[ruby-core:95596] [Ruby master Feature#16275] Revert `.:` syntax

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Issue #16275 has been updated by alanwu (Alan Wu).


`.:` has special power in that one cannot change its semantics by redefining a
method. It other words, it's a fundamental operation in the language like
`class << object` and `def object.method_name; end`. I think a fundamental
operation should only be introduced if there is a good supporting ecosystem for
it in the language.

I see reverting as a move to take a step back and do more design thinking about
functional programming and how it fits in with the rest of the language. With a
good ecosystem behind it, `.:` could be much more powerful than just a
shortcut.

On the other hand, keeping `.:` could be a way to crowd-source the design
process. @zverok already has some tickets that depend on `.:` for
expanding the FP part of Ruby. If `.:` hits a mainline release, more people
could come forward with ideas to gradually make `.:` less orphan and more
powerful. In the worst case scenario, no new paradigm is explored and we are
left with a syntactic sugar for an unpopular operation, I don't know if that is
an okay risk.

When it comes to primitive operations, less is more, so I think we should
revert for now.



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Feature #16275: Revert `.:` syntax
https://bugs.ruby-lang.org/issues/16275#change-82386

* Author: naruse (Yui NARUSE)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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`obj.:method` is introduced at r66667 by #12125 and #13581.
It encourages the functional programming style in Ruby.

But this shorthand syntax is just for methods of `self` without arguments.
It causes another feature requests like #16273 (and lambda compositions like #15428).

Such features will introduce a new view of Ruby but no one illustrates the whole picture yet.
I worried about such patch work may cause a conflict with future expansion of functional programing style or a just a garbage feature.

`.:` syntax is introduced in 2.7.0 preview1, not released in production yet.
How about reverting at this time and re-introduce with a big picture.



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