Veit's Blog

Carp Patterns II: Testing


In the second installment of my series on my personal view on current best practices in Carp, we’re going to talk about testing! This is one of my favorite topics, right after documentation.

The standard library of Carp comes with a testing framework that is good enough for the kind of unit tests that I write, although it is fairly spartan. Nonetheless, it will give you a good foundation on which to base your tests and make sure that the hardest part of your CI process is getting Carp installed on the CI server.

Baby’s first assertion

If you use the Carp project template on carpentry you already get a test file in test/tests.carp. It will contain one failing test:

(load "Test.carp")
(use Test)

(deftest test
  (assert-equal test
                "Implement me"
                "TODO: implement me"
Fig. 1: A simple failing test.

All flavors of assert from Test will take a state as the first argument, which will be created by the deftest macro. Then they take the things to compare or assert against—usually one, in the case of for instance assert-true, or two, in the case of comparisons—, and finally they take a message to display for the test. If we run the test script using carp -x test/tests.carp, we will get colorful output, telling us which tests pass and fail.

Often this is enough for my unit testing needs, especially in simple functional libraries. If I use assert-exit or assert-signal, I can even test exit codes and signal conditions with it.

Structuring your tests

I like putting all of my tests together, as a large battery. As the number of tests increases and their scope broadens this might get unwieldy. I have two go-to methods to rectify this.

If I want to split up my tests by module, I usually put the tests in different files. This ensures that I can easily trigger just the tests that target the module that I’m currently working on.

Inside these files, I can split up my test functions into multiple groups. I move from using deftest to with-test, like this:

; sorry for the long code example;
; i wanted to share how to split up a lot of code

(defn test-parsing []
    (println "## Parsing")
    (with-test test
      (assert-equal test
                    (scheme &(Uri.parse "scheme://domain.tld"))
                    "we can parse the domain"

(defn test-stringification []
    (println "## Stringification")
    (with-test test
      (assert-equal test
                    &(str &(Uri.parse "scheme://domain.tld"))
                    "we can stringify our representation again correctly"

(defn main []
  (Array.sum &[
Fig. 2: Testing a URI library.

Basically, you have a bunch of functions that run tests, and maybe the print out information about the test name in the beginning. But what is the Array.sum in main doing? I’ve found it to be a neat trick to set the exit status of failing test suites appropriately: since with-test returns the number of tests that have failed, summing the results of all of the test functions together will result in a non-zero exit code if at least one of the test cases failed, making sure CI notices.

Realistically, writing any of this by hand should not be necessary, but the Test module hasn’t been touched in a while, and noone has come up with a nicer API. I guess a macro that generates a bunch of the repetitive code in the test functions and another one that gathers all of the test functions by looking for the prefix test- could be enough for me personally, but I’ll leave this as an exercise for the reader.

A note on running your tests

Finally, I’ll leave you with a pattern that some in the community use to speed up their development workflow: they use file watchers to recompile and run the tests when a Carp file changes. If you have nodemon installed, for instance, you can use this spell to conjure up such a watcher process:

nodemon -e carp -x "carp -x --log-memory tests.carp || exit 1"

Fig. 3: Executing your tests every time a file changes.

There are many such file watchers around, so you can just use whichever you prefer.

An Unfinished Story

That’s all you need to get started writing tests for your Carp projects. The kinds of tests that you can write with any sort of framework are limited to unit tests, but integration tests and end-to-end tests are definitely possible with a little bit of elbow grease and possibly shell scripting.

In Carp proper, we have a few shell scripts that run Carp programs that are supposed to generate error messages, and compare the output to an expected file using diff. That’s simple, low maintenance, and does not require new contributors to install any software. If I had to do something like that for my other Carp projects, I would probably copy a page out of the compiler’s book.

Nonetheless, there are a lot of opportunities to make the testing story in Carp better, even with little effort. Try it!


This second installment of my series on patterns for Carp development again didn’t have much to do with actually writing code, but don’t fret! Next time we’re going to take a look at macros: how to write good, idiomatic, maintainable code that uses Carp’s powerful compile time system to the fullest!

Stay tuned!