Update nested maps in Elixir

Overview

Elixir is a dynamic, functional language designed for building scalable and maintainable applications.

Elixir leverages the Erlang VM, known for running low-latency, distributed and fault-tolerant systems, while also being successfully used in web development and the embedded software domain.

Elixir Maps

Maps are the "go to" key-value data structure in Elixir. Maps can be created with the %{} syntax, and key-value pairs can be expressed as key => value:

iex> %{}
%{}
iex> %{"one" => :two, 3 => "four"}
%{3 => "four", "one" => :two}

Key-value pairs in a map do not follow any order (that's why the printed map in the example above has a different order than the map that was created).

Maps do not impose any restriction on the key type: anything can be a key in a map. As a key-value structure, maps do not allow duplicated keys. Keys are compared using the exact-equality operator (===/2). If colliding keys are defined in a map literal, the last one prevails.

Being a functional programming language, Elixir is a bit different from other languages when it comes to update a nested data structure.

For example, in javascript we could do:

data = {
  inner: {
    one: {
      a: 1
    },
    two: {
      b: 45
    }
  }
}

data.inner.one.a = 2

In Elixir you have to build a new map with the updated information, for example:

data = %{
  inner: %{
    one: %{
      a: 1
    },
    two: %{
      b: 45
    }
  }
}

new_one = %{data.inner.one | a: 2}
new_inner = %{data.inner | one: new_one}
new_data = %{data | inner: new_inner}

which is not very handy.

In other functional languages like Haskell, there are libraries like Lenses that aims to solve the problem. In Elixir the kernel have an put_in function that acts in a similar way:

data = put_in(data, [:inner, :one, :a], 2)

You can find other similar functions in the Kernel documentation