Wal-Mart Stores Inc. announced Thursday that it is launching a mobile-payment function for stores in the United States. It said in a press release that certain stores will start accepting mobile payments from the Walmart app this month, and this service will be available across the U.S. within the first half of 2016. Walmart said the function is added to the existing Walmart app, which already allowed customers to check in when picking up online orders, refill pharmacy prescriptions and locate particular items in stories. It said the payment function can work with any major credit card,...