Add Define Something Only Once as a clojure til

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_418 TILs and counting..._
+_419 TILs and counting..._
 
 ---
 
@@ -44,6 +44,7 @@ _418 TILs and counting..._
 - [Aggregation Using merge-with](clojure/aggregation-using-merge-with.md)
 - [Argument Requirements For A Function](clojure/argument-requirements-for-a-function.md)
 - [Combinations Of Items From A Sequence](clojure/combinations-of-items-from-a-sequence.md)
+- [Define Something Only Once](clojure/define-something-only-once.md)
 - [Evaluate One Liners With lein-exec](clojure/evaluate-one-liners-with-lein-exec.md)
 - [Expanding Macros](clojure/expanding-macros.md)
 - [Get The Value Of An Environment Variable](clojure/get-the-value-of-an-environment-variable.md)

clojure/define-something-only-once.md

@@ -0,0 +1,22 @@
+# Define Something Only Once
+
+Clojure provides [`defonce`](https://clojuredocs.org/clojure.core/defonce)
+which is a macro that defines something only once. Once a variable has been
+bound to a value, subsequent attempts to do `defounce` for that variable
+will go unevaluated. This will have no implications for how the `def`
+special form works.
+
+Here is an example:
+
+```clojure
+(defonce stuff 5)
+#'user/stuff
+user=> (defonce stuff "what")
+nil
+user=> stuff
+5
+user=> (def stuff "okay")
+#'user/stuff
+user=> stuff
+"okay"
+```