def_style_spec.rb

describe SomeClass do
  describe '#some_method' do
    def actual(parameter)
      described_class.some_method(parameter)
    end

    it 'expresses behavior 1' do
      expect(
        actual('base_value')
      ).to eq('expected 1')
    end
    
    it 'expresses behavior 2' do
      expect(
        actual('other_value')
      ).to eq('expected 2')
    end

    it 'expresses behavior 1' do
      expect(
        actual('new_value')
      ).to eq('expected 3')
    end
  end
end

let_style_spec.rb

describe SomeClass do
  describe '#some_method' do
    let(:actual) { described_class.some_method(a_parameter) }
    let(:a_parameter) { 'base_value' }

    describe 'case 1' do
      it 'expresses behavior 1' do
        expect(actual).to eq('expected 1')
      end
    end

    describe 'case 2' do
      let(:a_parameter) { 'other_value' }
      it 'expresses behavior 2' do
        expect(actual).to eq('expected 2')
      end
    end

    describe 'case 3' do
      let(:a_parameter) { 'new_value' }
      it 'expresses behavior 3' do
        expect(actual).to eq('expected 3')
      end
    end
  end
end