Solutions

Author: ericelliott

source/index.js

@@ -0,0 +1,28 @@
+// (f . g)(x) = f(g(x))
+// compose(...fns) => x => y
+const pipe = (...fns) => x => fns.reduce((y, f) => f(y), x);
+
+export const flying = o => {
+  let isFlying = false;
+
+  return Object.assign({}, o, {
+    fly () {
+      isFlying = true;
+      return this;
+    },
+    isFlying: () => isFlying,
+    land () {
+      isFlying = false;
+      return this;
+    }
+  });
+};
+
+export const quacking = quack => o => Object.assign({}, o, {
+  quack: () => quack
+});
+
+export const createDuck = quack => pipe(
+  flying,
+  quacking(quack)
+)({});

source/index.test.js

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+import test from 'tape';
+
+import { flying, createDuck } from './index';
+
+// What is a functional mixin?
+/*
+flying(o) => m: o & {
+  fly() => m,
+  isFlying() => Boolean,
+  land() => m
+}
+*/
+test('flying.fly() and .isFlying()', assert => {
+  const msg = 'should set isFlying() to true';
+  const o = flying({});
+
+  const actual = o.fly().isFlying();
+  const expected = true;
+
+  assert.same(actual, expected, msg);
+  assert.end();
+});
+
+test('flying.land()', assert => {
+  const msg = 'should set isFlying() to false';
+  const o = flying({}).fly();
+  assert.same(o.isFlying(), true, 'starts out flying');
+
+  const actual = o.land().isFlying();
+  const expected = false;
+
+  assert.same(actual, expected, msg);
+  assert.end();
+});
+
+test('flying() mixin', assert => {
+  const msg = 'should mix into existing object';
+  const startingObject = { a: 'a' };
+
+  const actual = flying(startingObject).a;
+  const expected = 'a';
+
+  assert.same(actual, expected, msg);
+  assert.end();
+});
+
+test('createDuck()', assert => {
+  const msg = 'should have FLYING and quacking';
+  const quack = 'QUACK!';
+  const duck = createDuck(quack);
+
+  const actual = duck.fly().quack();
+  const expected = quack;
+
+  assert.same(actual, expected, msg);
+  assert.end();
+});
+
+// When should you use functional mixins?
+// * state / data privacy