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

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_695 TILs and counting..._
+_696 TILs and counting..._
 
 ---
 
@@ -511,6 +511,7 @@ _695 TILs and counting..._
 - [Generate Starter Reason Projects](reason/generate-starter-reason-projects.md)
 - [Helping The Compiler Help Us With Variants](reason/helping-the-compiler-help-us-with-variants.md)
 - [Inline Component Styles With Reason React](reason/inline-component-styles-with-reason-react.md)
+- [Is This A Directory Or A File](reason/is-this-a-directory-or-a-file.md)
 - [Making Things Mutable](reason/making-things-mutable.md)
 - [Multi-Argument Functions As Syntactic Sugar](reason/multi-argument-functions-as-syntactic-sugar.md)
 - [Pattern Match On Exceptions](reason/pattern-match-on-exceptions.md)

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

@@ -0,0 +1,38 @@
+# 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.