Transformation Priority Premise

Introduction to TPP

If we go back to how we evolve code in TDD, we have these methods:

Introduction to classic TDD

“Use obvious implementation” is ambiguous, what does “obvious implementation” mean? It may mean something for you and something else for another developer. The transformations on the following table are a way to clarify ambiguity the “obvious implementation”.

#	Transformation	Start code	End code
1	{} -> nil	{}	[return] nil
2	Nil -> constant	[return] nil	[return] “1”
3	Constant -> constant+	[return] “1”	[return] “1” + “2”
4	Constant -> scalar	[return] “1” + “2”	[return] argument
5	Statement -> statements	[return] argument	[return] min(max(0, argument), 10)
6	Unconditional -> conditional	[return] argument	if(condition) [return] argument else [return] 0
7	Scalar -> array	dog	[dog, cat]
8	Array -> container	[dog, cat]	{dog=”DOG”, cat=”CAT”}
9	Statement -> tail recursion	a + b	a + recursion
10	If -> loop	if(condition)	loop(condition)
11	Statement -> recursion	a + recursion	recursion
12	Expression -> function	today – birth	CalculateBirthDate()
13	Variable -> mutation	day	var Day = 10; Day = 11;
14	Case/Switch

Transformations on the top of the list should be preferred to those that are lower. It is better (or simpler) to change a constant into a variable than it is to add an if statement. So when making a test pass, you try to do so with transformations that are simpler (higher on the list) than those that are more complex. Please do not take this table literally. This is a starting point. Adapt this table to your language and environment.

Hands-on: Bowling Kata

Learning objectives

• You should be able to understand and remember the the three methods of moving forward in TDD

• Stick to the rules of TPP

Success criteria