Tavish Armstrong


Programming as argument

Sat 09 May 2015 14:26:00

Many people before me have argued that computer programs are secondarily about making a machine perform a task and primarily about telling other humans what you want the computer to do. Pseudocode takes this to the extreme – humans can discuss what a pseudocode program would do and whether or not it would be a good idea, without ever running the code on a computer. The pseudocode program is an argument – it argues that a program could be constructed to accomplish the task at hand. The problem with pseudocode is not that it doesn’t run on a computer. The problem with pseudocode is that it has no way of convincing you that it would work if it did run on a computer.

The reason why a pseudocode program of any respectable size is not a convincing argument is probably obvious to anyone who writes software for a living: it’s easy to write code that looks plausible to other humans, but unless you run the program, it’s hard to believe that it will actually work. Software that is easy to run is more convincing because you can watch it work. You provide inputs, it provides outputs. You can do this to prove or disprove hypotheses. In this case, though, the code stops being an argument that a program could be constructed to perform a task (as is the case with pseudocode). Instead, it becomes an argument that the compiled program will perform the task correctly. Testing the program manually is one way of convincing yourself that it will do that. But it is only one way.

An automated testing framework can be useful for making your code more convincing. You can show that given this input and that input, the program will produce the correct output. Other programmers can run the tests and feel more confident that the program will behave that way.

A few arguments are implicit in every test: these fake inputs are representative of real inputs. This is an accurate simulation of global state. These expected outputs are the correct outputs. When you use mocks or stubs in your tests, you argue that the dependencies you are simulating actually behave that way; a common mistake is over-mocking dependencies so much that you have to just trust that the code will work when connected to real dependencies. For example, if your tests don’t open a socket to a functioning server, can you really be sure that the code will do so correctly in production? If you can’t, you have to trust that the networking code works. The argument becomes “if we assume the networking code exists, the following code will write this message over the socket.” That if might be a very big if.

Code review makes this argumentation-model-of-programming more obvious. If you write useless tests, a reviewer will often tell you that they are terrible tests. The point they are making is not that your code doesn’t work. The point is that they have little reason to believe it will. They have not been convinced by your argument.

If code is an argument, what does that mean for working programmers? Well, it means that the rules of rhetoric apply when writing software. If you want your code to be merged, or you want your project to be adopted, you need it to be persuasive. This raises an important question: is persuasive code the same thing as good code?