public function test_it_should_concat_content_of_two_files_only_when_files_exists()
{
$concatF = f\liftM2(function ($first, $second) {
return $first . $second;
});
$concat = $concatF(read(__FILE__), read('aaa'));
$this->assertInstanceOf(Either\Left::class, $concat);
$this->assertEquals(e\left('File "aaa" does not exists'), $concat);
}