Execute code blocks inside markdown files
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Solved bug occuring when a top-level command outputs an empty line
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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 (mdx pp) and tests (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.

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

$ opam install mdx

Supported Extensions

Shell Scripts

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`

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

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"


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

For instance, given the following file.md document:

# print_endline "42"

Can be compiled and executed using:

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

This can be automated using dune:

 ((targets (file.ml))
  (deps    (file.md))
  (action  (with-stdout-to ${@} (run mdx pp ${<})))))

(executable ((name file)))


Cram Tests

Cram tests can be executed and checked with mdx test <file.md>.

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

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


To execute OCaml code and toplevel fragments, uses mdx test <file.md>.

# print_endline "42"

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

Integration with Dune

To test that the code blocks of file.md stay consistent, one can use dune's diff? stanza:

 ((name runtest)
  (deps (file.md))
  (action (progn
           (run mdx test ${<})
           (diff? ${<} ${<}.corrected)))))

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

$ dune runtest

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

$ dune runtest
------ file.md
++++++ file.md.corrected
File "file.md", 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

Non-deterministic Tests

Non-deterministic Outputs

mdx test supports non-deterministic outputs:

```sh 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 mdx test --non-deterministic <file> to come back to the default behaviour.

Non-deterministic Commands

mdx test supports non-deterministic commands:

```ocaml non-deterministic=command
# Random.int 10;;
- : int = 5

In that case, mdx test <file> will not run the command. Use 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.

```ocaml env=e1
let x = 1;;

```ocaml env=e1
module M = struct let k = 42 let f x = x * k end;;

x holds the value 3 in the environment e2.

```ocaml env=e2
let x = 3;;

We can retrieve the value of x in environment e1:

```ocaml 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:

```ocaml version<4.06
# let f x = x + 1
val f : int -> int = <fun>
# let f y =
val f : bytes -> bytes = <fun>

```ocaml 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


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 mdx pp -s foo will only consider the section matching the perl regular expression foo.

Dune rules (since mdx 1.1.0)

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

Consider the test/dune_rules.md file that contains blocks referring to files dune_rules_1.ml and dune_rules_2.ml, running:

$ mdx rule test/dune_rules.md

generates the following dune rules on the standard output:

 (name   runtest)
 (deps   (:x test/dune_rules.md)
         (:y1 dune_rules_1.ml)
         (:y0 dune_rules_2.ml)
         (source_tree foo))
 (action (progn
           (run mdx test --direction=infer-timestamp %{x})
           (diff? %{x} %{x}.corrected)
           (diff? %{y1} %{y1}.corrected)
           (diff? %{y0} %{y0}.corrected))))