View Source Enumerables and Streams

While Elixir allows us to write recursive code, most operations we perform on collections is done with the help of the Enum and Stream modules. Let's learn how.


Elixir provides the concept of enumerables and the Enum module to work with them. We have already learned two enumerables: lists and maps.

iex>[1, 2, 3], fn x -> x * 2 end)
[2, 4, 6]
iex>{1 => 2, 3 => 4}, fn {k, v} -> k * v end)
[2, 12]

The Enum module provides a huge range of functions to transform, sort, group, filter and retrieve items from enumerables. It is one of the modules developers use frequently in their Elixir code. For a general overview of all functions in the Enum module, see the Enum cheatsheet.

Elixir also provides ranges (see Range), which are also enumerable:

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

The functions in the Enum module are limited to, as the name says, enumerating values in data structures. For specific operations, like inserting and updating particular elements, you may need to reach for modules specific to the data type. For example, if you want to insert an element at a given position in a list, you should use the List.insert_at/3 function, as it would make little sense to insert a value into, for example, a range.

We say the functions in the Enum module are polymorphic because they can work with diverse data types. In particular, the functions in the Enum module can work with any data type that implements the Enumerable protocol. We are going to discuss Protocols in a later chapter, for now we are going to move on to a specific kind of enumerable called a stream.

Eager vs Lazy

All the functions in the Enum module are eager. Many functions expect an enumerable and return a list back:

iex> odd? = fn x -> rem(x, 2) != 0 end
#Function<6.80484245/1 in :erl_eval.expr/5>
iex> Enum.filter(1..3, odd?)
[1, 3]

This means that when performing multiple operations with Enum, each operation is going to generate an intermediate list until we reach the result:

iex> 1..100_000 |> * 3)) |> Enum.filter(odd?) |> Enum.sum()

The example above has a pipeline of operations. We start with a range and then multiply each element in the range by 3. This first operation will now create and return a list with 100_000 items. Then we keep all odd elements from the list, generating a new list, now with 50_000 items, and then we sum all entries.

The pipe operator

The |> symbol used in the snippet above is the pipe operator: it takes the output from the expression on its left side and passes it as the first argument to the function call on its right side. Its purpose is to highlight the data being transformed by a series of functions. To see how it can make the code cleaner, have a look at the example above rewritten without using the |> operator:

iex> Enum.sum(Enum.filter(, &(&1 * 3)), odd?))

Find more about the pipe operator by reading its documentation.


As an alternative to Enum, Elixir provides the Stream module which supports lazy operations:

iex> 1..100_000 |> * 3)) |> Stream.filter(odd?) |> Enum.sum()

Streams are lazy, composable enumerables.

In the example above, 1..100_000 |> * 3)) returns a data type, an actual stream, that represents the map computation over the range 1..100_000:

iex> 1..100_000 |> * 3))
#Stream<[enum: 1..100000, funs: [#Function<34.16982430/1 in>]]>

Furthermore, they are composable because we can pipe many stream operations:

iex> 1..100_000 |> * 3)) |> Stream.filter(odd?)
#Stream<[enum: 1..100000, funs: [...]]>

Instead of generating intermediate lists, streams build a series of computations that are invoked only when we pass the underlying stream to the Enum module. Streams are useful when working with large, possibly infinite, collections.

Many functions in the Stream module accept any enumerable as an argument and return a stream as a result. It also provides functions for creating streams. For example, Stream.cycle/1 can be used to create a stream that cycles a given enumerable infinitely. Be careful to not call a function like on such streams, as they would cycle forever:

iex> stream = Stream.cycle([1, 2, 3])
#Function<15.16982430/2 in Stream.unfold/2>
iex> Enum.take(stream, 10)
[1, 2, 3, 1, 2, 3, 1, 2, 3, 1]

On the other hand, Stream.unfold/2 can be used to generate values from a given initial value:

iex> stream = Stream.unfold("hełło", &String.next_codepoint/1)
#Function<39.75994740/2 in Stream.unfold/2>
iex> Enum.take(stream, 3)
["h", "e", "ł"]

Another interesting function is Stream.resource/3 which can be used to wrap around resources, guaranteeing they are opened right before enumeration and closed afterwards, even in the case of failures. For example,!/1 builds on top of Stream.resource/3 to stream files:

iex> stream =!("path/to/file")
  line_or_bytes: :line,
  modes: [:raw, :read_ahead, :binary],
  path: "path/to/file",
  raw: true
iex> Enum.take(stream, 10)

The example above will fetch the first 10 lines of the file you have selected. This means streams can be very useful for handling large files or even slow resources like network resources.

The Enum and Stream modules provide a wide range of functions, but you don't have to know all of them by heart. Familiarize yourself with, Enum.reduce/3 and other functions with either map or reduce in their names, and you will naturally build an intuition around the most important use cases. You may also focus on the Enum module first and only move to Stream for the particular scenarios where laziness is required, to either deal with slow resources or large, possibly infinite, collections.

Next, we'll look at a feature central to Elixir, Processes, which allows us to write concurrent, parallel and distributed programs in an easy and understandable way.