[ruby-core:118785] [Ruby master Feature#20664] Add `before` and `until` options to Enumerator.produce

["zverok (Victor Shepelev) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20664 has been updated by zverok (Victor Shepelev).


I am not sure about this API. 

I think in language core there aren’t many APIs that accept just a symbol of a necessary method (only `reduce(:+)` comes to mind, and I am still not sure why this form exists, because it seems to have been introduced at the same time when `Symbol#to_proc` was, so `reduce(:+)` and `reduce(&:+)` were always co-existing). 

Mostly callables are passed as a block (and therefore there can be only one); but some APIs accept another callable (any object with `#call` method, like Enumerator.new).

So, what if condition is _not_ an method of the sequence?.. Should we accept callables, too? Or, what if the method’s user expects it to be a particular value (like `until: 0`), or a pattern (like `before: 0..1`).

The alternative is 
```ruby
Enumerator.produce(File, &:superclass).take_until(&:nil?)
```
...which is more or less the same character-count-wise, more powerful (any block can be used), and more atomic.

The one problem we don’t currently have neither `Enumerable#take_until`, nor `Object#not_nil?`, to write something like
```ruby
# this wouldn’t work
Enumerator.produce(File, &:superclass).take_while(&:not_nil?)
# though one can use
Enumerator.produce(File, &:superclass).take_while(&:itself)
#=> [File, IO, Object, BasicObject]
```

...but in general, I suspect adding `Enumerable#take_until` to handle such cases ([and `#take_while_after`](https://bugs.ruby-lang.org/issues/18136) while we are on it :)) might be more powerful addition to the language, useful in many situations.

----------------------------------------
Feature #20664: Add `before` and `until` options to Enumerator.produce
https://bugs.ruby-lang.org/issues/20664#change-109340

* Author: knu (Akinori MUSHA)
* Status: Open
----------------------------------------
Enumerator.produce provides a nice way to generate an infinite sequence but is a bit awkward to define how to end a sequence.  It lacks a simple and easy way to produce typical finite sequences in an intuitive syntax.

This proposal attempts to solve the problem by adding these two options to the method:

- `before`: when provided, it is used as a predicate to determine if an iteration should end before a generated value gets yielded.
- `until`: when provided, it is used as a predicate to determine if an iteration should end until after a generated value gets yielded.

Any value that responds to `to_proc` and returns a `Proc` object is accepted in these options.

A typical use case for the `before` option is traversing a tree structure to iterate over the ancestors or following/preceding siblings of a node.

The `until` option can be used when there is a clear definition of the "last" value to yield.

```ruby
enum = Enumerator.produce(File, before: :nil?, &:superclass)
enum.to_a #=> [File, IO, Object, BasicObject]

enum = Enumerator.produce(3, until: :zero?, &:pred)
enum_to_a #=> [3, 2, 1, 0]
```

---Files--------------------------------
0001-Add-before-and-until-options-to-Enumerator.produce.patch (10.7 KB)


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