diff --git a/src/interpreter.ts b/src/interpreter.ts
index c2b4d9d..ea319ec 100644
--- a/src/interpreter.ts
+++ b/src/interpreter.ts
@@ -69,8 +69,12 @@ export function evaluate(ast: AST, env: Env): Value {
             };
 
         case 'let': {
-            const val = evaluate(ast.value, env);
-            const newEnv = new Map(env).set(ast.name, val);
+            const newEnv = new Map(env);
+
+            const val = evaluate(ast.value, newEnv);
+
+            newEnv.set(ast.name, val);
+
             return evaluate(ast.body, newEnv);
         }
 
diff --git a/src/main.ts b/src/main.ts
index 0703c19..b94d137 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -31,3 +31,9 @@ e('"abc" & "def"');
 e('n = 5; n | 5 \\ "five" | _ \\ "other"');
 e('list = [1, 2, 3]; list | [x, y, z] \\ x + y + z');
 e('point = {x = 5, y = 10}; point | {x = px, y = py} \\ px + py');
+
+e(`factorial = n \\ n
+  | 0 \\ 1
+  | n \\ n * factorial (n - 1);
+
+  factorial 5`)