Skip to content
Cram like framework for OCaml
Branch: master
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.

expect-test - a cram like framework for OCaml


Expect-test is a framework for writing tests in OCaml, similar to Cram. Expect-tests mimic the existing inline tests framework with the let%expect_test construct. The body of an expect-test can contain output-generating code, interleaved with %expect extension expressions to denote the expected output.

When run, these tests will pass iff the output matches what was expected. If a test fails, a corrected file with the suffix “.corrected” will be produced with the actual output, and the inline_tests_runner will output a diff.

Here is an example Expect-test program, say in

open Core

let%expect_test "addition" =
  printf "%d" (1 + 2);
  [%expect {| 4 |}]

When the test is run (as part of inline_tests_runner), will be produced with the contents:

open Core

let%expect_test "addition" =
  printf "%d" (1 + 2);
  [%expect {| 3 |}]

inline_tests_runner will also output the diff:
File "", line 5, characters 0-1:
  open Core

  let%expect_test "addition" =
    printf "%d" (1 + 2);
-|  [%expect {| 4 |}]
+|  [%expect {| 3 |}]

Diffs will be shown in color if the -use-color flag is passed to the test runner executable.

Expects reached from multiple places

A [%expect] can exist in a way that it is encountered multiple times, e.g. in a functor or a function:

let%expect_test _ =
  let f output =
    print_string output;
    [%expect {| hello world |}]
  f "hello world";
  f "hello world";

The [%expect] should capture the exact same output (i.e. up to string equality) at every invocation. In particular, this does **not** work:

let%expect_test _ =
  let f output =
    print_string output;
    [%expect {| \(foo\|bar\) (regexp) |}]
  f "foo";
  f "bar";

Output matching

Matching is done on a line-by-line basis. If any output line fails to match its expected output, the expected line is replaced with the actual line in the final output.


Inside %expect nodes, whitespace around patterns are ignored, and the user is free to put any amount for formatting purposes. The same goes for the actual output.

Ignoring surrounding whitespace allows to write nicely formatted expectation and focus only on matching the bits that matter.

To do this, ppx_expect strips patterns and outputs by taking the smallest rectangle of text that contains the non-whitespace material. All end of line whitespace are ignored as well. So for instance all these lines are equivalent:

  print blah;
  [%expect {|

  print blah;
  [%expect {|

  print blah;
  [%expect {|

However, the last one is nicer to read.

For the rare cases where one does care about what the exact output is, ppx_expect provides the %expect_exact extension point, which only succeed when the untouched output is exactly equal to the untouched pattern.

When producing a correction, ppx_expect tries to respect as much as possible the formatting of the pattern.

Output capture

The extension point [%expect.output] returns a string with the output that would have been matched had an [%expect] node been there instead.

An idiom for testing non-deterministic output is to capture the output using [%expect.output] and either post-process it or inspect it manually, e.g.,

show_process ();
let pid_and_exit_status = [%expect.output] in
let exit_status = discard_pid pid_and_exit_status in
print_endline exit_status;
[%expect {| 1 |}]

This is preferred over output patterns (see below).

Integration with Async, Lwt or other cooperative libraries

If you are writing expect tests for a system using Async, Lwt or any other libraries for cooperative threading, you need some preparation so that everything works well. For instance, you probably need to flush some stdout channel. The expect test runtime takes care of flushing Caml.stdout but it doesn’t know about Async.Writer.stdout, Lwt_io.stdout or anything else.

To deal with this, expect\_test provides some hooks in the form of a condifuration module Expect_test_config. The default module in scope define no-op hooks that the user can override. Async redefines this module so when Async is opened you can write async-aware expect test.

In addition to Async.Expect_test_config, there is an alternative, Async.Expect_test_config_with_unit_expect. That is easier to use than Async.Expect_test_config because [%expect] has type unit rather than unit Deferred.t. So one can write:

[%expect foo];

rather than:

let%bind () = [%expect foo] in

Expect_test_config_with_unit_expect arrived in 2019-06. We hope to transition from Expect_test_config to Expect_test_config_with_unit_expect, eventually renaming the latter as the former.


This is what you would need to write expect tests with Lwt:

module Expect_test_config
  : Expect_test_config.S with module IO = Lwt =
  module IO = Lwt
  let flush () = Lwt_io.(flush stdout)
  let run =

Comparing Expect-test and unit testing (e.g. let%test_unit)

The simple example above can be easily represented as a unit test:

let%test_unit "addition" = [%test_result: int] (1 + 2) ~expect:4

So, why would one use Expect-test rather than a unit test? There are several differences between the two approaches.

With a unit test, one must write code that explicitly checks that the actual behavior agrees with the expected behavior. %test_result is often a convenient way of doing that, but even using that requires:

  • creating a value to compare
  • writing the type of that value
  • having a comparison function on the value
  • writing down the expected value

With Expect-test, we can simply add print statements whose output gives insight into the behavior of the program, and blank %expect attributes to collect the output. We then run the program to see if the output is acceptable, and if so, replace the original program with its output. E.g we might first write our program like this:

let%expect_test _ =
  printf "%d" (1 + 2);
  [%expect {||}]

The corrected file would contain:

let%expect_test _ =
  printf "%d" (1 + 2);
  [%expect {| 3 |}]

With Expect-test, we only have to write code that prints things that we care about. We don’t have to construct expected values or write code to compare them. We get comparison for free by using diff on the output. And a good diff (e.g. patdiff) can make understanding differences between large outputs substantially easier, much easier than typical unit-testing code that simply states that two values aren’t equal.

Once an Expect-test program produces the desired expected output and we have replaced the original program with its output, we now automatically have a regression test going forward. Any undesired change to the output will lead to a mismatch between the source program and its output.

With Expect-test, the source program and its output are interleaved. This makes debugging easier, because we do not have to jump between source and its output and try to line them up. Furthermore, when there is a mismatch, we can simply add print statements to the source program and run it again. This gives us interleaved source and output with the debug messages interleaved in the right place. We might even insert additional empty %%expect attributes to collect debug messages.


Every %expect node in an Expect-test program becomes a point at which the program output is captured. Once the program terminates, the captured outputs are matched against the expected outputs, and interleaved with the original source code to produce the corrected file. Trailing output is appended in a new %expect node.

Build system integration

Follow the same rules as for ppx_inline_test. Just make sure to include ppx_expect.evaluator as a dependency of the test runner. The Jane Street tests contains a few working examples using oasis.

Output patterns

Lines in an %expect can end with a “tag” indicating the kind of match to perform. This functionality is deprecated because it interferes with the smooth expect-test workflow of accepting output. One should instead use output post-processing.

To enable support for output patterns, your jbuild file should have:

((inline_tests ((flags (-allow-output-patterns)))))

Here are the different kinds of output patterns.

The (regexp) tag will perform regexp matching on the given line:

printf "foo";
[%expect {| foo\|bar (regexp) |}]

Similarly, the (glob) tag will perform glob matching on the given line:

printf "foobarbaz";
[%expect {| {foo,hello}* (glob) |}]

The (literal) tag will force a literal match on a line, and can be useful in edge cases:

printf "foo*bar (regexp)";
[%expect {| foo*bar (regexp) (literal) |}]

The (escaped) tag will treat the line as an escaped literal string, which can be useful for matching unprintable characters. It doesn’t support escaped newlines right now.

You can’t perform that action at this time.