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Feature #17006

Let `Kernel#Hash` take a block to provide the default value

Added by sawa (Tsuyoshi Sawada) 3 months ago. Updated 3 months ago.

Status:
Open
Priority:
Normal
Assignee:
-
Target version:
-
[ruby-core:99042]

Description

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:

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

A more sophisticated way is this:

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:

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

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