feat: test monad base

__tests__/monad/base.spec.ts

@@ -0,0 +1,29 @@
+import { Maybe, Just, Fun, lift, bind, pure } from '../../src';
+
+describe('monad base', () => {
+  describe('lift', () => {
+    test('returns applicative pure result', () => {
+      expect(lift(1).valueOf()).toEqual(pure(1).valueOf());
+    });
+  });
+
+  test('left identity', () => {
+    const fun: Fun<number, Maybe<number>> = (x: number) => new Just(x + 1);
+    const x = 1;
+    expect(bind(lift(x), fun).valueOf()).toEqual(fun(x).valueOf());
+  });
+
+  test('right identity', () => {
+    const stringJust: Maybe<string> = new Just('hello!');
+    expect(bind(stringJust, lift).valueOf()).toEqual(stringJust.valueOf());
+  });
+
+  test('associativity', () => {
+    const numberJust: Maybe<number> = new Just(1);
+    const funF: Fun<number, Maybe<number>> = (x: number) => new Just(x + 1);
+    const funG: Fun<number, Maybe<string>> = (x: number) => new Just(String(x));
+    const left = bind(bind(numberJust, funF), funG);
+    const right = bind(numberJust, (x) => bind(funF(x), funG));
+    expect(left.valueOf()).toEqual(right.valueOf());
+  });
+});