[ruby-core:95513] [Ruby master Feature#16274] Transform hash keys by a hash

[shevegen@gmail.com]

Issue #16274 has been updated by shevegen (Robert A. Heiler).


Personally I have had a need to transform keys (and values) in a hash quite a bit. We
also have strange thingies such as HashWithIndifferentAccess so that may be indicative
of people wondering about strings/symbols as keys for a hash in general. :)

To sawa's suggestion: if the choice is solely between option 1 and option 2,
I personally would favour 1, mostly because I only rarely use slice in 
general (primarily I may use slice in regards to String, but even then I tend
to prefer e. g. [] start, end positions normally, even if it may not be the
same, such as if we include .slice!() use cases). But this may differ on
an individual choice, by different ruby users, so I can not generalize it;
it is only an opinion if the choice is purely between #1 and #2 alone.

I do not know how frequent the use case may be, but ignoring this for the
moment I think this suggestion may be useful.

----------------------------------------
Feature #16274: Transform hash keys by a hash
https://bugs.ruby-lang.org/issues/16274#change-82289

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
We have `Hash#transform_keys` and its bang version to change the keys of a hash, but that requires passing a block, which assumes that the mapping from the old keys to the new keys follows some rule. But in reality, we frequently want to change the keys where it is difficult to provide a rule. For example, suppose we have:

```
hash = {created: 2019-10-23 17:54:46 +0900, updated: 2019-10-23 17:59:18 +0900, author: "foo"}
```

and want to achieve:

```
{created_at: 2019-10-23 17:54:46 +0900, update_time: 2019-10-23 17:59:18 +0900, author: "foo"}
```


I request an option to change the keys of a hash not by giving a block, but by passing a hash. I came up with two options.

### 1. Argument for `Hash#transform_keys` and its bang version

Allow `Hash#transform_keys` to optionally take a hash argument instead of a block.

```
hash.transform_keys({created: :created_at, updated: :update_time})
# => {created_at: 2019-10-23 17:54:46 +0900, update_time: 2019-10-23 17:59:18 +0900, author: "foo"}
```

### 2. Argument for `Hash#slice` and the counterparts in other classes

Since `Hash#slice` is often the first step of modifying a hash into some other hash form, it makes sense to let it take an optional hash argument.

```
hash.slice(:created, :author, transform_keys: {created: :created_at})
# => {created_at: 2019-10-23 17:54:46 +0900, author: "foo"}
```


With option 1, it could make sense to even allow a hash argument and a block simultaneously:

```
hash.transform_keys({created: :created_at, updated: :update_time}, &:to_s)
# => {"created_at" => 2019-10-23 17:54:46 +0900, "update_time" => 2019-10-23 17:59:18 +0900, "author" => "foo"}
```




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