Elixir provides a set of functions that enumerate over collections which implement the Enumerable protocol. These collections include lists and maps.

Enum.filter/2 takes two parameters:

  • a collection, and
  • a function which returns a boolean

Filtering a list:

iex> numbers = [1,2,3,4]
iex> evens = Enum.filter(numbers, fn(x) -> (rem x, 2) == 0 end)

Filtering maps:

iex> map = %{a: 1, b: 2}
iex> result = Enum.filter(map, fn {k, v} -> k == :a  end)
[a: 1]

Note that the functions in the Enum module are eager. The Stream module allows lazy enumeration of enumerables and provides infinite streams.

A lazy filter using streams:

iex> stream = 1..4
iex> evens = Stream.filter(stream, fn(x) -> (rem x, 2) == 0 end)
#Stream<[enum: 1..4, funs: [#Function<6.100286901/1 in Stream.filter/2>]]>
iex> Enum.to_list(evens)

Additional reading: