Lua: left eval or

common/space_lua/eval.test.ts

@@ -45,7 +45,13 @@ Deno.test("Evaluator test", async () => {
   assertEquals(evalExpr(`true and false`), false);
   assertEquals(evalExpr(`true or false`), true);
   assertEquals(evalExpr(`not true`), false);
-
+  // Test eager evaluation of left operand
+  assertEquals(
+    evalExpr(
+      `true or (function() error("this should not be evaluated") end)()`,
+    ),
+    true,
+  );
   // Tables
   const tbl = evalExpr(`{3, 1, 2}`);
   assertEquals(tbl.get(1), 3);

common/space_lua/eval.ts

@@ -53,6 +53,22 @@ export function evalExpression(
       case "Nil":
         return null;
       case "Binary": {
+        if (e.operator === "or") {
+          // Special case: eagerly evaluate left before even attempting right
+          const left = evalExpression(e.left, env, sf);
+          if (left instanceof Promise) {
+            return left.then((left) => {
+              if (luaTruthy(left)) {
+                return left;
+              }
+              return evalExpression(e.right, env, sf);
+            });
+          } else if (luaTruthy(left)) {
+            return left;
+          } else {
+            return evalExpression(e.right, env, sf);
+          }
+        }
         const values = evalPromiseValues([
           evalExpression(e.left, env, sf),
           evalExpression(e.right, env, sf),