AoC19 Day 2 optimization attemt

excerpt.hs

runProgramAt :: Int -> Memory -> Memory
runProgramAt pc m = let opcode = m M.! pc
                        left = m M.! (m M.! (pc + 1))
                        right = m M.! (m M.! (pc + 2))
                        result = m M.! (pc + 3)
                        next = pc + 4 in
        case opcode of
                -- 99 signals the end of execution, so just return the memory
                99 -> m
                -- 1 indicates addition (left + right -> result)
                1 -> runProgramAt next $ M.alter (Just (left + right)) result m
                -- 2 indicates multiplication (left * right -> result)
                2 -> runProgramAt next $ M.alter (Just (left * right)) result m
                -- otherwise, we return memory itself.
                otherwise -> m