Add Either types (#26)

index.ts

@@ -30,3 +30,5 @@ export type {
   AppConfig,
   RedisConfig,
 } from './src/config/configTypes'
+
+export type { Either } from './src/errors/either'

src/errors/either.ts

@@ -0,0 +1,18 @@
+type Left<T> = {
+  error: T
+  result?: never
+}
+
+type Right<U> = {
+  error?: never
+  result: U
+}
+
+/**
+ * Either is a functional programming type which is used to communicate errors happening in potentially recoverable scenarios.
+ * It can return either an error (Left side) or a resolved result (Right side), but not both.
+ * It is up to caller of the function to handle received error or throw (Public)NonRecoverableError if it cannot.
+ *
+ * @see {@link https://antman-does-software.com/stop-catching-errors-in-typescript-use-the-either-type-to-make-your-code-predictable Further reading on motivation for Either type}
+ */
+export type Either<T, U> = NonNullable<Left<T> | Right<U>>