From 27f8c00210e526ec2c5197be3d3309f0bc8182c0 Mon Sep 17 00:00:00 2001
From: Hugo Saracino <hugo@inato.com>
Date: Tue, 8 Dec 2020 16:52:47 +0100
Subject: [PATCH] =?UTF-8?q?=E2=9C=A8=20Add=20an=20exercise=200=20for=20pip?=
 =?UTF-8?q?e=20and=20flow?=
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---
 src/exo0/exo0.test.ts | 48 +++++++++++++++++++++++++++++
 src/exo0/exo0.ts      | 71 +++++++++++++++++++++++++++++++++++++++++++
 src/exo0/index.ts     |  1 +
 3 files changed, 120 insertions(+)
 create mode 100644 src/exo0/exo0.test.ts
 create mode 100644 src/exo0/exo0.ts
 create mode 100644 src/exo0/index.ts

diff --git a/src/exo0/exo0.test.ts b/src/exo0/exo0.test.ts
new file mode 100644
index 0000000..fafec90
--- /dev/null
+++ b/src/exo0/exo0.test.ts
@@ -0,0 +1,48 @@
+import { isOddF, isOddP, next, next3 } from './exo0';
+
+describe('exo0', () => {
+  describe('isOddP', () => {
+    it('should return true if the provided number is odd', () => {
+      const oddValue = 1;
+
+      expect(isOddP(oddValue)).toBe(true);
+    });
+
+    it('should return false if the provided number is even', () => {
+      const oddValue = 2;
+
+      expect(isOddP(oddValue)).toBe(false);
+    });
+  });
+
+  describe('isOddF', () => {
+    it('should return true if the provided number is odd', () => {
+      const oddValue = 1;
+
+      expect(isOddF(oddValue)).toBe(true);
+    });
+
+    it('should return false if the provided number is even', () => {
+      const oddValue = 2;
+
+      expect(isOddF(oddValue)).toBe(false);
+    });
+  });
+
+  describe('next', () => {
+    it('should return the correct next element in the Collatz sequence', () => {
+      expect(next(4)).toBe(2);
+      expect(next(2)).toBe(1);
+      expect(next(1)).toBe(4);
+    });
+  });
+
+  describe('next3', () => {
+    it('should return the correct element in the Collatz sequence 3 steps ahead', () => {
+      expect(next3(12)).toBe(10);
+      expect(next3(10)).toBe(8);
+      expect(next3(8)).toBe(1);
+      expect(next3(1)).toBe(1);
+    });
+  });
+});
diff --git a/src/exo0/exo0.ts b/src/exo0/exo0.ts
new file mode 100644
index 0000000..6c7fb58
--- /dev/null
+++ b/src/exo0/exo0.ts
@@ -0,0 +1,71 @@
+// `fp-ts` training introduction
+// Composing computations with `pipe` and `flow`
+
+// Functional programming is all about composing small functions together like
+// lego bricks to build more and more complex computations.
+//
+// Strictly speaking, composing two functions `f` and `g` means applying the
+// result of the first one to the second one. By applying this composition
+// over and over you can chain multiple functions together.
+//
+// The `fp-ts` library provides to helpers to do that:
+// - `pipe` which first needs to be fed a value to start the pipe and then
+//   any number of functions to be applied sequentially.
+// - `flow` which is the same thing but were we do not have to provide the
+//   first value. It will then return a function which will expect that value
+//   to be provided
+//
+// ```ts
+// flow(f, g, h) === x => pipe(x, f, g, h)
+// pipe(x, f, g, h) === flow(f, g, h)(x)
+// ```
+//
+// NOTES:
+// - `flow(f) === f`
+// - `pipe(x, f) === f(x)`
+
+import { unimplemented } from '../utils';
+
+export const isEven = (value: number) => value % 2 === 0;
+
+export const not = (value: boolean) => !value;
+
+///////////////////////////////////////////////////////////////////////////////
+//                               PIPE & FLOW                                 //
+///////////////////////////////////////////////////////////////////////////////
+
+// For this exercise each function needs to be implemented using both forms:
+// - a `pipe` implementation (P suffix)
+// - a `flow` implementation (F suffix)
+
+// Using only the two helpers `isEven` and `not` at the top of this file (and
+// `pipe` or `flow`), write the function `isOdd` that checks if a number is
+// odd.
+
+export const isOddP: (value: number) => boolean = unimplemented;
+
+export const isOddF: (value: number) => boolean = unimplemented;
+
+// The Collatz conjecture: https://en.wikipedia.org/wiki/Collatz_conjecture
+//
+// We will write a function that for any given number, computes the next
+// one according to the following rules:
+// - if n is even => divide it by two
+// - if n is odd  => triple it and add one
+//
+// Below is the functional equivalent of the control flow statement if-else.
+
+export const ifThenElse = <A>(onTrue: () => A, onFalse: () => A) => (
+  condition: boolean,
+) => (condition ? onTrue() : onFalse());
+
+// Using `pipe` the function that computes the next step in the Collatz
+// sequence.
+
+export const next: (value: number) => number = unimplemented;
+
+// Using only `flow` and `next`, write the function that for any given number
+// a_n from the Collatz sequence, returns the number a_n+3 (ie. the number
+// three steps ahead in the sequence).
+
+export const next3: (value: number) => number = unimplemented;
diff --git a/src/exo0/index.ts b/src/exo0/index.ts
new file mode 100644
index 0000000..a862d82
--- /dev/null
+++ b/src/exo0/index.ts
@@ -0,0 +1 @@
+export * from './exo0';