Issue #17006 has been updated by sawa (Tsuyoshi Sawada).


nobu (Nobuyoshi Nakada) wrote in #note-2:
> The description mentions the default value first, but the proposal will solve the default proc only.

Default value is a special (static) case of default proc, so I think having the latter would be enough:

```ruby
Hash({a: 1, b: 2}){0}
```

If there is concern for efficiency, at least for strings, we can turn on the frozen string mode.


```ruby
# frozen_string_literal: true

Hash({1 => "a", 2 => "b"}){"c"}
```


----------------------------------------
Feature #17006: Let `Kernel#Hash` take a block to provide the default value
https://bugs.ruby-lang.org/issues/17006#change-86425

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
----------------------------------------
Sometimes, we need to create a hash that has explicit key-value pairs as well as a default value, but there has not been a way to do that at once. The most naive way is to do like this:

```ruby
h = {a: 1, b: 2}
h.default_proc = ->(h, k){raise "Unknown key #{k}"}
h # => ...
```

A more sophisticated way is this:

```ruby
Hash.new{|h, k| raise "Unknown key #{k}"}.merge({a: 1, b: 2})
```

But that is still not short enough, and it also looks backwards, i.e., we usually want to specify the explicit key-value pairs before the default value.

My proposal is to allow `Kernel#Hash` to take a block that provides the default value in the same way as the block of `Hash.new`:

```ruby
Hash({a: 1, b: 2}){|h, k| raise "Unknown key #{k}"}
```




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