll typically access it via Enum.into and when using comprehensions (which we cover in the next section). For example, we can inject the elements of a range into an empty list using
| | iex> Enum.into 1..5, [] |
| | [1, 2, 3, 4, 5] |
If the list is not 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 to standard output:
| | iex> Enum.into IO.stream(:stdio, :line), IO.stream(:stdio, :line) |