### What’s it all about?

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}}
data = %{a: 1, b: 2}
reducer = fn {key, value}, acc ->
{:cont, Map.merge(acc, %{key => value + 1})}
end
KeywordLens.reduce_while(data, [:a, :b], %{}, reducer)
%{a: 2, b: 3}
```

The implementation uses zipper like traversal so it is memory efficient.