Add Is This A Directory Or A File as a reason til

reason/is-this-a-directory-or-a-file.md

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+# Is This A Directory Or A File?
+
+When compiling [ReasonML](https://reasonml.github.io/) natively, we have access to a variety of
+additional modules including the `Unix` module. We can interact with
+directories and files using functions on `Unix`.
+
+```reason
+let current_dir = Unix.opendir(Unix.getcwd());
+let first_file = Unix.readdir(current_dir);
+/* is first_file a directory or a file? */
+Unix.closedir(current_dir);
+```
+
+Here we open the current working directory, grab the first thing out of that
+directory -- maybe it's a file, maybe it's a directory, maybe it is
+something else. Lastly, we close the directory.
+
+```reason
+let current_dir = Unix.opendir(Unix.getcwd());
+let first_file = Unix.readdir(current_dir);
+
+switch(Unix.stat(first_file)) {
+| Unix.stats({ st_kind: Unix.S_REG }) => print_endline("Regular File")
+| Unix.stats({ st_kind: Unix.S_DIR }) => print_endline("Directory")
+| Unix.stats({ st_kind: Unix.S_LINK }) => print_endline("Link")
+| Unix.stats({ st_kind: Unix.S_SOCK }) => print_endline("Socket")
+| _ => print_endline("Something else")
+};
+
+Unix.closedir(current_dir);
+```
+
+There are a variety of kinds of files to switch on. Here, we are switching
+on _Regular Files_, _Directories_, _Links_, and _Sockets_. Everything else
+falls through.
+
+See the [`Unix` module docs](https://reasonml.github.io/api/Unix.html) for
+more details.