From 9479b61c09795997896bfa8eeaa2597a3fec1a9d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C3=ABl=20Lemaire?= <michael@thunderk.net>
Date: Thu, 17 Oct 2019 23:26:50 +0200
Subject: [PATCH] Added "not"

---
 README.md         |  6 +++++-
 package.json      |  2 +-
 src/index.test.ts | 15 ++++++++++++++-
 src/index.ts      |  6 ++++++
 4 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index 2a27f5d..6d31bf7 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,7 @@ always()  // => true
 never()  // => false
 ```
 
-**and** and **or** combines predicates
+**and**, **or** and **not** allow to combine or negate predicates
 
 ```typescript
 const a = and((x: number) => x > 2, (x: number) => x < 5);
@@ -38,6 +38,10 @@ o(1)  // => true
 o(2)  // => false
 o(3)  // => false
 o(4)  // => true
+const n = not((x: number) => x == 1);
+n(0)  // => true
+n(1)  // => false
+n(2)  // => true
 ```
 
 **attr** gets an object's attribute:
diff --git a/package.json b/package.json
index fa0f3db..6afedd6 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "tk-functional",
-  "version": "0.2.0",
+  "version": "0.2.1",
   "description": "Typescript/Javascript helpers for functional-style programming",
   "source": "src/index.ts",
   "main": "dist/tk-functional.umd.js",
diff --git a/src/index.test.ts b/src/index.test.ts
index 6df142f..da1ff89 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -1,4 +1,4 @@
-import { always, and, attr, bool, cmp, fmap, identity, is, isinstance, never, nn, nnu, nop, nu, or, partial, pipe } from "./index";
+import { always, and, attr, bool, cmp, fmap, identity, is, isinstance, never, nn, nnu, nop, not, nu, or, partial, pipe } from "./index";
 
 describe(nop, () => {
   it("does nothing", () => {
@@ -52,6 +52,19 @@ describe(never, () => {
   });
 });
 
+describe(not, () => {
+  it("negates predicate", () => {
+    const iseven = (x: number) => x % 2 == 0;
+    const isodd = not(iseven);
+    expect(iseven(0)).toBe(true);
+    expect(isodd(0)).toBe(false);
+    expect(iseven(1)).toBe(false);
+    expect(isodd(1)).toBe(true);
+    expect(iseven(2)).toBe(true);
+    expect(isodd(2)).toBe(false);
+  });
+});
+
 describe(and, () => {
   it("combines predicates", () => {
     const f = and((x: number) => x % 2 == 0, (x: number) => x > 5);
diff --git a/src/index.ts b/src/index.ts
index eadd832..7ccdafa 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -48,6 +48,12 @@ export const always = (...args: any[]) => true
  */
 export const never = (...args: any[]) => false
 
+/**
+ * Negate a predicate
+ */
+export function not<A extends any[]>(predicate: (...args: A) => boolean): (...args: A) => boolean {
+  return (...args) => !predicate(...args);
+}
 
 /**
  * Apply a boolean "and" to merge predicates