HopperMon

HopperMon = {
    hopper = peripheral.wrap("right"),
    modem = rednet.open("top"),
    sorterCompID = 5,
}

function HopperMon:checkHopper()
    local size = self.hopper.getSizeInventory()
    size = size - 1 -- slot number starts at 0 so subtract 1

    for slot = 0, size do
        local slotContents = self.hopper.getStackInSlot(slot)
        if slotContents == nil then
            return true
        end
    end

    return false
end

function HopperMon:hasSpace()
    local boolhasSpace = self:checkHopper()
    local hasSpace = ""
    if boolhasSpace then
        print("has space")
        hasSpace = "space"
    else
        print("has no space")
        hasSpace = "noSpace"
    end
    local message = '' .. tostring(os.getComputerID()) .. ':' .. hasSpace
    print("Sending " .. message)
    rednet.send(self.sorterCompID, message)
end

hm = HopperMon

while true do
    event, param1, param2, param3 = os.pullEvent()
    if event == 'rednet_message' then -- number senderid, string message, number distance
        if param1 == hm.sorterCompID then
            print(param2)
            if param2:find('hasSpace') then
                hm:hasSpace()
            end
        end
    end
end