From: mame@...
Date: 2018-06-21T10:21:29+00:00
Subject: [ruby-core:87567] [Ruby trunk Feature#14781] Enumerator#generate

Issue #14781 has been updated by mame (Yusuke Endoh).


zverok (Victor Shepelev) wrote:
> Though, I should add that `Enumerator.generate` (seen this way, not just `.generate` alone) seems to clearly state "generate enumerator" :)

"generate" seems too general.  It looks the most typical or primitive way to create an enumerator, but it is not.

Haskell provides "iterate" function for this feature, but it resembles an iterator in Ruby.

----------------------------------------
Feature #14781: Enumerator#generate
https://bugs.ruby-lang.org/issues/14781#change-72573

* 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+|[-+*/]}) }.take_while { !scanner.eos? }.to_a
# => ["7", "+", "38", "/"]

# 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.



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