issue CatchTheTornado#404

 - test case for regular access of a list and object using 'at'
 - implementation of flow which detects our of bound error
   when using 'at' to access a lists element by its index

src/askvm/resources/core/__tests__/at.test.ts

@@ -16,7 +16,15 @@ function ask(code: string) {
 }
 
 describe(`at`, function () {
-  it(`should throw when trying to access undefined property`, async function () {
+  it(`Should retrieve a defined property from an object.`, async function () {
+    await expect(
+      ask(
+        `ask(const('cocoObject',object('title','Coco')),call(get('at'),get('cocoObject'),'title'))`
+      )
+    ).resolves.toStrictEqual(`Coco`);
+  });
+
+  it(`Should throw when trying to access an undefined property of an object.`, async function () {
     await expect(
       ask(
         `ask(const('myobject',object('title','Coco')),call(get('at'),get('myobject'),'description'))`
@@ -26,9 +34,21 @@ describe(`at`, function () {
     );
   });
 
-  it(`should throw when accessing index which is out of lists bounds.`, async function () {
+  it(`Should retrieve an element from a list.`, async function () {
+    await expect(
+      ask(
+        `ask(const('yummyList',list('Coco','Papaya','Mango')),call(get('at'),get('yummyList'),0))`
+      )
+    ).resolves.toStrictEqual(`Coco`);
+  });
+
+  it(`Should throw when accessing index which is out of lists bounds.`, async function () {
     await expect(
-      ask(`ask(const('myarray',array(1,2,3)),call(get('at'),get('myarray'),3))`)
-    ).rejects.toThrow(`Index out of bounds mate.`);
+      ask(
+        `ask(const('yummyList',list('coco','papaya','mango')),call(get('at'),get('yummyList'),3))`
+      )
+    ).rejects.toThrow(
+      `Sorry, but index 3 is out of bounds for a list with 3 elements.`
+    );
   });
 });

src/askvm/resources/core/at.ts

@@ -3,11 +3,19 @@ import { any, resource, typed } from '../../lib';
 export const at = resource({
   type: any,
   async resolver(listOrObject: any, key: any): Promise<any> {
-    // TODO what is list comes as listOrObject
-    // if the requested index is out of bounds, then 'out of bounds' error should be thrown
+    if (
+      Array.isArray(listOrObject) &&
+      Number.isInteger(key) &&
+      listOrObject.length <= key
+    )
+      throw Error(
+        'Sorry, but index ' +
+          key +
+          ' is out of bounds for a list with ' +
+          listOrObject.length +
+          ' elements.'
+      );
 
-    if (Array.isArray(listOrObject) && listOrObject.length < 0 + key)
-      throw Error('Index out of bounds.');
     if (
       listOrObject !== null &&
       typeof listOrObject === 'object' &&