Jan 22 (Reuters) - Canada’s main stock index opened lower on Friday, reversing almost all of its weekly gains, as upbeat domestic retail sales data failed to offset the weakness in energy and materials stocks. * At 9:31 a.m. ET (1431 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 104.32 points, or 0.58%, at 17,811.88. (Reporting by Amal S in Bengaluru; Editing by Ramakrishnan M.)