Executable code blocks inside markdown files

mdx -- executable code blocks inside markdown files

mdx allows to execute code blocks inside markdown files.
There are (currently) two sub-commands, corresponding
to two modes of operations: pre-processing (ocaml-mdx pp)
and tests (ocaml-mdx test).

The pre-processor mode allows to mix documentation and code,
and to practice "literate programming" using markdown and OCaml.

The test mode allows to ensure that shell scripts and OCaml fragments
in the documentation always stays up-to-date.

The blocks in markdown files can be parameterized by mdx-specific labels, that
will change the way mdx interprets the block.
The syntax is: <!-- $MDX labels -->, where labels is a list of valid labels
separated by a comma. This line has to immediately precede the block it is
attached to. Examples are given in the following sections.
This syntax is the recommended way to define labels since mdx 1.7.0, to use the previous syntax please refer to the mdx 1.6.0 README.

mdx is released as a single binary (called ocaml-mdx) and
can be installed using opam:

$ opam install mdx

If you want to contribute or hack on the project, please see the

Supported Extensions

Shell Scripts

ocaml-mdx interprets shell scripts inside sh code blocks as cram-like tests. The
syntax is the following:

  • Lines beginning with a dollar sign and a space are
    commands and will be run in the shell.

  • Multi-lines commands end by \ and continue with two spaces and
    a > sign on the next line:

     $ <line1> \
     > <line2> \
     > <line3>
  • Commands support the heredoc syntax (<<):

     $ cat <<EOF \
     > hello\
     > world\
     > EOF
  • Lines beginning without a dollar sign are considered command outputs.

  • Command outputs can contain ellipses: .... These will
    match any possible outputs (on zero, one or multiple lines).

  • Arbitrary padding with whitespace is supported, as long as it is consistent
    inside a code block.

Here is an example of a markdown file using shell scripts inside code blocks,
with a padding of 3:

   $ for i in `seq 1 10`

ocaml-mdx will also consider exit codes when the syntax [<exit code>]is used:

$ exit 1

Note that nothing will be displayed when the exit code is 0 (e.g. in case
of success).

OCaml Code

ocaml-mdx interprets OCaml fragments. It understands normal code fragments and
toplevel code fragments (starting with a # sign and optionally ending with
;;). Arbitrary whitespace padding is supported, at long as it stays
consistent within a code block.

Toplevel fragments interleave OCaml code and their corresponding outputs.

Here is an example of normal OCaml code:

print_endline "42"

Here is an examples of toplevel OCaml code:

# print_endline "42"

File sync

mdx is also capable of including content from files in fenced code blocks
using the label file. When an OCaml file is included it can be automatically
sliced if it contains annotations such as [@@@part "partName"] and if the
block has the label part=partName:

<!-- $MDX,part=partName -->

Non-OCaml files can also be read and included in a block:

<!-- $MDX file=any_file.txt -->

However, part splitting is only supported for OCaml files.


ocaml-mdx pp allows to transform a markdown file into a valid
OCaml file, which can be passed to OCaml using the -pp

For instance, given the following document:

# print_endline "42"

Can be compiled and executed using:

$ ocamlc -pp 'ocaml-mdx pp' -impl -o file.exe
$ ./file.exe

This can be automated using dune:

 ((targets (
  (deps    (
  (action  (with-stdout-to ${@} (run ocaml-mdx pp ${<})))))

(executable ((name file)))


Cram Tests

Cram tests can be executed and checked with ocaml-mdx test <>.

 $ for i in `seq 1 10`; do echo $i; done

If the output is not consistent with what is expected,
<>.corrected is generated.


To execute OCaml code and toplevel fragments, uses ocaml-mdx test <>.

# print_endline "42"

If the output is not consistent with what is expected
<>.corrected is generated.

Integration with Dune

To test that the code blocks of stay consistent, one can use
dune's mdx stanza:


This allows to test the consistency of a markdown file using the normal dev

$ dune runtest

will display a diff of the output if something has changed. For instance:

$ dune runtest
File "", line 23, characters 0-1:
-| $ for i in `seq 1 3`; do echo $i; done
+| $ for i in `seq 1 4`; do echo $i; done
 | 1
 | 2
 | 3
+| 4

And the changes can then be accepted using:

$ dune promote

For further details about the mdx stanza you should read the
according section
in the dune documentation.

Non-deterministic Tests

Non-deterministic Outputs

ocaml-mdx test supports non-deterministic outputs:

<!-- $MDX non-deterministic=output -->
$ <command>

In that case, ppx test <file> will run the command but will not
generate <file>.corrected if the new output differs from the one
described in the file. Use ocaml-mdx test --non-deterministic <file> to come
back to the default behaviour.

Non-deterministic Commands

ocaml-mdx test supports non-deterministic commands:

<!-- $MDX non-deterministic=command -->
# 10;;
- : int = 5

In that case, ocaml-mdx test <file> will not run the command. Use ocaml-mdx test --non-deterministic <file> to come back to the default behaviour.

Named execution environments (since mdx 1.1.0)

Separate environments can be defined for blocks:

x holds the value 1 in the environment e1.

<!-- $MDX env=e1 -->
let x = 1;;

<!-- $MDX env=e1 -->
module M = struct let k = 42 let f x = x * k end;;

x holds the value 3 in the environment e2.

<!-- $MDX env=e2 -->
let x = 3;;

We can retrieve the value of x in environment e1:

<!-- $MDX env=e1 -->
# print_int x;;
- : unit = ()
# print_int M.k;;
- : unit = ()
# M.f;;
- : int -> int = <fun>
Matching on the OCaml version (since mdx 1.2.0)

Blocks can be processed or ignored depending on the current version of OCaml.

For example to have a different outcome whether we are past OCaml 4.06:

<!-- $MDX version<4.06 -->
# let f x = x + 1
val f : int -> int = <fun>
# let f y =
val f : bytes -> bytes = <fun>

<!-- $MDX version>=4.06 -->
# let f x = x + 1
val f : int -> int = <fun>
# let f y =
val f : string -> string = <fun>

The available operators are <>, >=, >, <=, < and =.
The version number can be of the following forms:

  • *

  • X

  • X.Y

  • X.Y.Z

Environment variables declaration

Environment variables can be declared at the beginning of a block:

<!-- $MDX set-FOO=bar,set-BAR=foo -->
# print_endline (Sys.getenv "FOO")
- : unit = ()
# print_endline (Sys.getenv "BAR")
- : unit = ()

Those variables are then available in the subsequent blocks

# print_endline (Sys.getenv "FOO")
- : unit = ()


It is possible to test or execute only a subset of the file using
sections using the --section option (short name is -s). For
instance ocaml-mdx pp -s foo will only consider the section matching the
perl regular expression foo.

Dune rules (since mdx 1.1.0)

ocaml-mdx can generate dune rules to synchronize .md files with .ml files.

Consider the test/ file that contains blocks referring to files and, running:

$ ocaml-mdx rule test/

generates the following dune rules on the standard output:

 (name   runtest)
 (deps   (:x test/
         (source_tree foo))
 (action (progn
           (run ocaml-mdx test %{x})
           (diff? %{x} %{x}.corrected)
           (diff? %{y1} %{y1}.corrected)
           (diff? %{y0} %{y0}.corrected))))
12 Feb 2021
with-test & < "1.1.0"
>= "1.5.0" & < "2.0.0"
>= "2.3.0"
>= "1.7.2"
>= "1.0.0"
>= "0.5.0"
>= "1.3.2"
build & >= "1.1.0"
>= "0.8.5"
>= "1.7.2"
>= "4.02.3" & < "4.13"
>= "2.2"
Reverse Dependencies