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A lightweight and colourful test framework

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A lightweight and colourful test framework.

Alcotest exposes a simple interface to perform unit tests. It exposes a simple

module type, a
function to assert test predicates and a
function to perform a list of
unit -> unit
test callbacks.

Alcotest provides a quiet and colorful output where only faulty runs are fully displayed at the end of the run (with the full logs ready to inspect), with a simple (yet expressive) query language to select the tests to run. See the manpage for details.

The API documentation can be found [here][docs]. For information on contributing to Alcotest, see

OCaml-CI Build Status docs


A simple example (taken from


Generated by the following test suite specification:

(* Build with `ocamlbuild -pkg alcotest simple.byte` *)

(* A module with functions to test *) module To_test = struct let lowercase = String.lowercase_ascii let capitalize = String.capitalize_ascii let str_concat = String.concat "" let list_concat = List.append end

(* The tests *) let test_lowercase () = Alcotest.(check string) "same string" "hello!" (To_test.lowercase "hELLO!")

let test_capitalize () = Alcotest.(check string) "same string" "World." (To_test.capitalize "world.")

let test_str_concat () = Alcotest.(check string) "same string" "foobar" (To_test.str_concat ["foo"; "bar"])

let test_list_concat () = Alcotest.(check (list int)) "same lists" [1; 2; 3] (To_test.list_concat [1] [2; 3])

(* Run it *) let () = let open Alcotest in run "Utils" [ "string-case", [ test_case "Lower case" Quick test_lowercase; test_case "Capitalization" Quick test_capitalize; ]; "string-concat", [ test_case "String mashing" Quick test_str_concat ]; "list-concat", [ test_case "List mashing" Slow test_list_concat ]; ]

The result is a self-contained binary which displays the test results. Use

exec examples/simple.exe -- --help
to see the runtime options.

Here's an example of a of failing test suite:

By default, only the first failing test log is printed to the console (and all test logs are captured on disk). Pass

to print all error messages.

Selecting tests to execute

You can filter which tests to run by supplying a regular expression matching the names of the tests to execute, or by passing a regular expression and a comma-separated list of test numbers (or ranges of test numbers, e.g.

): ``
$ ./simple.native test '.*concat*'
Testing Utils.
[SKIP]     string-case            0   Lower case.
[SKIP]     string-case            1   Capitalization.
[OK]       string-concat          0   String mashing.
[OK]       list-concat            0   List mashing.
The full test results are available in
build/tests`. Test Successful in 0.000s. 2 tests run.

$ ./simple.native test 'string-case' '1..3' Testing Utils. [SKIP] string-case 0 Lower case. [OK] string-case 1 Capitalization. [SKIP] string-concat 0 String mashing. [SKIP] list-concat 0 List mashing. The full test results are available in

. Test Successful in 0.000s. 1 test run. ```

Note that you cannot filter by test case name (i.e.

Lower case
), you must filter by test name & number instead.

See the examples folder for more examples.

Quick and Slow tests

In general you should use

tests: tests that are ran on any invocations of the test suite. You should only use
tests for stress tests that are ran only on occasion (typically before a release or after a major change). These slow tests can be suppressed by passing the
flag on the command line, e.g.:
$ ./test.exe -q # run only the quick tests
$ ./test.exe    # run quick and slow tests

Passing custom options to the tests

In most cases, the base tests are

unit -> unit
functions. However, it is also possible to pass an extra option to all the test functions by using
'a -> unit
, where
is the type of the extra parameter.

In order to do this, you need to specify how this extra parameter is read on the command-line, by providing a Cmdliner term for command-line arguments which explains how to parse and serialize values of type

(note: do not use positional arguments, only optional arguments are supported).

For instance:

let test_nice i = Alcotest.(check int) "Is it a nice integer?" i 42

let int = let doc = "What is your prefered number?" in Cmdliner.Arg.(required & opt (some int) None & info ["n"] ~doc ~docv:"NUM")

let () = Alcotest.run_with_args "foo" int [ "all", ["nice", `Quick, test_nice] ]

Will generate

such that:
$ test.exe test
test.exe: required option -n is missing

$ test.exe test -n 42 Testing foo. [OK] all 0 int.


Alcotest provides an

module that you could use to wrap Lwt test cases. The basic idea is that instead of providing a test function in the form
unit -> unit
, you provide one with the type
unit -> unit Lwt.t
and alcotest-lwt calls
for you.

However, there are a couple of extra features:

  • If an async exception occurs, it will cancel your test case for you and fail it (rather than exiting the process).

  • You get given a switch, which will be turned off when the test case finishes (or fails). You can use that to free up any resources.

For instance:

let free () = print_endline "freeing all resources"; Lwt.return ()

let test_lwt switch () = Lwt_switch.add_hook (Some switch) free; Lwt.async (fun () -> failwith "All is broken"); Lwt_unix.sleep 10.

let () = @@ "foo" [ "all", [ Alcotest_lwt.test_case "one" `Quick test_lwt ] ]

Will generate:

$ test.exe
Testing foo.
[ERROR]             all          0   one.
-- all.000 [one.] Failed --
in _build/_tests/all.000.output:
freeing all resources
[failure] All is broken

Comparison with other testing frameworks

The README is pretty clear about that:

Alcotest is a lightweight and colourful test framework.

Alcotest is the only testing framework using colors!

More seriously, Alcotest is similar to ounit but it fixes a few of the problems found in that library:

  • Alcotest has a nicer output, it is easier to see what failed and what succeeded and to read the log outputs of the failed tests;

  • Alcotest uses combinators to define pretty-printers and comparators between the things to test.

Other nice tools doing different kind of testing also exist:

  • qcheck qcheck does random generation and property testing (e.g. Quick Check)

  • crowbar and bun are similar to qcheck, but use compiler-directed randomness, e.g. it takes advantage of the AFL support the OCaml compiler.

  • ppx_inline_tests
    allows to write tests in the same file as your source-code; they will be run only in a special mode of compilation.

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