foci

Foci is a lodash inspired lens library.

A lens is a path into an object, which can be used to extract its values, or even "modify" them in place (by creating a new object with the value changed).

When writing immutable code, we very commonly end up with deeply nested data stores, e.g.:

const store = {
	users: [
		{name: 'Jack Sparrow', 
		 posts: [
		 	{title: 'Why is the rum always gone? A dissertation on Carribean trade deficit'}
		 	],
		 	...
		 },
	...
	]
}

And updating a deeply nested structure will require heavy usage of the spread operator (or Object.assign). E.g. To capitalize the title of the first post of the first user, you would write:

const userIdx = 0;
const postIdx = 0;
const capitalize = (string) => {...}

{...store,
	users: store.users.map((user, idx) => (
		idx === userIdx 
		? {...user, 
			  posts: user.posts.map((post, idx) =>
					idx === postIdx
					? {...post,
	 						title: capitalize(post.title)
					  }
					: user)
		  }
		 : user
		)
}

This is an enormous amount of obfuscating boiler plate code for a very simple update.

With lenses, we could write this update much more declaratively:

mod(`users[${userIdx}].posts[${postIdx}]`)
	(capitalize)
	(store)
	

API

lens

A lens is simply a string which describes a path into an object. It can include object accesses and array indicies.

The focus of the lens is the final value referenced in the path.

Lenses can be constructed with the lens function, or passed as string literals into the lens consumer functions (get, set, mod)

Combining lenses with ES6 template strings can be a concise way to use environment variables to create a dynamic path.

> "a.b[3].d" // focus is the d field

> const idx = 10
> `a.b[${idx}]` // focus is the 11th element of b

get :: (string or lens) -> obj -> focus

get consumes a lens and produces a function that takes in an object obj and outputs the focus of its lens.

> get('a.b.c')({a: {b: {c: 7}}})
7

set :: (string or lens) -> const -> obj -> obj

set consumes a lens and produces a function that takes in a constant value const, and produces a function consuming an object obj and outputs a clone of obj with the focus of the lens replaced with const

> set('a.b.c')(10)({a: {b: {c: 7}}})
{a: {b: {c: 10}}}

mod :: (string or lens) -> (a -> a) -> obj -> obj

mod consumes a lens and produces a function that takes in a modifiying function m for the focus of the lens, and produces a function consuming an object obj, then outputs a clone of obj with the focus of the lens replaced with m's output.

> const inc = n => n + 1
> mod('a.b.c')(inc)({a: {b: {c: 7}}})
{a: {b: {c: 8}}}

lens :: string -> lens

lens consumes a path into an object and produces a corresponding lens function. All lens consumers (get, set, mod) will accept a literal string. However, lenses are composable via compose, so lenses can be built piece by piece.

compose :: (lens or string)* -> lens

compose composes the foci of multiple lenses

> const l = compose('a.b', 'c.d')
> get(l)({a: {b: {c: {d: 10}}}})
10