From: "zverok (Victor Shepelev)" <noreply@...>
Date: 2021-09-03T12:21:50+00:00
Subject: [ruby-core:105139] [Ruby master Feature#18136] take_while_after

Issue #18136 has been updated by zverok (Victor Shepelev).


@ufuk
I see two problems with your proposal:
* the `upto`, as somebody already pointed to me, is existing in Ruby with different meaning (`1.upto(5)`), so, unfortunately can't be reused here
* "like while, but with condition negated" is usually called `until`; but the "also takes the last item" is NOT part of until definition people used to

I don't like `_after` postfix much, but I don't see a definitely good alternative: in imperative code, the problem is usually solved with "postfix `while`" (e.g. `do action while condition`), I don't recall the term for "while including the matched item" in either Ruby or other language.

----------------------------------------
Feature #18136: take_while_after
https://bugs.ruby-lang.org/issues/18136#change-93548

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
----------------------------------------
Sorry, I already tried that once (#16441) but I failed to produce the persuasive example.
So I am back with a couple of them, much simpler and clear than my initial.

**The proposal itself:** Have `take_while_after` which behaves like `take_while` but also includes the last element (first where the condition failed). Reason: there are a lot of cases where "the last good item" in enumeration is the distinctive one (one where enumeration should stop, but the item is still good.

**Example 1:** Take pages from paginated API, the last page will have less items than the rest (and that's how we know it is the last):

```ruby
(0..).lazy
  .map { |offset| get_page(offset, limit) }
  .take_while_after { |response| response.count == limit } # the last will have, say, 10 items, but should still be included!
  .map { process response somehow }
```

**Example 2:** Same as above, but "we should continue pagination" is specified with a separate data key "can_continue":
```ruby
(0..).lazy
  .map { |offset| get_page(offset, limit) }
  .take_while_after { |response| response['can_continue'] } # the last will have can_continue=false, but still has data
  .map { process response somehow }
```

**Exampe 3:** Taking a sentence from a list of tokens like this:
```ruby
tokens = [
  {text: 'Ruby', type: :word},
  {text: 'is', type: :word},
  {text: 'cool', type: :word},
  {text: '.', type: :punctuation, ends_sentence: true},
  {text: 'Rust', type: :word},
  # ...
]

sentence = tokens.take_while_after { _1[:ends_sentence] }
```

(I can get more if it is necessary!)

Neither of those can be solved by "Using `take_while` with proper condition.", as @matz suggested here: https://bugs.ruby-lang.org/issues/16441#note-9

I typically solve it by `slice_after { condition }.first`, but that's a) uglier and b) greedy when we are working with lazy enumerator (so for API examples, all paginated pages would be fetched at once, and only then processed).

Another consideration in #16441 was an unfortunate naming.
I am leaving it to discussion, though I tend to like `#take_upto` from #16446.



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