ppx_expect

ppx_expect is Jane Street’s inline expect-test framework for OCaml. Tests are written as let%expect_test with [%expect {|...|}] assertions.

How it works

Place @mdexp.snapshot before the [%expect] block. mdexp extracts the block string content and emits it in the document. Here is what an annotated test looks like (the + lines highlight the mdexp directives):

  let%expect_test "greeting" =
    print_endline "Hello, World!";
+   (* @mdexp.snapshot *)
    [%expect {| Hello, World! |}]
  ;;

The let%expect_test wrapper and [%expect] assertion are not included in the output — only the block string content appears. In this case, the rendered document shows:

Hello, World!

Multi-line output

Multi-line output works the same way. The block string content is dedented automatically:

  let%expect_test "list" =
    List.iter ~f:print_endline [ "First"; "Second"; "Third" ];
+   (* @mdexp.snapshot { lang: "txt" } *)
    [%expect
      {|
      First
      Second
      Third |}]
  ;;

This produces:

First
Second
Third

Snapshot configuration

By default, the snapshot content is emitted as plain text. As we’ve seen above, an inline annotation can change this. Here is another example:

{ lang: "json" } wraps the content in a fenced code block with the given language tag:

  let%expect_test "json output" =
    print_endline {|{ "name": "mdexp", "version": "1.0" }|};
+   (* @mdexp.snapshot { lang: "json" } *)
    [%expect {|{ "name": "mdexp", "version": "1.0" }|}]
  ;;

This produces:

{ "name": "mdexp", "version": "1.0" }

{ block: true } wraps the content in a plain fence without a language tag:

  let%expect_test "block mode" =
    print_endline "some output";
+   (* @mdexp.snapshot { block: true } *)
    [%expect {|some output|}]
  ;;

This produces:

some output