You are (probably) already doing TDD

October 4, 2013

In a previous post I listed three arguments in favor of this rather bold statement:

Test Driven Design is always the best technique for software development.

I plan to explore all three arguments by expressing each as a syllogism, and then analyzing the syllogism. This post will explore the third argument:

We are already practicing Test Driven Design, so why not replace ourselves with a small shell script?

Defining the syllogism##

As I explored in an earlier post, syllogisms are often a useful tool to analyze a statement or to guide an argument in a productive direction. For this argument, I have written the following syllogism:

We verify changes to the code via ad-hoc tests,

Test Driven Design is more productive than ad-hoc testing,

therefore, Test Driven Design is a more productive way to verify changes to the code.


To analyze a syllogism, we first need to agree on the definition of the terms used in the syllogism. Here are my definitions.

  • Verify changes to the code: The action of proving to a developer that a given change to the code has the desired impact on the run-time behavior.
  • Ad-hoc tests: Non-automated verification of some run-time state, via visual inspection in the UI or the debugger
  • Test Driven Design: The process of
    • Accessing publicly exposed APIs, to write the minimum unit test code to cause a unit test to fail
    • Writing the minimum production code to cause the unit test to pass
    • Iteratively repeating the first two steps to generalize the code


In order for a syllogism to be valid, we must agree that the premises are true. I’ll list the two premises separately, and state my justification with each of them.

Major premise###

We verify changes to the code via ad-hoc tests.

Here the major term is “verify changes to the code” and the middle term is “ad-hoc tests”. The vast majority of developers I have met never commit changes to production code when they are unsure of the run-time behavior caused by those changes. Prior to exposing changes to our client, we always do some verification.

Minor premise###

Test Driven Design is more productive than ad-hoc testing.

Here the minor term is “Test Driven Design” and the middle term is “ad-hoc testing”. The increased productivity of Test Driven Design comes for a few reasons:

  • Test Driven Design captures the effort applied to testing and allows it to be used over and over.
    • One way to measure the cost of writing a test is to divide the time to write the test by the number of times it is executed.
    • Each time a test is executed, the cost of writing the test decreases.
  • Test Driven Design works at the public API level, so it is more specific than ad-hoc testing.
    • All of the test cases can be more clearly determined.
    • The contract with the component (including semantics) can be verified.
  • Test Driven Design externalizes the effort applied to testing and allows others to use it.
    • It allows others to change code with confidence.
    • It exposes details about contract for a component to the clients of the component.


The conclusion of the syllogism is this statement: “Test Driven Design is a more productive way to verify changes to the code.” In this simple syllogism, the minor term becomes the subject of the conclusion and the major term becomes the predicate.

How to disagree##

As with any syllogism, an argument against this conclusion has three grounds for disagreement:

  • The terms are not defined correctly
  • The premises are not true
  • The logic is not valid

For example, I may have defined the terms too narrowly. Maybe a broader definition of some term will render the conclusion less useful, or you may not believe the truth of the major or the minor premise.

When should we avoid using Test Driven Design?##

Since Test Driven Design often has a higher initial cost than other software development techniques, I believe that we need to determine when this analysis indicates that it should not be used. Based on this analysis, we should not use Test Driven Design when:

  • We do not want to capture the effort used to verify our changes.
    • The code will not change in the future.
    • We are attempting to verify unchanging, working legacy code.
  • We do not need to verify the contract for a public API.

Thanks to a number of my colleagues at ANSYS for their input on this topic.

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