sandwall builder

function placeline()
    while turtle.detectDown() ~= true do
        os.sleep(0.3)
        placesand()
    end
end

function placesand()
    if turtle.getItemCount(turtle.getSelectedSlot()) ~= 0 then
        turtle.placeDown()
    elseif turtle.getSelectedSlot() == 16 then
            os.shutdown()
    else do
        turtle.select(turtle.getSelectedSlot() + 1)
        end
    end
end
turtle.select(1)
while turtle.detect() ~= true do
    placeline()
    turtle.forward()
end