See gem 'accessory' on Rubygems Yard Docs Documentation coverage

Accessory is a Ruby re-interpretation of Elixir's particular implementation of functional lenses.

  • Like Elixir, Accessory provides functions called get_in, update_in, pop_in, put_in, get_and_update_in, etc., that each take a "lens path" and traverse it.

  • Like Elixir, Accessory's lens paths are composed of "accessors" — you'll find most of the same ones from Elixir's Access module (insofar as they make sense in Ruby), and also some uniquely-Ruby accessors as well.

  • Like Elixir, these lens paths are detached from any source, and so are reusable (they can be e.g. set as module-level constants.)

  • Unlike Elixir, where lenses are plain lists and accessors are closures, Accessory's lenses and accessors are objects. The traversal functions are all methods on the Accessory::LensPath.

  • Unlike Elixir, Accessory's mutative traversals (update_in, put_in, etc.) modify the input document in place. (Non-in-place accessors are also planned.)


$ gem install accessory


require 'accessory'


An Accessory::LensPath is a "free-floating" lens (not bound to a subject document.)

Accessory::LensPath[:foo, "bar", 0]

Accessors are classes named like Accessory::FooAccessor. You can create and use accessors directly in a LensPath:


...or use the convenience module-functions on Accessory::Access:

include Accessory
LensPath[Access.subscript(:foo)] # equivalent to the above

Also, as an additional convenience, LensPath will wrap any raw objects (objects not descended from Accessory::Accessor) in a SubscriptAccessor. So another way to write the above is:


You can define your own accessor classes by inheriting from Accessory::Accessor, and use them in a LensPath as normal.

class MyAccessor < Accessory::Accessor
  # ...


Extending LensPaths

Existing LensPaths may be "extended" with additional path-components using .then:

lp = LensPath[Access.first, :foo, Access.all]
lp.then("bar") # => #LensPath[Access.first, :foo, Access.all, "bar"]

LensPath instances are created frozen; each use of .then produces a new child LensPath, rather than affecting the original. This allows you to reuse "base" LensPaths.

LensPaths may also be concatenated using +, again producing a new LensPath instance:

lp1 = LensPath[Access.first, :foo]
lp2 = LensPath[Access.all, "bar"]
lp1 + lp2 # => #LensPath[Access.first, :foo, Access.all, "bar"]

+ also allows "bald" accessors, or plain arrays of accessors:

LensPath[:foo] + :bar + [:baz, :quux] # => #LensPath[:foo, :baz, :baz, :quux]

Another name for + is /. This allows for a traveral syntax similar to Pathname instances:

LensPath.empty / :foo / :bar # => #LensPath[:foo, :bar]

Fluent API

Methods with the same names as the module-functions in Access are included in LensPath. Calling these methods has the same effect as calling the relevant module-function and passing it to .then.

These methods allow you to construct a LensPath through a chain of method-calls that closely resembles a concrete traversal of a container-object.

include Accessory

# the following are equivalent:
LensPath[Access.first, :foo, Access.all, "bar"]


You can combine your own accessors with the fluent methods by using .then:



A LensPath may be bound to a subject document with LensPath#on to produce a Lens:

doc = {foo: 1}
doc_foo = LensPath[:foo].on(doc) # => #<Lens on={:foo=>1} [:foo]>

Alternately, you can use Lens.on(doc) to create an identity Lens:

doc = {foo: 1}
Lens.on(doc) # => #<Lens on={:foo=>1} []>

A Lens exposes the same traversal methods as a LensPath, but does not require that you pass in a document, as it already has its own:

doc_foo.get_in # => 1
doc_foo.put_in(2) # => {:foo=>2}
doc # => {:foo=>2}

A Lens also exposes all the extension methods of its LensPath. Like a LensPath, a Lens is frozen, so these methods return a new Lens wrapping a new LensPath:

doc = {}
doc_root = Lens.on(doc)     # => #<Lens on={} []>
doc_foo  = doc_root / :foo  # => #<Lens on={} [:foo]>

The .lens refinement

By using Accessory, a .lens method is added to all Objects, which has the same meaning as passing the object to Lens.on.

using Accessory
{}.lens[:foo][:bar].put_in(5) # => {:foo=>{:bar=>5}}

The .lens method also accepts additional variadic arguments, representing either a LensPath or a list of accessors to use to construct one:

using Accessory

{}.lens(:foo, :bar).put_in(5) # => {:foo=>{:bar=>5}}

foo_bar = LensPath[:foo, :bar]
{}.lens(foo_bar).put_in(3) # => {:foo=>{:bar=>3}}

Default inference for intermediate accessors

Every accessor knows how to construct a valid, empty value of the type it expects to receive as input. For example, the AllAccessor expects to operate on Enumerables, and so defines a default constructor of

When a LensPath is created, the default constructors for each accessor are "fed back" through to their predecessor accessor. The predecessor stores the default constructor to use as a fall-back default (i.e. a default for when you didn't explicitly specify a default.)

This means that you usually don't need to specify defaults in your accessors, because sensible values are inferred from the next operation in the LensPath traversal chain.

Let's annotate a LensPath with the inferred defaults for each traversal-step:

  :foo,               # (FirstAccessor)
  Access.first,       # (AttributeAccessor)
  Access.attr(:name), # (SubscriptAccessor)
  :bar                # nil (no successor)

Using put_in with this lens will have the result you'd expect:

doc = {}
(doc.lens / :foo / Access.first / Access.attr(:name) / :bar).put_in(1)
doc # => {:foo=>[#<OpenStruct name={:bar=>1}>]}