feat(ch18): add a factory of numerical data types

src/ch-18_composable_data_types/18.1_composable_data_types.test.js

@@ -0,0 +1,25 @@
+import {expect} from 'chai';
+
+describe("Composable custom Data Types", () => {
+    it("should describe a factory that returns instances (functions) of numerical data types", () => {
+        const t = value => {
+            const add = n => t(value + n);
+
+            return Object.assign(add, {
+                toString: () => `t(${value})`,
+                valueOf: () => value,
+            })
+        };
+
+        const [x, y, z] = [1, 2, 3];
+
+        // Identity
+        expect(t(x)(0).valueOf()).equal(1);
+        expect(t(x)(0).valueOf()).equal(t(x).valueOf());
+
+        // Associativity
+        expect(t(x)(t(y))(t(z)).valueOf()).equal(6);
+        expect(t(x)(t(y)(t(z))).valueOf()).equal(6);
+        expect(t(x)(t(y))(t(z)).valueOf()).equal(t(x)(t(y)(t(z))).valueOf());
+    });
+});