Added solutions to 04

Author: mattpocock

exercises/04-isValidEmail.exercise.ts

@@ -264,8 +264,24 @@ export const onSubmit = async (values: { email: string }) => {
  * for plenty of type magic on their own, but when combined with opaque types
  * that can be extremely useful.
  *
- * 🕵️‍♂️ Discuss what you might use opaque types for. Some ideas:
+ */
+
+/**
+ * 🕵️‍♂️ Stretch goal 1:
+ *
+ * Create two Opaque types - one called PostId, another called UserId.
+ * Create two functions - getPostById and getUserById. Make each only
+ * take their specific types of id as a parameter.
+ *
+ * Try calling getUserById("123"); You'll notice that you get an error:
+ *
+ * Argument of type 'string' is not assignable to parameter of type 'UserId'.
+ *
+ * Discuss the best way to solve this. You could consider:
+ *
+ * A type predicate.
+ * Casting using as.
+ * An assertion function.
  *
- * - Ensuring a user exists in the database before acting on it
- * - Separating out different types of entity ids (PostId, UserId)
+ * Solution #1
  */

exercises/04-isValidEmail.solutions.ts

@@ -0,0 +1,36 @@
+/**
+ * #1 below:
+ */
+
+export type Opaque<TValue, TOpaque> = TValue & {
+  __: TOpaque;
+};
+
+type PostId = Opaque<string, "PostId">;
+type UserId = Opaque<string, "UserId">;
+
+const getUserById = (id: UserId) => {};
+const getPostById = (id: PostId) => {};
+
+// Method 1 - assertion function
+
+function assertIsUserId(id: any): asserts id is UserId {}
+
+const id = "123";
+assertIsUserId(id);
+getUserById(id);
+
+// Method 2 - type predicate
+
+function isUserId(id: any): id is UserId {
+  return true;
+}
+
+const id2 = "123";
+if (isUserId(id2)) {
+  getUserById(id2);
+}
+
+// Method 3 - casting
+
+getUserById("123" as UserId);