let vs def

1_let.rb

desc "A user's comment" do
  let(:user) { User.create! name: "John" }
  let(:comment) { user.comments.create! }

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

2_def.rb

desc "A user's comment" do
  def user
    @user ||= User.create! name: "John"
  end

  def comment
    @comment ||= user.comments.create!
  end

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

3_def_condensed.rb

desc "A user's comment" do
  def user; @user ||= User.create! name: "John"; end
  def comment; @comment ||= user.comments.create!; end

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

4_motivation.rb

# the original motivation for `let` was to make this scenario easier:

# step 1
describe Thing do
  it "does something" do
    thing = Thing.new
    thing.should do_something
  end
end

# step 2 - add another example
describe Thing do
  it "does something" do
    thing = Thing.new
    thing.should do_something
  end

  it "does something else" do
    thing = Thing.new
    thing.should do_something_else
  end
end

# step 3 - refactor - without let

# 3a1
describe Thing do
  before do
    @thing = Thing.new
  end

  it "does something" do
    thing = Thing.new
    thing.should do_something
  end

  it "does something else" do
    thing = Thing.new
    thing.should do_something_else
  end
end

# 3a2
describe Thing do
  before do
    @thing = Thing.new
  end

  it "does something" do
    thing.should do_something
  end

  it "does something else" do
    thing = Thing.new
    thing.should do_something_else
  end
end

# 3a3
describe Thing do
  before do
    @thing = Thing.new
  end

  it "does something" do
    @thing.should do_something
  end

  it "does something else" do
    thing = Thing.new
    thing.should do_something_else
  end
end

# 3a4
describe Thing do
  before do
    @thing = Thing.new
  end

  it "does something" do
    @thing.should do_something
  end

  it "does something else" do
    thing.should do_something_else
  end
end

# 3a5
describe Thing do
  before do
    @thing = Thing.new
  end

  it "does something" do
    @thing.should do_something
  end

  it "does something else" do
    @thing.should do_something_else
  end
end

# and now with let - fewer steps, no adding @ signs

# 3b1
describe Thing do
  let(:thing) { Thing.new }

  it "does something" do
    thing = Thing.new
    thing.should do_something
  end

  it "does something else" do
    thing = Thing.new
    thing.should do_something_else
  end
end

# 3b2
describe Thing do
  let(:thing) { Thing.new }

  it "does something" do
    thing.should do_something
  end

  it "does something else" do
    thing = Thing.new
    thing.should do_something_else
  end
end

# 3b2
describe Thing do
  let(:thing) { Thing.new }

  it "does something" do
    thing.should do_something
  end

  it "does something else" do
    thing.should do_something_else
  end
end

I find 3a5 the most readable and consistent with the Four-Phase Test. Not sure I understand the gains of the memoization and agree with @tenderlove that the hidden conditional is a little freaky.