[ruby-core:118558] [Ruby master Feature#20625] Object#chain_of

11 Jul 2024

      Issue #20625 has been updated by zverok (Victor Shepelev).

```ruby
breadcrumbs = Enumerator.produce(parent, &:parent_directory).take_while { !_1.nil? }
hierarchy = Enumerator.produce(employee, &:member).take_while { !_1.nil? }
```

My initial proposal for what became `Enumerator.produce` (it is the opposite of `Enumerable#reduce`, hence the name) was to make it an Object’s method. 

But it was decided that increasing Object’s API is too breaking change, so it became a separate `Enumerator` method... Which, to the best of my estimation, not a lot of people are aware about (because even when somebody looks for “method like this”, it is not obvious where to look for it).

----------------------------------------
Feature #20625: Object#chain_of
https://bugs.ruby-lang.org/issues/20625#change-109081

* Author: matheusrich (Matheus Richard)
* Status: Open
----------------------------------------
## Motivation

It's often common to traverse a tree/list-like structure in order to get a chain
of elements. This proposal is to add a method to `Object` that allows collecting
a chain of elements by applying a block to each element. It doesn't require the
root element to be an instance of a specific class or respond to a specific
protocol.

I think this method could be useful in many cases, since hierarchies like this
are common in codebases (e.g. file systems, organizations structures, commit
histories, breadcrumbs in web apps, configuration hierarchies, etc.).

Here are some examples extracted from real codebases (simplified for the sake of
the example/privacy):

```rb
# Given a file system structure, get the breadcrumbs of a file or directory
def breadcrumbs(root)
  crumbs = []
  current = root

  while current
    crumbs << current
    current = current.parent_dir
  end

  crumbs
end

# Given an employee, get the hierarchy of managers
def hierarchy(employee)
  hierarchy = []
  current = employee

  while current
    hierarchy << current
    current = current.manager
  end

  hierarchy
end
```

## Implementation

The implementation in Ruby could look like this:

```rb
class Object
  def chain_of(&block)
    chain = []
    current = self

    while current
      chain << current
      current = block.call(current)
    end

    chain
  end
end
```

Here's an example use:

```rb
class ListNode
  attr_accessor :value, :parent

  def initialize(value, parent = nil)
    @value = value
    @parent = parent
  end

  def ancestors
    chain_of(&:parent).shift
  end
end

root = ListNode.new("root")
child1 = ListNode.new("child1", root)
child2 = ListNode.new("child2", child1)

puts child2.ancestors.map(&:value)
# => ["child1", "root"]
```

The examples from the motivation section could be rewritten as:

```rb
breadcrumbs = root.chain_of(&:parent_directory)
hierarchy = employee.chain_of(&:manager)
```

## Considerations

- While I'm including the object by default in the chain, it could be more
intuitive to exclude it. In any case, it's easy to remove or add it with
`shift`/`unshift`.

- On a different note, the method could be named differently (I do like
`chain_of`, though). Some alternatives I've considered are `map_chain`,
`traverse`, and `trace_path`.

- The method assumes that the traversal will finish at some point. If the user
has a cyclic structure, it will loop indefinitely. We could stop looping if we
find the same element twice. I don't think it's worth the extra complexity.

- I'm not sure `Object` is the best place for this method. While it's very
general, I think it gives power to the user to decide how to traverse a chain
like this without having to rely on a specific class. Maybe a mixin
(`Traversable`/`Chainable`) would be more appropriate? Could this fit in
`Enumerable`, somehow?

Of course, I'm open to suggestions and feedback. Thanks for reading!

-- 
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