# 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:
```elixir
[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:

```elixir
[{"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.

```elixir
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.

```elixir
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

```elixir
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:

```elixir
%{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](https://hex.pm/docs/publish), the package can be installed
by adding `keyword_lens` to your list of dependencies in `mix.exs`:

```elixir
def deps do
  [
    {:keyword_lens, "~> 0.1.0"}
  ]
end
```

Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
be found at [https://hexdocs.pm/keyword_lens](https://hexdocs.pm/keyword_lens).