expect-test - a Cram-like framework for OCaml

Introduction

Expect-test is a framework for writing tests in OCaml, similar to Cram.

Expect-tests mimic the (now less idiomatic) inline test framework in providing a 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, expect-tests pass iff the output matches the expected output. If a test fails, the inline_tests_runner outputs a diff and creates a file with the suffix ".corrected" containing the actual output.

Here is an example expect-test in foo.ml:

open! Core

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

When the test runs, the inline_tests_runner creates foo.ml.corrected with contents:

open! Core

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

inline_tests_runner also outputs:

------ foo.ml
++++++ foo.ml.corrected
File "foo.ml", line 6, characters 0-1:
 |open! Core
 |
 |let%expect_test "addition" =
 |  printf "%d" (1 + 2);
-|  [%expect {| 4 |}]
+|  [%expect {| 3 |}]
 |;;
 |

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

Common usage

Each [%expect] block matches all the output generated since the previous [%expect] block (or the beginning of the test). In this way, when multiple [%expect] blocks are interleaved with test code, they can help show which part of the test produced which output.

The following test:

let%expect_test "interleaved" =
  let l = [ "a"; "b"; "c" ] in
  printf "A list [l]\n";
  printf "It has length %d\n" (List.length l);
  [%expect {| A list [l] |}];
  List.iter l ~f:print_string;
  [%expect
    {|
    It has length 3
    abc
    |}]
;;

is rewritten as

let%expect_test "interleaved" =
  let l = [ "a"; "b"; "c" ] in
  printf "A list [l]\n";
  printf "It has length %d\n" (List.length l);
  [%expect
    {|
    A list [l]
    It has length 3
    |}];
  List.iter l ~f:print_string;
  [%expect {| abc |}]
;;

When there is "trailing" output at the end of a let%expect_test (output that has yet to be matched by some [%expect] block), a new [%expect] block is appended to the test with the trailing output:

let%expect_test "trailing output" =
  print_endline "Hello";
  [%expect {| Hello |}];
  print_endline "world"
;;

becomes:

let%expect_test "trailing output" =
  print_endline "Hello";
  [%expect {| Hello |}];
  print_endline "world";
  [%expect {| world |}]
;;

Matching behavior

You might have noticed that the contents of the [%expect] blocks are not exactly the program output; in some of the examples above, they contain a different number of leading and trailing newlines, and are indented to match the code indentation. We say the contents of a block [%expect str] (where str is some string literal) match the output at that block if the output, after we format it to standardize indentation and other whitespace, is identical to the contents of str after it has been similarly formatted .

The formatting applied depends on the type of delimiter used in str (i.e. whether it a "quoted string" or a {xxx| delimited string |xxx}). To summarize:

• Output containing only whitespace is formatted as [%expect {| |}] or [%expect ""].
• Output where only one line contains non-whitespace characters is formatted onto a single line, as [%expect {| output |}] or [%expect "output"].
• Output where multiple lines contain non-whitespace characters is formatted so that:
  • There is no trailing whitespace on lines with content.
  • The relative indentation of the lines is preserved.
  • In {| delimited strings |}, the least-indented line with content (the "left margin" of the output) is aligned to be two spaces past the indentation of the [%expect] block.
  • In "quoted string", the least-indented line is indented by exactly one space (this plays the nicest with ocamlformat's existing decisions about how to format string literals).
  • There is one empty line before and one empty line after the contents.

Here is an example containing several cases of output that are subject to distinct formatting rules and how they appear in [%expect] and [%expect_exact] blocks:

Expand examples

let%expect_test "matching behavior --- no content" =
  printf "     ";
  [%expect {| |}];
  printf "     ";
  [%expect ""];
  printf "     ";
  [%expect_exact {|     |}];
  printf "     ";
  [%expect_exact "     "]
;;

let%expect_test "matching behavior --- one line of content" =
  printf "\n   This is one line\n\n";
  [%expect {| This is one line |}];
  printf "\n   This is one line\n\n";
  [%expect "This is one line"];
  printf "\n   This is one line\n\n";
  [%expect_exact
    {|
   This is one line

|}];
  printf "\n   This is one line\n\n";
  [%expect_exact "\n   This is one line\n\n"]
;;

let%expect_test "matching behavior --- multiple lines of content" =
  printf
    {|
Once upon a midnight dreary,
  while I pondered, weak and weary,
Over many a quaint and curious
  volume of forgotten lore |};
  [%expect
    {|
    Once upon a midnight dreary,
      while I pondered, weak and weary,
    Over many a quaint and curious
      volume of forgotten lore
    |}];
  printf
    {|
Once upon a midnight dreary,
  while I pondered, weak and weary,
Over many a quaint and curious
  volume of forgotten lore |};
  [%expect
    " \n\
    \ Once upon a midnight dreary,\n\
    \   while I pondered, weak and weary,\n\
    \ Over many a quaint and curious\n\
    \   volume of forgotten lore\n\
    \ "];
  printf
    {|
Once upon a midnight dreary,
  while I pondered, weak and weary,
Over many a quaint and curious
  volume of forgotten lore |};
  [%expect_exact
    {|
Once upon a midnight dreary,
  while I pondered, weak and weary,
Over many a quaint and curious
  volume of forgotten lore |}];
  printf
    {|
Once upon a midnight dreary,
  while I pondered, weak and weary,
Over many a quaint and curious
  volume of forgotten lore |};
  [%expect_exact
    "\n\
     Once upon a midnight dreary,\n\
    \  while I pondered, weak and weary,\n\
     Over many a quaint and curious\n\
    \  volume of forgotten lore "]
;;

Expect-test is by default permissive about this formatting, so that a [%expect] block that is correct modulo formatting is accepted. However, passing -expect-test-strict-indentation=true to the ppx driver makes the test runner issue corrections for blocks that do not satisfy the indentation rules. For example, the following:

let%expect_test "bad formatting" =
  printf "a\n    b";
  [%expect
    {|
a
    b |}]
;;

is corrected to:

let%expect_test "bad formatting" =
  printf "a\n    b";
  [%expect
    {|
    a
        b
    |}]
;;

(to add the required indentation and trailing newline)

Reachability

Expects reached from multiple places

A [%expect] extension can be encountered multiple times if it is in e.g. a functor or a function:

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

The test passes if the [%expect] block matches the output each time it is encountered, as described in the section on matching behavior.

If the outputs are not consistent, then the corrected file contains a report of all of the outputs that were captured, in the order that they were captured at runtime.

For example, calling f in the snippet above with inconsistent arguments will produce:

let%expect_test "function" =
  let f output =
    print_string output;
    [%expect
      {|
      (* expect_test: Test ran multiple times with different test outputs *)
      ============================ Output 1 / 4 ============================
      hello world
      ============================ Output 2 / 4 ============================
      goodbye world
      ============================ Output 3 / 4 ============================
      once upon
      a midnight dreary
      ============================ Output 4 / 4 ============================
      hello world
      |}]
  in
  f "hello world";
  f "goodbye world";
  f "once upon\na midnight dreary";
  f "hello world"
;;

Unreached expects

Every [%expect] and [%expect_exact] block in a let%expect_test must be reached at least once if that test is ever run. Failure for control flow to reach a block is not treated like recording empty output at a block. The extension expression [%expect.unreachable] is used to indicate that some part of an expect test shouldn't be reached; if control flow reaches that point anyway, the corrected file replaces the [%expect.unreachable] with a plain old expect containing the output collected until that point. Conversely, if control flow never reaches some [%expect] block, that block is turned into a [%expect.unreachable]. For example:

let%expect_test "unreachable" =
  let interesting_bool = 3 > 5 in
  printf "%b\n" interesting_bool;
  if interesting_bool
  then [%expect {| true |}]
  else (
    printf "don't reach\n";
    [%expect.unreachable])
;;

becomes:

let%expect_test "unreachable" =
  let interesting_bool = 3 > 5 in
  printf "%b\n" interesting_bool;
  if interesting_bool
  then [%expect.unreachable]
  else (
    printf "don't reach\n";
    [%expect
      {|
      false
      don't reach
      |}])
;;

Note that, for an expect block that is sometimes reachable and sometimes not, that block passes if the output captured at that block matches every time the block is encountered. For example, the following test passes:

module Test (B : sig
    val interesting_opt : int option
  end) =
struct
  let%expect_test "sometimes reachable" =
    match B.interesting_opt with
    | Some x ->
      printf "%d\n" x;
      [%expect {| 5 |}]
    | None -> [%expect {| |}]
  ;;
end

module _ = Test (struct
    let interesting_opt = Some 5
  end)

module _ = Test (struct
    let interesting_opt = None
  end)

module _ = Test (struct
    let interesting_opt = Some 5
  end)

Exceptions

When an exception is raised by the body of an expect-test, the inline_test_runner shows it (and, if relevant, any output generated by the test that had not yet been captured) in a [@@expect.uncaught_exn] attribute attached to the corresponding let%expect_test. [%expect] blocks in the test are treated according to the usual rules: those reached before the exception is raised capture output as usual, and those that "would have" been reached after are marked as unreachable:

let%expect_test "exception" =
  Printexc.record_backtrace false;
  printf "start!";
  [%expect {| |}];
  let sum = 2 + 2 in
  if sum <> 3
  then (
    printf "%d" sum;
    failwith "nope");
  printf "done!";
  [%expect {| done! |}]
;;

becomes:

let%expect_test "exception" =
  Printexc.record_backtrace false;
  printf "start!";
  [%expect {| start! |}];
  let sum = 2 + 2 in
  if sum <> 3
  then (
    printf "%d" sum;
    failwith "nope");
  printf "done!";
  [%expect.unreachable]
[@@expect.uncaught_exn
  {|
  (Failure nope)
  Trailing output
  ---------------
  4
  |}]
;;

Unlike [%expect] blocks, which might be reached on some runs of a test and not others, a test with an [@@expect.uncaught_exn] attribute must raise every time it is run. Changing the None branch of the functorized test from before to raise gives:

module Test' (B : sig
    val interesting_opt : int option
  end) =
struct
  let%expect_test "sometimes raises" =
    match B.interesting_opt with
    | Some x ->
      printf "%d\n" x;
      [%expect {| 5 |}]
    | None -> failwith "got none!"
  [@@expect.uncaught_exn
    {|
    (* expect_test: Test ran multiple times with different uncaught exceptions *)
    =============================== Output 1 / 3 ================================
    <expect test ran without uncaught exception>
    =============================== Output 2 / 3 ================================
    (Failure "got none!")
    =============================== Output 3 / 3 ================================
    <expect test ran without uncaught exception>
    |}]
  ;;
end

module _ = Test' (struct
    let interesting_opt = Some 5
  end)

module _ = Test' (struct
    let interesting_opt = None
  end)

module _ = Test' (struct
    let interesting_opt = Some 5
  end)

Output capture

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

One idiom for testing non-deterministic output is to capture the output using [%expect.output] and post-process it:

(* Suppose we want to test code that attaches a timestamp to everything it prints *)
let print_message s = printf "%s: %s\n" (Time_float.to_string_utc (Time_float.now ())) s

let%expect_test "output capture" =
  (* A simple way to clean up the non-determinism is to 'X' all digits *)
  let censor_digits s = String.map s ~f:(fun c -> if Char.is_digit c then 'X' else c) in
  print_message "Hello";
  [%expect.output] |> censor_digits |> print_endline;
  [%expect {| XXXX-XX-XX XX:XX:XX.XXXXXXZ: Hello |}];
  print_message "world";
  [%expect.output] |> censor_digits |> print_endline;
  [%expect {| XXXX-XX-XX XX:XX:XX.XXXXXXZ: world |}]
;;

Other uses of [%expect.output] include:

• Sorting lines of output printed in nondeterministic order.
• Passing output that is known to be a sexp to t_of_sexp and performing tests on the resulting structure.
• Performing some sort of additional validation on the output before printing it to a normal [%expect] block.

Configuration

Expect-test exposes hooks for configuring how the bodies of expect tests are run, which can be used to set up and tear down test environments, sanitize output, or embed [%expect] expressions in a monadic computation, like a Deferred.t.

Each let%expect_test reads these configurations from the module named Expect_test_config in the scope of that let binding. The default module in scope defines no-op hooks that the user can override. To do so, first include the existing Expect_test_config, then override a subset of the following interface:

module type Expect_test_config = sig
  (** The type of the expression on the RHS of a [let%expect_test]
      binding is [unit IO.t] *)
  module IO : sig
      type 'a t

      val return : 'a -> 'a t
  end

  (** Run an IO operation until completion *)
  val run : (unit -> unit IO.t) -> unit

  (** [sanitize] can be used to map all output strings, e.g. for cleansing. *)
  val sanitize : string -> string

  (** This module type actually contains other definitions, but they
      are for internal testing of [ppx_expect] only. *)
end

For example, Async exports an Expect_test_config equivalent to:

module Expect_test_config = struct
  include Expect_test_config

  module IO = Async_kernel.Deferred

  let run f = Async_unix.Thread_safe.block_on_async_exn f
end

If we want to consistently apply the same sanitization to all of the output in our expect test, like we did in the timestamp example above, we can override Expect_test_config.sanitize. This cleans up the testing code and removes the need to use [%expect.output].

(* Suppose we want to test code that attaches a timestamp to everything it prints *)
let print_message s = printf "%s: %s\n" (Time_float.to_string_utc (Time_float.now ())) s

module Expect_test_config = struct
  include Expect_test_config

  (* A simple way to clean up the non-determinism is to 'X' all digits *)
  let sanitize s = String.map s ~f:(fun c -> if Char.is_digit c then 'X' else c)
end

let%expect_test "sanitization" =
  print_message "Hello";
  [%expect {| XXXX-XX-XX XX:XX:XX.XXXXXXZ: Hello |}];
  print_message "world";
  [%expect {| XXXX-XX-XX XX:XX:XX.XXXXXXZ: world |}]
;;

Build system integration

Follow the same rules as for ppx_inline_test.