Issue #16183 has been updated by Eregon (Benoit Daloze).


zverok (Victor Shepelev) wrote:
> So, I propose side-effect-less method, that **is** acceptable for chaining, I believe.

That means a copy of the Hash is necessary on each call to #with_default.
I.e., it would be the same as:
```ruby
h = {a: 1, b: 2}
h2 = h.dup.tap { _1.default = 3 }
```

I think it's better to keep a potentially expensive copy of the Hash explicit with the `.dup` call + `default=`.
But I agree there is some beauty to change the default value in a safe, non-mutable way with a single method call.


----------------------------------------
Feature #16183: Hash#with_default
https://bugs.ruby-lang.org/issues/16183#change-81754

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Reasons: there is no way, currently, to *declaratively* define Hash with default value (for example, to store it in constant, or use in an expression). Which leads to code more or less like this:


```ruby
FONTS = {
  title: 'Arial',
  body: 'Times New Roman',
  blockquote: 'Tahoma'
}.tap { |h| h.default = 'Courier' }.freeze

# Grouping indexes:
ary.each_with_object(Hash.new { |h, k| h[k] = [] }).with_index { |(el, h), idx| h[el.downcase] << idx }
```

With proposed method:
```ruby
FONTS = {
  title: 'Arial',
  body: 'Times New Roman',
  blockquote: 'Tahoma'
}.with_default('Courier').freeze

ary.each_with_object({}.with_default { [] }).with_index { |(el, h), idx| h[el.downcase] << idx }
```

About the block synopsys: I am not 100% sure, but I believe that _most_ of the time when `default_proc` provided, it looks like `{ |h, k| h[k] = some_calculation }`. So, I believe for this "declarative simplification" of defaults, it is acceptable to assume it as the only behavior (pass only key to block, and always store block's result); more flexible form would still be accessible with `Hash.new`.



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