let_is_leaking_to_parallel_example_spec.rb

# Do you expect this spec to fail? If not - give it a try:

#   mkdir -p spec &&
#      curl # "paste in the raw url here" > spec/bad_let_spec.rb &&
#      rspec spec/bad_let_spec.rb

# A hint: this spec can be turned into green
# - without changing any code in the context sections
# - without changing any let (in case you have the same bad feeling about a `let` being
#   initialized from an `@variable` from a `before(:all)`)

# "Solution" below

describe "..." do
  before(:all) {
    @a = "before all"
    a
  }

  let(:a) {@a}

  context "context 1" do
    let(:b) {"context 1"}

    it "..." do
      b.should == "context 1"
      a.should == "before all"
    end
  end

  context "context 2" do
    let(:b) {"context 2"}

    it "..." do
      b.should == "context 2"
      a.should == "before all"
    end
  end
end

# This would already turn the spec into green:
#
#
#
#
#
#
#
#
# <   @a = "before all"
# >   a = @a = "before all"

# This will defer the let(:a) to be executed not before the "example run". And somehow, the example let will then not leak.