Added frontend to the repo

shared/types.ts

@@ -0,0 +1,22 @@
+/**
+ * PhantomData akin to Rust, because sometimes you need to be smarter than
+ * the compiler.
+ */
+export class PhantomData<P> { private __PHANTOM__: P }
+
+/**
+ * Opaque type, similar to `opaque type` in Flow, or new types in Rust/C.
+ * These should be produced only by manually casting `t as Opaque<T, P>`.
+ *
+ * `P` can be anything as it's never actually used. Using strings is okay:
+ *
+ * ```
+ * type MyType = Opaque<number, 'MyType'>;
+ * ```
+ */
+export type Opaque<T, P> = T & PhantomData<P>;
+
+/**
+ * Just a readable shorthand for null-ish-able types, akin to `T?` in Flow.
+ */
+export type Maybe<T> = T | null | undefined;