[ruby-core:120219] [Ruby master Feature#20818] Allow passing a block to Hash#store (to update current value)

Issue #20818 has been updated by AMomchilov (Alexander Momchilov).


This is related to my `Hash#exchange_value` proposal, so I'll link that here. https://bugs.ruby-lang.org/issues/20300

I guess the difference is whether you already know the replacement you want to use instead, or if you'd like to compute it lazily when the key is already set.

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Feature #20818: Allow passing a block to Hash#store (to update current value)
https://bugs.ruby-lang.org/issues/20818#change-110983

* Author: furunkel (Julian Aron Prenner)
* Status: Open
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I would like to propose a block form for `Hash#store`. In addition to passing a value, it should also be allowed to pass a block.
If passed a block instead of a value, the block is called with the current value or, if unset, the hash's default value; the block's return value will be the value that is stored.
This is similar to e.g., `computeIfPresent`/`computeIfAbsent` in Java (I think).

I can think of several situations where this would be useful, in particular for caches and counters.
For instance:
```ruby
counts = {}
elements.each do |element|
  counts.store(element){ (_1 || 0) + element.weight}
end
```

or even more elegant with a default value:
```ruby
counts = {}
counts.default = 0
elements.each do |element|
  counts.store(element){ _1 + element.weight}
end
```

Moreover, using the block form we should be able to do operations such as  `h[k] ||= x`, `h[k] -= x`, `h[k] += x`, or more generally `h[k] = f(h[k])`, with a single "hashing round-trip".
 If I'm not mistaken, currently these involve two separate calls to `#[]` and `#[]=` (with two calls to `#hash`?).

Finally, this makes `#store` a proper dual of `#fetch` which, similarly, can be passed a block.

I have an experimental implementation of this (GitHub PR) at: https://github.com/ruby/ruby/pull/11956
 



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