jonathan chow

dynamically creating tests with ScalaTest

» 12 May 2013

At the Code Retreat run at Movio this weekend we watched Corey Haines do the Roman Numerals kata in Ruby. An interesting thing he did was to list all conversion in a hash and iterate over it to dynamically create the test for each conversion:

describe "Converting arabic numbers to roman numerals" do
  {
    1 => "I",
    2 => "II",
    5 => "V"
    # ...
  }.each_pair do |arabic, roman|
    it "converts #{arabic} to #{roman}" do
      expect(convert(arabic)).to eq(roman)
    end
  end
end

Later while attempting to create Conway’s Game of Life using TDD I came across some tests that were repetitive in a similar manner:

describe ("alive cells") {
  val cell = Cell(Alive)
  it ("should become Dead when there are 0 live neighbours") {
    cell.next(0) should be (Cell(Dead))
  }
  it ("should become Dead when there are 1 live neighbours") {
    cell.next(1) should be (Cell(Dead))
  }
  it ("should become Alive when there are 2 live neighbours") {
    cell.next(2) should be (Cell(Alive))
  }
  it ("should become Alive when there are 3 live neighbours") {
    cell.next(3) should be (Cell(Alive))
  }
  // ...
}

It turns out that ScalaTest also supports creating tests using the same style:

describe ("alive cells") {
  val cell = Cell(Alive)

  Seq(
    (0, Dead),
    (1, Dead),
    (2, Alive),
    (3, Alive)
    // ...
  ) foreach { case (count, state) =>
    it (s"should become $state when there are $count live neighbours") {
      cell.next(count) should be (Cell(state))
    }
  }
}

It might take some getting used to, but I think it’s a rather nice way to run different inputs for the same test. It lets you isolate each input into its own test case without any code duplication and concisely lists all your test cases and expected outputs together.

You can see how I did Corey’s Roman Numerals kata in Scala (along with other katas I have done/will do) on my GitHub.