chris blogs

November 2005

06nov2005 · DSLs for TDD and BDD

At current, it is popular to invent domain specific languages for Test Driven Development and Behavior Driven Development.

Of course, I had to give that a try too. I don’t have any code for my DSL yet, it doesn’t even have a real name either. For comparision, let’s have a look at a sample of Test::Unit, taken from the Ruby Test First Challenge:

require 'test/unit'
require 'SimpleSpread'

class TestSpread < Test::Unit::TestCase
  def test_that_cells_are_empty_by_default
    sheet = Sheet.new()
    assert_equal("", sheet.get("A1"))
    assert_equal("", sheet.get("ZX347"))
  end

  def test_that_text_cells_are_stored
    sheet = Sheet.new()
    a1 = "A string"
    sheet.put("A1", a1)
    assert_equal(a1, sheet.get("A1"))
    sheet.put("A1", "foo")
    assert_equal("foo", sheet.get("A1"))
    sheet.put("A1", "")
    assert_equal("", sheet.get("A1"))
  end
end

If you don’t understand this, you probably are wrong here; it’s standard Test::Unit usage.

Have a look at the structure of that test case. Tests are grouped into test cases that inherit from Test::Unit::TestCase. Then, you define methods, the actual tests (which have a name) inside. All these methods start with test_. There are two special methods setup and teardown that are run before and after the test. Every test has actions and assertions. If assertions fail, the test ends and Test::Unit will report it to have failed.

  • testcase: TestSpread
    • test: test_that_cells_are_empty_by_default
      • setup
      • assertion
      • assertion
    • test: test_that_text_cells_are_stored
      • setup
      • action
      • assertion
      • action
      • assertion
      • action
      • assertion

Fast forward to my testing language, for now called Desire. Here is the code:

require 'desire'
require 'SimpleSpread'

tag Sheet, :interface do
  given { @sheet = Sheet.new }.expect {
    cells_are_empty_by_default {
      doing { @sheet.get("A1") }.
      results_in ""

      doing { @sheet.get("ZX347") }.
      results_in ""
    }

    text_cells_are_stored {
      a1 = "A string"
      doing { @sheet.put "A1", a1 }.
      results_in { a1 == @sheet.get("A1") }

      doing { @sheet.put "A1", "foo" }.
      results_in { "foo" == @sheet.get("A1") }

      doing { @sheet.put "A1", "" }.
      results_in { "" == @sheet.get("A1") }
    }
  }
end

In Desire, things work differently. First, note that you simply can read out the source and it will make sense. You don’t need to create a class to hold tests, selection purely works by tagging tests. Tags can be any Ruby object, in above case we use a class and a symbol to tag the following tests. To the reader of the testcase, this means: We test related to the class Sheet and the interface (because the test is independent from the implementation).

After this, we declare a basic situation—a common ground—we wish to use for our tests. Here, it consists of initializing an instance variable (more about that later) to a new instance of Sheet.

Then, an expect block follows. All tests in Desire are declared inside expect blocks. Here, we declare five(!) tests, which are grouped using two descriptions. To define a test, you use two methods, doing and results_in. doing calls the block and saves its return value. With results_in, you either can check against a predefined value (by passing it as argument) or by providing a check on your own (by passing a block that should return true).

Every pair of doing/results_in defines a new test, which is run despite of the outcome of the other tests. Therefore, we have this structure:

  • tagged: Sheet, :interface
    • setup
    • description: cells_are_empty_by_default
      • test
        • action
        • assertion
      • test
        • action
        • assertion
    • description: text_cells_are_stored
      • test
        • action
        • assertion
      • test
        • action
        • assertion
      • test
        • action
        • assertion

This is, at least in my humble opinion, far clearer and more structured than the former one.

If I get around to actually implement Desire, I could well imagine basing it on Test::Unit, though. It’s a solid base and easy enough to be used with metaprogramming.

Problems left open for now are the use of instance variables to communicate the common ground with the tests and minor syntactic issues.

Other ideas for defining assumptions:

doing { File.open "feeble" }.
raises Errno::ENOENT

doing { a }.
or { b }.
or { c }.
results_in common_result  # or: results_in_same

I’m curious what others think about it.

NP: Dan Bern—Five Nothing Lead

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