diff --git a/main.janet b/main.janet
new file mode 100644
index 0000000..1d7aabd
--- /dev/null
+++ b/main.janet
@@ -0,0 +1,14 @@
+(defn diff [expr var]
+  (cond
+    (symbol? expr) (if (= expr var) 1 0)
+    (number? expr) 0
+    (match expr
+      [op & args] (case op
+        '+ ['+ ;(map |(diff $ var) args)]
+        '* (let [[a b] args]
+             ['+ ['* (diff a var) b]
+                        ['* a (diff b var)]])
+        (errorf "unknown op: %v" op)))))
+
+(pp (diff '(+ (* x x) 3) 'x))
+(pp (diff '(+ x 3) 'x))