package ppx_dryunit

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A detection tool for traditional unit testing in OCaml


Dune Dependency






Dryunit is a detection tool for traditional test suites. This is an optional extension that provides similar functionallity as the main package and command line dryunit (which is the recommended way to get started).


opam install ppx_dryunit


It's activated appending this to the end of your tests/

let () = [%dryunit]

Custom definitions are given using a record. All fields are optional and might be in any order.

let () =
    { cache_dir   = ".dryunit"
    ; cache       = true
    ; framework   = "alcotest"
    ; ignore      = ""
    ; filter      = ""
    ; detection   = "file"
    ; ignore_path = "self"

For more information, checkout the repository.

Published: 20 Oct 2017



Dryunit is a tool that allows you to test OCaml code using Convention over Configuration.

Your tests are put first, so TDD can get out of your way. We wanted to get the project right and be dry. That is why the first implementations do not implement a test framework. You are invited to use Alcotest or OUnit for that.

The big advantage of traditional testing over alternatives is that you get to use pure OCaml. Free of enhancements. Using the exact same syntax you do everything else. Your tests are independent but can use anything in their context. It's just OCaml, do whatever way you want.


Conventions are minimal, but necessary. They allow for a good visual distinction when you are interacting with non-test code. They also make configuration simpler.

  • All files containing tests should be either called or

  • All test function names must start with test.

  • By default, test executables are created per directory and are called main. But you do not need to ever see this file.


Install the command line in your system:

opam install dryunit

Dryunit works with jbuilder out of the box:

mkdir tests
dryunit init > tests/jbuild

No other configuration is required. The generated rules will define the executable tests/main.exe ready for the default framework. You can also make the framework explicit by using dryunit init alcotest.


This is the output of the command dryunit init:

 ((names (main))
  (libraries (alcotest))))

 ((targets (
  (deps ( (glob_files * (glob_files * ))
  (action  (with-stdout-to ${@} (run dryunit gen
    --framework alcotest
    ;; --filter "space separated list"
    ;; --ignore "space separated list"
    ;; --ignore-path "space separated list"

As you see, this is the place to customize to customize your test executable. The definitions in the comments provide a template for common filters, but you can find more information about customizations using dryunit help or dryunit COMMAND - - help.

About the extension

This project was originated as a PPX. It turns out this setup introduces the unnecessary preprocess of every test file, is more expensive to setup incremental compilation, and that's not what most users need.

It is still available as the optional package ppx_dryunit. Currently the extension provides roughly the same functionality as the command line, plus the possibility to detect tests only in the current file, which is its recommended setup.

The simplest way to use it is adding this line at the end of your file

let () = [%dryunit]

That will generate a default configuration that only sees the current file. You can override any default behavior passing arguments through a record, as shown below. All fields are optionals and might be in any order.

let () =
    { cache_dir   = ".dryunit"
    ; cache       = true
    ; framework   = "alcotest"
    ; ignore      = ""
    ; filter      = ""
    ; detection   = "file"
    ; ignore_path = "self"

Implementation details

  • At build time, dryunit will check anything that looks like a test file in the build context and check its internal cache mechanism for preprocessed suites.

  • If none is found, an instance of OCaml parser will be created to extract a structured representation of the test file.

  • Cache is done in one file for the whole directory. Updated according to timestamps and compiler version. Default directory is (_build/.dryunit).

  • The extension does nothing if outside a build directory.

Dependencies (5)

  1. ppx_tools_versioned
  2. ocaml-migrate-parsetree build & < "2.0.0"
  3. cppo build
  4. jbuilder >= "1.0+beta7"
  5. ocaml >= "4.02.3" & < "4.06"

Dev Dependencies


Used by (1)

  1. dryunit < "0.4.0"




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