A gem for finding non-exhaustive case statements.

case_match.rb

module CaseMatch
  class Error < StandardError; end

  class Case
    EMPTY_BLOCK = -> {}

    def initialize(obj, cases)
      @obj = obj
      @cases = cases
      @values = []
      @blocks = []
    end

    def when(*values, &block)
      @values |= values
      if values.any? { |v| @obj === v }
        @blocks << block || EMPTY_BLOCK
      end
      self
    end

    def else(&block)
      @values = @cases
      @blocks << block || EMPTY_BLOCK
      self
    end

    def esac
      raise Error if @cases != @values
      @blocks.first.call
    end
    alias :end :esac
  end
end

class User
  def case(attr)
    CaseMatch::Case.new(public_send(attr), statuses.keys)
  end

  # Simulate ActiveRecord::Enum
  def status
    "active"
  end

  def statuses
    { "active" => 0, "archived" => 1 }
  end
end

user = User.new
user.case(:status)
  .when("active") { puts "active" }
  .when("archived") { puts "archived" }
  .end
#=> "active"

user = User.new
user.case(:status)
  .when("active") { puts "active" }
  .when("archived")
  .end
#=> "active"

user = User.new
user.case(:status)
  .when("active") { puts "active" }
  .end
#=> CaseMatch::Error