[stcredzero]

Once a function is ready you are not supposed to change it much. If you do you should change the inner workings and not the input/output.

This presupposes that you will always come up with an adequate design, and that you won't have to refactor this library of functions. For large enough systems, this is almost never the case. For sizeable systems in production, you will always come up against things you haven't thought of before.

Granted, proper factoring of your functions in the first place is going to help a lot. (Small composable functions.) However, if you claim that you can code such that you never ever have to change your mind about function signatures, never have to propagate changes across the system, even in large production systems, then you're either not seeing something, or you have some technique which you should be selling or basing a consulting practice around. (And/or, maybe I have something new to learn.)

EDIT: I have written projects in Clojure, including a multiplayer game server. This is the experience I'm basing this comment on.

[ignorabilis]

In the previous post I started with in theory. What this means is that theoretically things can go like this and in practice they have gone for me until now (with Clojure, so +1 for you too :)). It doesn't mean that it will work for everybody or even for me in the future.

Then: Granted, proper factoring of your functions in the first place is going to help a lot. hence -> Once a function is ready you are not supposed to change it much. much not= at all.

And you are absolutely correct - eventually you will have to change some of your functions. And when you need to do so you should focus on the logic and take your time until you understand it completely. Only then refactor the funciton. So the emphasis should be on the developer completely understanding the task and not on the developer finding a cheap way to change the code and rely on tests to catch his errors.

Furthermore if you change the signature of a function, tests are just another place where you need to change the function as well. In practice the tests that you wrote are not doing you any good because you cannot test the reinvented function with the old set of tests.

Sure, tests might catch a few errors here and there at some point. But after all what is more time consuming - maintaining hundreds of thousands of tests or focusing on your single, small, composable function and your problem?

I am part of a team that develops a very large .NET (with C#) product with hundreds of projects in the solution. So far the primary use of tests is to be maintained. They caught a few bugs, true, but nothing that our QAs were not going to see anyway. In imperative OOP land tests are a necessary evil, but with time I started doubting even that thesis.

[stcredzero]

In the previous post I started with in theory.

One thing I've found is that what would help tremendously with large production projects results from epiphenomena in code, and is sometimes not what is sexy from a language design standpoint.

I am part of a team that develops a very large .NET (with C#) product with hundreds of projects in the solution. So far the primary use of tests is to be maintained. They caught a few bugs, true, but nothing that our QAs were not going to see anyway.

I was a fly on the wall when Extreme Programming was being formulated in Smalltalk. (Not part of the Chrysler C3 project, but I was working for a Smalltalk vendor and heard about what was going on.) Test Driven Development, or at least good Unit Test coverage, is essential for an agile style project in a language like Smalltalk. I'm not so sure unit tests are as essential for languages like C#. TDD does produce more testable code, but it has considerable "cultural overhead."

Two important questions to ask are: What aids refactoring? What stops refactoring? The answers to these questions are also different for large codebases as opposed to small ones. I have noticed that Swift's enums are tremendously useful in this regard, but they are not "sexy" so this doesn't get discussed very much.

[AnimalMuppet]

> Two important questions to ask are: What aids refactoring? What stops refactoring? The answers to these questions are also different for large codebases as opposed to small ones.

Part of the point of thorough unit tests that really test all the external behaviors of all your classes (or functions, if you're doing FP) is that it can give you the courage to refactor. If all the tests run for this class, then I didn't change any external behavior of the class. If the whole project's unit tests all pass, then I can be highly confident that my change did not introduce any bugs.

Note well the condition, though: if the tests really test all the external behaviors of all the classes. For that to be true, you almost have start with TDD from the beginning of the project.

[ignorabilis]

As long as there is some sort of logic tests can be written. The question is can we prevent writing tests at all by designing better our applications and by taking advantage of FP languages and tools like the REPL? And I believe at least in theory we can. In practice life happens so this theory is certainly not applicable to every case on the planet.

When things are decoupled refactoring is not that dangerous. If all pieces of your program are small, intelligible functions you can always just refactor a single, simple function and you should not need tests to rely on - all you have to do is make sure that this function is working like it worked before the refactoring (or better).

[Dewie3]

> I have noticed that Swift's enums are tremendously useful in this regard, but they are not "sexy" so this doesn't get discussed very much.

Not sexy to whom? Algebraic data types are quite sexy right now, I'd say.