Issue #14781 has been updated by knu (Akinori MUSHA).

File enumerator_from.rb added

I've been thinking about this, and I have some ideas I want to share:

- To recursively traverse ancestors of a node is one of the most typical use cases, so that should be made easy to do.
- When and how to end a sequence may vary, so there should be some flexibility in defining an end.  For example, nil is not always the dead end.  It could mean something; you might even want to end a sequence with an explicit nil as sentinel.  Rescuing an exception and treating it as an end might not be a good option because that would make debugging hard, but StopIteration should be a good fit as a signal for an end.
- Sometimes you'd need to look back two or more steps to generate a new value (not to mention Fibonacci series), so the constructor should preferably take multiple seeds.
- Sometimes seeds are not subject of yielding; it would be handy if you could specify how many leading seeds to skip.

In the original proposal, there are some tricks needed to define an end of a sequence or to look back multiple preceding terms, so I've come up with an alternative API that builds them in as keyword options:

```
Enumerator.from(seeds, drop: 0, allow_nil: false) { |*preceding_terms| next_term }

  seeds: Array of objects to be used as seeds (required, but can be an empty array)
  drop: How many leading terms to skip
  allow_nil: True if nil should not end the enumerator
```

I wrote an experimental implementation and test cases in the attached file.

I'll be working on it further in this weekends' hackathon, so any input is appreciated!

----------------------------------------
Feature #14781: Enumerator#generate
https://bugs.ruby-lang.org/issues/14781#change-74498

* Author: zverok (Victor Shepelev)
* Status: Feedback
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
This is alternative proposal to `Object#enumerate` (#14423), which was considered by many as a good idea, but with unsure naming and too radical (`Object` extension). This one is _less_ radical, and, at the same time, more powerful.

**Synopsys**: 
* `Enumerator.generate(initial, &block)`: produces infinite sequence where each next element is calculated by applying block to previous; `initial` is first sequence element;
* `Enumerator.generate(&block)`: the same; first element of sequence is a result of calling the block with no args.

This method allows to produce enumerators replacing a lot of common `while` and `loop` cycles in the same way `#each` replaces `for`.

**Examples:**

With initial value

```ruby
# Infinite sequence
p Enumerator.generate(1, &:succ).take(5)
# => [1, 2, 3, 4, 5]

# Easy Fibonacci
p Enumerator.generate([0, 1]) { |f0, f1| [f1, f0 + f1] }.take(10).map(&:first)
#=> [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

require 'date'

# Find next Tuesday
p Enumerator.generate(Date.today, &:succ).detect { |d| d.wday == 2 }
# => #<Date: 2018-05-22 ((2458261j,0s,0n),+0s,2299161j)>

# Tree navigation
# ---------------
require 'nokogiri'
require 'open-uri'

# Find some element on page, then make list of all parents
p Nokogiri::HTML(open('https://www.ruby-lang.org/en/'))
  .at('a:contains("Ruby 2.2.10 Released")')
  .yield_self { |a| Enumerator.generate(a, &:parent) }
  .take_while { |node| node.respond_to?(:parent)  }
  .map(&:name)
# => ["a", "h3", "div", "div", "div", "div", "div", "div", "body", "html"]

# Pagination
# ----------
require 'octokit'

Octokit.stargazers('rails/rails')
# ^ this method returned just an array, but have set `.last_response` to full response, with data
# and pagination. So now we can do this:
p Enumerator.generate(Octokit.last_response) { |response| 
    response.rels[:next].get                         # pagination: `get` fetches next Response
  } 
  .first(3)                                          # take just 3 pages of stargazers
  .flat_map(&:data)                                  # `data` is parsed response content (stargazers themselves)
  .map { |h| h[:login] }
# => ["wycats", "brynary", "macournoyer", "topfunky", "tomtt", "jamesgolick", ...
```

Without initial value

```ruby
# Random search
target = 7
p Enumerator.generate { rand(10) }.take_while { |i| i != target }.to_a
# => [0, 6, 3, 5,....]

# External while condition
require 'strscan'
scanner = StringScanner.new('7+38/6')
p Enumerator.generate { scanner.scan(%r{\d+|[-+*/]}) }.slice_after { scanner.eos? }.first
# => ["7", "+", "38", "/", "6"]

# Potential message loop system:
Enumerator.generate { Message.receive }.take_while { |msg| msg != :exit }
```

**Reference implementation**: https://github.com/zverok/enumerator_generate

I want to **thank** all peers that participated in the discussion here, on Twitter and Reddit.

---Files--------------------------------
enumerator_from.rb (3.16 KB)


-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>