String & list concatenation

src/interpreter.ts

@@ -188,8 +188,21 @@ function evaluateBuiltIn(op: string, left: Value, right: Value): Value {
         return evaluate(right.body, callEnv);
     }
 
-    if (op === '&')
-        throw new Error('TODO Not implemented yet');
+    if (op === '&') {
+        if (left.kind == 'list' && right.kind === 'list') {
+            return {
+                kind: 'list',
+                elements: [...left.elements, ...right.elements]
+            };
+        }
+
+        if (left.kind == 'string' && right.kind === 'string') {
+            return {
+                kind: 'string',
+                value: left.value + right.value
+            };
+        }
+    }
 
     throw new Error(`Unknown built-in: ${op}`);
 }

src/main.ts

@@ -19,3 +19,5 @@ e('c = 5; { a = 3, b = c }');
 e('rec = { a = 3, b = 5 }; rec.a');
 e('rec = { a = 3, b = 5 }; rec{ a = 10 }');
 e('add1 = (x \\ x + 1); 3 > add1');
+e('[1, 2] & [3, 4]');
+e('"abc" & "def"');