fix: introduce Maybe monad export

Author: patrickmichalina

src/modules/util/monads/index.ts

@@ -0,0 +1 @@
+export { Maybe } from './maybe'

src/modules/util/monads/maybe.ts

@@ -0,0 +1,41 @@
+// tslint:disable:no-this
+// tslint:disable-next-line:no-class
+export class Maybe<T> {
+  private constructor(private value?: T) {}
+
+  static some<T>(value: T) {
+    // tslint:disable-next-line:no-if-statement
+    if (!value) {
+      throw Error('Provided value must not be empty')
+    }
+    return new Maybe(value)
+  }
+
+  static none<T>() {
+    return new Maybe<T>(undefined)
+  }
+
+  static fromValue<T>(value: T) {
+    return value
+      ? Maybe.some<NonNullable<T>>(value as NonNullable<T>)
+      : Maybe.none<NonNullable<T>>()
+  }
+
+  getOrElse(defaultValue: T) {
+    return this.value === undefined ? defaultValue : this.value
+  }
+
+  map<R>(f: (wrapped: T) => R): Maybe<R> {
+    return this.value === undefined
+      ? Maybe.none<R>()
+      : Maybe.some(f(this.value))
+  }
+
+  doIfSome<R>(f: (wrapped: T) => R): void {
+    this.value && f(this.value)
+  }
+
+  flattenMap<R>(f: (wrapped: T) => Maybe<R>): Maybe<R> {
+    return this.value === undefined ? Maybe.none<R>() : f(this.value)
+  }
+}