KeywordLens

A keyword lens is a nested keyword-like structure used to describe paths into certain data types. It is similar to the list you can provide to Ecto's Repo.preload/2

You can describe a KeywordLens like this:

[a: :b, c: [d: :e]]

Such a list is handy for describing subsets of nested data structures. For example, you can imagine the following KeywordLens: [a: :b] applied to this map: %{a: %{b: 1}} points to the value 1. In contrast this KeywordLens: [:a, :b] applied to this map %{a: 1, b: 2} points to both values 1 and 2.

It's not a proper Keyword list because we allow any key for convenience, so these are valid:

[{"a", :b}]
[a: [{"b", [c: :d]}]
[{%{}, :b}]
[{1, {2, 3}}]

One KeywordLens can point to many different values inside a given data structure.

Here are some examples of different KeywordLenses and the unique set of lenses they represent.

keyword_lens = [a: :b]
lenses = [[:a], [:b]]

keyword_lens = [a: [b: [:c, :d]]]
lenses = [[:a, :b, :c], [:a, :b, :d]]

keyword_lens = [a: [:z, b: [:c, d: :e]]]
lenses = [[:a, :z], [:a, :b, :c], [:a, :b, :d, :e]]

keyword_lens = [:a, "b", :c]
lenses = [[:a], ["b"], [:c]]

You can use KeywordLens.Helpers.expand/1 to see which unique lenses are encoded in a given KeywordLens.

KeywordLens.Helpers.expand([a: :b])
[:a, :b]

KeywordLens.Helpers.expand([a: [b: [:c, :d]]])
[[:a, :b, :c], [:a, :b, :d]]

This library provides a protocol you can implement for your own data structures and structs. We provide a map implementation to get started.

Examples

KeywordLens.map(%{a: %{b: 1}}, [a: :b], &(&1 + 1))
%{a: %{b: 2}}

Can't I just use get_in / update_in

You could, but the syntax becomes a bit verbose and repetitive:

%{a: %{b: 1}, c: %{d: 1, e: 1}}
|> update_in([:a, :b], & &1 + 1)
|> update_in([:c, :d], & &1 + 1)
|> update_in([:c, :e], & &1 + 1)
%{a: %{b: 2}, c: %{d: 2, e: 2}}

# Vs

KeywordLens.map(%{a: %{b: 1}, c: %{d: 1, e: 1}}, [a: :b, c: [:d, :e]], & &1+1)
%{a: %{b: 2}, c: %{d: 2, e: 2}}

Installation

If available in Hex, the package can be installed by adding keyword_lens to your list of dependencies in mix.exs:

def deps do
  [
    {:keyword_lens, "~> 0.1.1"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/keyword_lens.