Add `is` predicate to provide custom validation functions (#55)

source/lib/predicates/predicate.ts

@@ -51,6 +51,20 @@ export class Predicate<T = any> {
 		return not(this);
 	}
 
+	/**
+	 * Test if the value matches a custom validation function. The validation function should return `true` if the value
+	 * passes the function. If the function either returns `false` or a string, the function fails and the string will be
+	 * used as error message.
+	 *
+	 * @param fn Validation function.
+	 */
+	is(fn: (value: T) => boolean | string) {
+		return this.addValidator({
+			message: (value, error) => error || `Expected \`${value}\` to pass custom validation function`,
+			validator: value => fn(value)
+		});
+	}
+
 	/**
 	 * Register a new validator.
 	 *

source/test/test.ts

@@ -8,6 +8,16 @@ test('not', t => {
 	t.throws(() => m('' as any, m.string.not.empty), '[NOT] Expected string to be empty, got ``');
 });
 
+test('is', t => {
+	const greaterThan = (max: number, x: number) => {
+		return x > max || `Expected \`${x}\` to be greater than \`${max}\``;
+	};
+
+	t.notThrows(() => m(1, m.number.is(x => x < 10)));
+	t.throws(() => m(1, m.number.is(x => x > 10)), 'Expected `1` to pass custom validation function');
+	t.throws(() => m(5, m.number.is(x => greaterThan(10, x))), 'Expected `5` to be greater than `10`');
+});
+
 test('reusable validator', t => {
 	const checkUsername = m.create(m.string.minLength(3));