scrabble_spec.rb

module ScoreHelpers
  def the_letter(letter, options)
    score = options[:is_worth]
    # TODO Remember the letter's score for the current example group.
  end
end

module ScoreMatchers
  extend RSpec::Matchers::DSL

  matcher :score do |expected_score|
    def scores
      Hash.new(1) # TODO Return a hash of each letter's score for the current example group.
    end

    def score_for(word)
      scores.values_at(*word.chars).inject(0, :+)
    end

    match do |word|
      score_for(word) == expected_score
    end
  end
end

describe 'Scrabble' do
  extend ScoreHelpers
  include ScoreMatchers

  context 'with the letters D, O and G' do
    the_letter 'D', is_worth: 2
    the_letter 'O', is_worth: 1
    the_letter 'G', is_worth: 2

    specify { expect('DOG').to score 5 }

    context 'with the letter E' do
      the_letter 'E', is_worth: 1

      specify { expect('DOGE').to score 6 }
    end
  end

  context 'with the letters C, A and T' do
    the_letter 'C', is_worth: 3
    the_letter 'A', is_worth: 1
    the_letter 'T', is_worth: 1

    specify { expect('CAT').to score 5 }

    context 'with C on a triple letter score' do
      the_letter 'C', is_worth: 9

      specify { expect('CAT').to score 11 }
    end
  end
end

@h-lame had a clever answer: just wrap let and before, so that RSpec deals with sharing and inheritance.