TDD: avoid getting stuck or what’s the next test?

One central point of practicing TDD is to determine what is the next test. Choosing the wrong path can lead you into the infamous impasse

One central point of practicing TDD is to determine what is the next test. Choosing the wrong path can lead you into the infamous impasse: to make the next test pass you need to make not baby but giant steps. Some time ago Uncle Bob introduced a principle called the transformation priority premise. To make a test pass you need to change the implementation. These changes are transformations. There are at least the following transformations (taken from his blog post):

  • ({}–>nil) no code at all->code that employs nil
  • (nil->constant)
  • (constant->constant+) a simple constant to a more complex constant
  • (constant->scalar) replacing a constant with a variable or an argument
  • (statement->statements) adding more unconditional statements.
  • (unconditional->if) splitting the execution path
  • (scalar->array)
  • (array->container)
  • (statement->recursion)
  • (if->while)
  • (expression->function) replacing an expression with a function or algorithm
  • (variable->assignment) replacing the value of a variable.

To determine what the next test should be you look at the possible next tests and the changes in the implementation necessary to make that test pass. The required transformations should be as high in the list as possible. If you always choose the test which causes the highest transformations you avoid getting stuck, the impasse.
This seems to work but I think this is pretty complicated and expensive. Shouldn’t there be an easier way?
Let’s take a look at his case study: the word wrap kata. Word wrap is a function which takes two parameters: a string, and a column number. It returns the string, but with line breaks inserted at just the right places to make sure that no line is longer than the column number. You try to break lines at word boundaries.
The first three tests (nil, empty string and one word which is shorter than the wrap position) are obvious and easy but the next test can lead to an impasse:

public void twoWordsLongerThanLimitShouldWrap() throws Exception {
  assertThat(wrap("word word", 6), is("word\nword"));

With the transformation priority premise you can “calculate” that this is the wrong test and another one is simpler meaning needs transformations higher in the list. But let me introduce another concept: the facets or dimensions of tests.
Each test in a TDD session tests another facet of your problem. And only one more. What a facet is is determined by the problem domain. So you need some domain knowledge but usually to solve that problem you need this nevertheless. Back to the word wrap example: what is a facet? The first test tests the nil input, it changes one facet. The empty input test changes another facet. Then comes one word shorter than the wrap position (one facet changed again) and the fourth test uses two words longer than the wrap position. See it? The fourth tests introduces changes in two facets: one word to two word and shorter to longer than. So what can you do instead? Just change one facet. According to this the next test would be to use one word longer than the wrap position (facet: longer) which is proposed as a solution. Or you can use two words shorter than the wrap position (facet: word count) but this test will just pass without modifications to the implementation code. So facets of the word wrap kata could be: word count, shorter/longer, number of breaks, break position.
I know this is a very informal way of finding the next tests. It leans on your experience and domain knowledge. But I think it is less expensive than the transformations. And even better it can be combined with the transformation priority premise to check and verify your decisions.
What are you experiences with getting stuck in TDD? Do you think the proposed facets of TDD could be of help? Is it too informal? Too vague?

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