add Topiary to “Formatting Your Code”

Author: toastal

data/tutorials/platform/1_07_code_formatting.md

@@ -25,3 +25,49 @@ In addition to editor plugins that use OCamlFormat for automatic code formatting
 ```shell-session
 $ opam exec -- dune fmt
 ```
+
+
+## Using Topiary
+
+[Topiary](https://topiary.tweag.io) is a Tree-sitter-based code formatter supporting multiple languages, including OCaml & OCamllex.
+It can be invoked with
+
+```shell-session
+$ topiary format source.ml
+```
+
+Topiary does not require an empty configuration file to operate & has its own set of defaults, however, it can be [configured](https://topiary.tweag.io/book/cli/configuration.html).
+
+### Example configuration setup
+
+This example configuration will override the default configuration to use 1 tab character for indentation by creating a `.topiary.ncl` Nickel configuration file.
+
+
+```shell-session
+$ touch .topiary.ncl
+$ $EDITOR .topiary.ncl
+```
+
+```nickel
+{
+	languages = {
+		nickel.indent | priority 1 = "	",
+		ocaml.indent | priority 1 = "	",
+		ocaml_interface.indent | priority 1 = "	",
+		ocamllex.indent | priority 1 = "	",
+	},
+}
+```
+
+Then this file needs to be exported to the environment such as `export TOPIARY_CONFIG_FILE=".topiary.ncl"` in Bash/ZSH or `set -x TOPIARY_CONFIG_FILE ".topiary.ncl"` in Fish.
+
+TIP: If using Direnv, the environment variable can also be added to the user’s personal `.envrc` so it is exported on switching to the project directory by appending with `echo 'export TOPIARY_CONFIG_FILE=".topiary.ncl"' >> .envrc`.
+
+Afterwards, `--merge-configuration` will always merge in the example configuration.
+Invoke Topiary’s formatting with
+
+```shell-session
+$ topiary format --merge-configuration source.ml
+```
+
+and/or configure your editor to run this command on saving the file.