Add support for ml (#827, WIP)

TODO: use block comment instead of line comment

docs/languages.md

@@ -14,6 +14,7 @@ Jupytext works with notebooks in any of the following languages:
 - Javascript
 - Julia
 - Matlab
+- OCaml
 - Octave
 - PowerShell
 - Python

jupytext/languages.py

@@ -58,6 +58,7 @@
     ".java": {"language": "java", "comment": "//"},
     ".groovy": {"language": "groovy", "comment": "//"},
     ".sage": {"language": "sage", "comment": "#"},
+    ".ml": {"language": "ocaml"},  # no line comment for OCaml
 }
 
 _COMMENT_CHARS = [

tests/notebooks/ipynb_ocaml/ocaml_notebook.ipynb

@@ -0,0 +1,83 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "ba111577",
+   "metadata": {},
+   "source": [
+    "# Example of an OCaml notebook"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "id": "04192beb",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "val sum : int -> int -> int = <fun>\n"
+      ]
+     },
+     "execution_count": 3,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "let sum x y = x + y"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "e184eee6",
+   "metadata": {},
+   "source": [
+    "Let's try our function:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "id": "99b4cac0",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "- : bool = true\n"
+      ]
+     },
+     "execution_count": 4,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "sum 3 4 = 7 + 0"
+   ]
+  }
+ ],
+ "metadata": {
+  "jupytext": {
+   "formats": "ipynb:markdown,md"
+  },
+  "kernelspec": {
+   "display_name": "OCaml default",
+   "language": "OCaml",
+   "name": "ocaml-jupyter"
+  },
+  "language_info": {
+   "codemirror_mode": "text/x-ocaml",
+   "file_extension": ".ml",
+   "mimetype": "text/x-ocaml",
+   "name": "OCaml",
+   "nbconverter_exporter": null,
+   "pygments_lexer": "OCaml",
+   "version": "4.12.0"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}