Feature #17006: Let `Kernel#Hash` take a block to provide the default value

• Status: Open
• Assignee: -
• Target version: -

[sawa (Tsuyoshi Sawada)]

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}"}

[sawa (Tsuyoshi Sawada)]

• Description updated (diff)

[nobu (Nobuyoshi Nakada)]

The description mentions the default value first, but the proposal will solve the default proc only.

[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:

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

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

# frozen_string_literal: true

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