Example of how to deal with Ruby case statements

ruby_case_statements_rase_when_unmatched.rb

# https://twitter.com/josh_cheek/status/1522312126995611651
def UnmatchedCase(...)
  UnmatchedCase.new(...)
end

class UnmatchedCase < RuntimeError
  PENDING = Struct.new(:inspect).new('(PENDING)')

  attr_reader :description, :value
  def initialize(description, value = PENDING)
    @description, @value = description, value
    super "#{description}: #{value.inspect}"
  end

  def ===(case_value)
    initialize description, case_value
    set_backtrace caller.drop(1)
    raise self
  end
end

# Note that the colons in the original tweet did not work for me. I do remember
# something like that in the past, but looking at `parse.y`, it does not seem
# to be a thing: https://github.com/ruby/ruby/blob/v3_1_2/parse.y#L4053
def describe_music(year)
  case year
  when 1970..1979 then 'disco'
  when 1980..1989 then 'eighties'
  when 1990..1999 then 'dance'
  when 2000..     then 'RnB'
  when UnmatchedCase('Unknown musical genre time frame')
  end
end

describe_music 2020 # => "RnB"
describe_music 1994 # => "dance"
describe_music 1983 # => "eighties"
describe_music 1970 # => "disco"
describe_music 1955 # => UnmatchedCase: Unknown musical genre time frame: 1955

# ~> UnmatchedCase
# ~> Unknown musical genre time frame: 1955
# ~>
# ~> program.rb:38:in `<main>'