[ruby-core:109207] [Ruby master Feature#18915] New error class: NotImplementedYetError or scope change for NotImplementedYet

Issue #18915 has been reported by Quintasan (Michał Zając).

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Feature #18915: New error class: NotImplementedYetError or scope change for NotImplementedYet
https://bugs.ruby-lang.org/issues/18915

* Author: Quintasan (Michał Zając)
* Status: Open
* Priority: Normal
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# Abstract

Introduce `NotImplementedYetError` exception that should be used in case when the codepath has not been implemented by the developer for some reason (maybe they're designing an abstract class or are designing some sort of interface to reuse later on) OR extend the meaning of `NotImplementedYet` to cover those usecases so we don't have to introduce another exception

# Background

`NotImplementedError` is supposed to be raised `if the underlying operating system or Ruby runtime does not support them` (https://ruby-doc.org/core-3.1.2/NotImplementedError.html)

However it appears that many people are misusing this exception by raising this in a superclass from which they later inherit from. I do realize that Ruby promotes duck-typing (the default RuboCop style guide has a cop for this – https://github.com/rubocop/ruby-style-guide#duck-typing). However I have seen this being discussed numerous times:

* https://github.com/rubocop/ruby-style-guide/issues/458
* http://chrisstump.online/2016/03/23/stop-abusing-notimplementederror/
* https://oleg0potapov.medium.com/ruby-notimplementederror-dont-use-it-dff1fd7228e5
* https://gitlab.com/gitlab-org/gitlab/-/issues/354314 (which I'm the author of)
* https://github.com/rmosolgo/graphql-ruby/issues/2067 (here the author actually confused it with Python's `NotImplementedYet`)
* https://stackoverflow.com/questions/13668068/how-to-signal-not-implemented-yet

# Proposal

Create `NotImplementedYetError` exception

OR

Allow raising `NotImplementedError` in cases other than OS or Ruby runtime incompatibilities

# Evaluation

### Add `NotImplementedYetError`

I think a new exception is a better idea than changing the usage of an existing one just because "everyone is using it". That said it would require people to refactor their code which might prevent wider adoption of the new exception.

### Change scope of `NotImplementedYet`

This would require the least amount of changes possible (only a documentation change) and I believe there would be no compatibility problems whatsoever.



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