package dryunit

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A detection tool for traditional unit testing in OCaml







Dryunit is a small command line tool and a ppx that detects your unit tests and auto-generate the correspondent bootstrap code on the fly in your build directory. As a result, you get to use plain old OCaml and all the tooling you already use.

Published: 12 Oct 2017



Dryunit is a small tool that detects your unit tests and autogenerate the correspondent bootstrapping code on the fly in your build directory.

As a result, you get to use plain old OCaml and all the tooling you already use. It currently works with Alcotest and OUnit and has a template to simplify jbuild integration.


Install the extension and command line in your system:

opam install dryunit

If you use jbuilder, you can use the command dryunit init to generate a template config. For example, the commands below will generate the executable tests/main.exe for tests based on alcotest:

mkdir tests
dryunit init > tests/jbuild

That will make jbuilder generate the file "tests/" in your build directory. To add tests, just write a function with a name stating with "test" in any file inside the tests directory.

About configuration

This is the content of the command dryunit init:

 ((names (main))
  (libraries (alcotest))
  (preprocess (pps (ppx_dryunit)))))

;; This rule generates the bootstrapping
 ((targets (
  ;; Uncomment for change detection:
  ;; (deps (
  (action  (with-stdout-to ${@} (run
    dryunit gen --framework alcotest
    ;; Uncomment to configure:
    ;;  --ignore "space separated list"
    ;;  --filter "space separated list"

It defaults to a configuration for a test executable main.exe based on Alcotest. By default, this file does not need to be created among your test files.

It also shows helpful information on comments, describing how to setup simple changing detection for a list of files, and in the end, how to filter or ignore some tests.

Nuances of changing detection

There two basic ways you can setup change detection:

  1. List all test files in the deps in the tests/jbuild file.

  2. Create locally the file tests/ and make sure it is recompiled at every build.

Listing test files

The upside of listing the files is that it only requires jbuilder to work. You also don't need the maintain the file tests/ among your test files.

Creating locally the file tests/

If you don't want to keep a list of current test files in the configuration, you need to create the file in the same directory your tests live. This file should never be cached - it needs to be recompiled at every build. To make sure jbuilder does that it, there must be a random modification between builds.

Here's a template for a task in the Makefile:

	@dryunit gen --framework alcotest > tests/

test: dryunit

Since this file will change frequently, you should put it in the list of ignored files in your VCS.

How it works

The project has two main components:

  • The command line dryunit: the main user interface, responsible for the configuration and the pluggable workflow.

  • The extension ppx_dryunit: it does all the "heavy lifting", including test detection, caching and Ast rewriting.


The caching is important because the main executable needs to be re generated with some random modification at build time.

More importantly, the detection is done by calling a shell instance to ask OCaml parser to generate a structured representation of each test file. This adds an extra cost and should not be done for unmodified files. That's why the extension will dump the memory representation of the detected test suites in a cache file.

The primary form to detect changes is the timestamp of the test file. This is why by default, you can't detect tests from the file that activates the extension.

The cache is also aware of the version of the compiler used at each build, so executing opam switch doesn't break nor removes existing cache.

How DRY can you be?

The initial idea for the project was to create a nearly invisible test framework that would require no bootstrap code whatsoever. But to be truly dry you shouldn't need to change existing test code, build system nor frameworks.

This is a simplified version of Alcotest's sample test at the time of writing.

module MyLib = struct
  let capit letter = Char.uppercase letter
  let plus int_list = List.fold_left (fun a b -> a + b) 0 int_list

let test_capit () =
  Alcotest.(check char) "same chars"  'A' (Mylib.capit 'a')

let test_plus () =
  Alcotest.(check int) "same ints" 7 ( [1;1;2;3])

All functions starts with test and must be in the same directory of the file used to activate ppx_dryunit. If you want to create it manually, it looks like:

  This file is supposed to be generated before build with a random ID.
  ID = 597588864186
let () =
    { cache_dir = ".dryunit"
    ; cache     = true
    ; framework = "alcotest"
    ; ignore    = ""
    ; filter    = ""
    ; detection = "dir"

If you just need to detect tests from one file, you can skip the command line and add the code above at the end of your main executable, with a slight change: detection = "file".

Under the hood

When processing the extension, the following happens:

  • Dryunit checks if it's running from a *build/* directory, and returns () otherwise.

  • Look at the directory where the extension was declared and find all *.ml files other than the current file.

  • Extract a structured representation of each file, using OCaml's parser. This is fast, but produce boilerplate.

  • Create a test suite with all module-level functions starting with "test".

  • Replace [%dryunit... ] in the AST with the apropriate code to bootstrap Alcotest.

The future of the project

This project is framework independent. It changes the AST, but relies on the user environment to provide and validate the appropriate dependencies. The project itself remains light, even if support for new test frameworks supporting similar workflows is added in the future.

This is project is low maintenance. Thanks to OCaml's parser and the Migrate-Parsetree project, this ppx knows very little about the actual source syntax and already works on all major OCaml versions supporting ppx.

Dependencies (4)

  1. cmdliner >= "1.0.2"
  2. ppx_dryunit
  3. jbuilder >= "1.0+beta7"
  4. ocaml >= "4.02.3" & < "4.06"

Dev Dependencies


Used by