[ruby-core:119591] [Ruby master Feature#15381] Let double splat call `to_h` implicitly

["jeremyevans0 (Jeremy Evans) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #15381 has been updated by jeremyevans0 (Jeremy Evans).


There are certainly backwards compatibility issues from changing `**` from calling `to_hash` to `to_h`.  I'm sympathetic to the argument that calling `to_h` fits better, as the conversion is explicit and not implicit, but the backwards compatibility costs are probably too high.  Maybe in Ruby 4?

One possible approach is defining `**@` as an operator method, something like:

```ruby
class BasicObject
  def **@ = to_hash
end
```

Using the unary `**` operator on an object would call the `**@` method, which should return a hash (or raise an exception). The method name itself is designed to be similar to `+@` and `-@`, which are called when you use the unary `+` and `-` operators.  Users could then change the `**@` method to call `to_h` instead of `to_hash` if they want, and get the explicit conversion behavior (either for all objects, or for specific objects/classes).

I proposed `*@` as the method called by the unary `*` operator in #2013, with a working patch, before `**` was introduced for keywords.  This was the first issue I filed, back in 2009.  It was eventually rejected as there was not much discussion on it, but it was still thought an interesting idea at the time it was rejected.

----------------------------------------
Feature #15381: Let double splat call `to_h` implicitly
https://bugs.ruby-lang.org/issues/15381#change-110212

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
----------------------------------------
The single splat calls `to_a` implicitly on the object (if it is not an array already) so that, for example, we have the convenience of writing conditions in an array literal:

```ruby
a = [
  *(:foo if some_condition),
  *(:bar if another_condition),
]
```

And the ampersand implicitly calls `to_proc` on the object (if it is not a proc already) so that we can substitute a block with an ampersand followed by a symbol:

```ruby
some_method(&:some_method_name)
```

Unlike the single splat and ampersand, the double splat does not seem to implicitly call a corresponding method. I propose that the double splat should call `to_h` implicitly on the object if it not already a Hash so that we can, for example, write a condition in a hash literal as follows:

```ruby
h = {
  **({a: 1} if some_condition),
  **({b: 2) if another_condition),
}
```

There may be some other benefits of this feature that I have not noticed yet.



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