Add `not` predicate (#12)

source/lib/operators/not.ts

@@ -0,0 +1,22 @@
+import { Predicate, Validator, validatorSymbol } from '../predicates/predicate';
+
+/**
+ * Operator which inverts all the validations.
+ *
+ * @param predictate Predicate to wrap inside the operator.
+ */
+export const not = <T extends Predicate>(predicate: T) => {
+	predicate['addValidator'] = (validator: Validator<any>) => {		// tslint:disable-line:no-string-literal
+		const fn = validator.validator;
+		const message = validator.message;
+
+		validator.message = (x: any) => `[NOT] ${message(x)}`;
+		validator.validator = (x: any) => !fn(x);
+
+		predicate[validatorSymbol].push(validator);
+
+		return predicate;
+	};
+
+	return predicate;
+};

source/lib/predicates/predicate.ts

@@ -1,4 +1,5 @@
 import * as is from '@sindresorhus/is';
+import { not } from '../operators/not';
 
 export interface Validator<T> {
 	message(value: T): string;
@@ -12,6 +13,7 @@ export interface Context {
 export const validatorSymbol = Symbol('validators');
 
 export abstract class Predicate<T = any> {
+
 	constructor(
 		type: string,
 		private context: Context = { validators: [] }
@@ -26,6 +28,13 @@ export abstract class Predicate<T = any> {
 		return this.context.validators;
 	}
 
+	/**
+	 * Invert the following validators.
+	 */
+	get not() {
+		return not(this);
+	}
+
 	/**
 	 * Register a new validator.
 	 *

source/test/test.ts

@@ -0,0 +1,9 @@
+import test from 'ava';
+import * as m from '..';
+
+test('not', t => {
+	t.notThrows(() => m(1, m.number.not.infinite));
+	t.notThrows(() => m(1, m.number.not.infinite.greaterThan(5)));
+	t.notThrows(() => m('foo!', m.string.not.alphanumeric));
+	t.throws(() => m('', m.string.not.empty), '[NOT] Expected string to be empty, got ``');
+});