From: "ioquatix (Samuel Williams)" <noreply@...>
Date: 2022-10-17T06:59:39+00:00
Subject: [ruby-core:110345] [Ruby master Feature#19063] Hash.new with non-value objects should be less confusing

Issue #19063 has been updated by ioquatix (Samuel Williams).


@sawa I appreciate your technical knowledge and insight.

However... the practical reality is the vast majority of the users are using this interface incorrectly. This might imply that the theoretical basis for how `Hash.new(default_value)` and `Array.new(count, default_value)` work, is impractical and not what users expect.

There are different ways you can frame this, all equally valid from a technical point of view. However, the current framing of the problem is causing real problems in production code.

So we need to propose how to

1. introduce a safer interface (`dup` or `assign` style options, `{}` constructor with 0-arity), and
2. inform existing users of the problem (warning, deprecation, etc),

Your proposal can help with (1), but I'm not sure it can help with (2). The question is, are there valid use cases for `Hash.new(default_value)` and/or `Array.new(count, default_value)` where the user expects mutable values to be shared? I have not yet seen a single compelling use case.

----------------------------------------
Feature #19063: Hash.new with non-value objects should be less confusing
https://bugs.ruby-lang.org/issues/19063#change-99636

* Author: baweaver (Brandon Weaver)
* Status: Open
* Priority: Normal
----------------------------------------
Related to #10713 and #2764.

Ruby's `Hash.new` accepts either a block or a param for its default value. In the case of non-value objects this leads to unexpected behaviors:

```ruby
bad_hash_with_array_values = Hash.new([])
good_hash_with_array_values = Hash.new { |h, k| h[k] = [] }
```

While, as @hsbt has said in the past, this is behaving as intended for the Ruby language it has caused a lot of confusion in the community over the years and is a known sharp-edge.

My assertion is that this is not the intended behavior, and I cannot find a legitimate usecase in which someone intends for this to happen. More often new users to Ruby are confused by this behavior and spend a lot of time debugging.

We must consider the impact to Ruby users, despite what the intent of the language is, and make the language more clear where possible.

Given that, I have a few potential proposals for Ruby committers.

### Proposal 1: Do What They Meant

When people use `Hash.new([])` they mean `Hash.new { |h, k| h[k] = [] }`. Can we make that the case that if you pass a mutable or non-value object that the behavior will be as intended using `dup` or other techniques?

When used in the above incorrect way it is likely if not always broken code.

### Proposal 2: Warn About Unexpected Behavior

As mentioned above, I do not believe there are legitimate usages of `Hash.new([])`, and it is a known bug to many users as they do not intend for that behavior. It may be worthwhile to warn people if they do use it.

### Proposal 3: Require Frozen or Values

This is more breaking than the above, but it may make sense to require any value passed to `Hash.new` to either be `frozen` or a value object (e.g. `1` or `true`)

## Updating RuboCop

Failing this, I am considering advocating for RuboCop and similar linters to warn people against this behavior as it is not intended in most to all cases:

https://github.com/rubocop/rubocop/issues/11013

...but as @ioquatix has mentioned on the issue it would make more sense to fix Ruby rather than put a patch on top of it. I would be inclined to agree with his assessment, and would rather fix this at a language level as it is a known point of confusion.

## Final Thoughts

I would ask that maintainers consider the confusion that this has caused in the community, rather than asserting this "works as intended." It does work as intended, but the intended functionality can make Ruby more difficult for beginners. We should keep this in mind.



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