Feature #14423
Updated by zverok (Victor Shepelev) about 6 years ago
Sometimes (or rather often), there is a programming pattern of "start from one object, do something, look at the result, do the same, look at the result (and so on)".
Examples:
* fetch page by URL, if pagination present, fetch next page;
* take 10 jobs from the queue, process them, exit when queue is empty;
Typically, those are represented by `while` or `loop` + `break` which somehow feels "not functional enough", and even "not Ruby enough", so much less expressive than `map` and other `Enumerable`/`Enumerator`-based cycles.
In some functional languages or libraries, there is function named `FixedPoint` or `converge`, whose meaning is "take an initial value, repeat the block provided on the result on prev computation, till it will not 'stable'". I believe this notion can be useful for Rubyists too.
Reference implementation (name is disputable!):
```ruby
class Object
def converge(&block)
Enumerator.new { |y|
prev = self
y << self
loop do
cur = block.call(prev)
raise StopIteration if cur == prev
y << cur
prev = cur
end
}
end
end
```
Examples of usage:
```ruby
# Functional kata: find the closest number to sqrt(2):
1.0.converge { |x| (x + 2 / x) / 2 }.to_a.last # => 1.414213562373095
Math.sqrt(2) # => 1.4142135623730951
# Next page situation:
get(url).converge { |page| page.next }
# => returns [page, page.next, page.next.next, ...til the result is nil, or same page repeated]
# Job queue situation:
queue.top(10).converge { |jobs|
jobs.each(&:perform)
queue.top(10)
}
# => takes top 10 jobs, till queue is empty (`[]` is returned two successful times)
# Useful for non-converging situations, too:
2.converge { |x| x ** 2 }.take(4)
# => [2, 4, 16, 256]
# Idiomatic Fibonacci:
[0, 1].converge { |f0, f1| [f1, f0 + f1] }.take(10).map(&:first)
# => [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
```
Reference to similar things:
* Clojure [iterate](https://clojuredocs.org/clojure.core/iterate) "Returns a lazy sequence of `x`, `(f x)`, `(f (f x))` etc." No converging, just infinite sequence... And maybe that is even more basic and idiomatic. The name is nice, too.
* WolframLang [FixedPoint](http://reference.wolfram.com/language/ref/FixedPoint.html)
* Ramda [converge](http://ramdajs.com/docs/#converge)
* Elixir [Stream#unfold](https://hexdocs.pm/elixir/Stream.html#unfold/2) (ends iteration when `nil` is returned)
* Scala [Iterator#iterate](https://www.scala-lang.org/api/current/scala/collection/Iterator$.html#iterate%5BT%5D%28start%3AT%29%28f%3AT%3D%3ET%29%3AIterator%5BT%5D) (just infinite sequence)
Possible call-seq:
* If converges: `Object#converge(&block)`, `Enumerator.converge(object, &block)`;
* If just an infinite sequence: `Object#iterate(&block)`, `Object#deduce(&block)` (as opposed to `reduce`), `Enumerator.iterate(object, &block)`, `Enumerator#enumerate(object, &block)`.
WDYT?..
PS: Can imagine somebody already proposed that, yet can't find nothing similar in the tracker for all keywords I've tried.