feat(ch09): add inverse trace and flip

src/ch-09_curry-and-fn-composition/9.1_curry-and-fn-composition.js

@@ -7,6 +7,13 @@ export const trace = label => value => {
     return value;
 };
 
+export const traceInv = value => label => {
+    console.log(`${label}: ${value}`);
+    return value;
+};
+
 export const pipe = (...fns) => x => fns.reduce((acum, f) => f(acum), x);
 
 export const map = fn => mappable => mappable.map(fn);
+
+export const flip = fn => a => b => fn(b)(a);

src/ch-09_curry-and-fn-composition/9.1_curry-and-fn-composition.test.js

@@ -1,5 +1,5 @@
 import {assert, expect} from 'chai';
-import {compose, map, pipe, sum, trace} from "./9.1_curry-and-fn-composition";
+import {compose, flip, map, pipe, sum, trace, traceInv} from "./9.1_curry-and-fn-composition";
 import {spy} from "sinon";
 
 describe("Curry and Function Composition", () => {
@@ -92,5 +92,23 @@ describe("Curry and Function Composition", () => {
         const doubled = doubleAll(array);
 
         expect(doubled).to.deep.equal([2, 4, 6, 8, 10]);
-    })
+    });
+
+    it("should flip the order of two parameters in an inverse trace function", () => {
+        const f = a => a * 2;
+        const g = b => b + 1;
+
+        const flippedTrace = flip(traceInv);
+
+        const h = pipe(
+            g,
+            flippedTrace('after g'),
+            f,
+            flippedTrace('after f'),
+        );
+
+        expect(h(30)).to.equal(62);
+        assert(console.log.calledWith('after g: 31'), 'It is not after g: 31');
+        assert(console.log.calledWith('after f: 62'), 'It is not after f: 62');
+    });
 });