Memory Model

By default, variables in Lua scripts are not saved when the player saves the game. This means that all the values you have set up will be cleared if the player saves and loads. This can lead to problems with scripts that do the following:

local dest

function create ()
   dest = spob.get("Caladan")

   -- ...

   hook.land( "land" )
end

function land ()
   if spob.cur() == dest then -- This is wrong!
      -- ...
   end
end

In the above script, a variable called dest is created, and when the mission is created, it gets set to spob.get("Caladan"). Afterwards, it gets used in land which is triggered by a hook when the player lands. For this mission, the value dest will be set as long as the player doesn’t save and load. When the player saves and loads, the value dest gets set to nil by default in the first line. However, upon loading, the create function doesn’t get run again, while the hook is still active. This means that when the player lands, spob.cur() will be compared with dest will not have been set, and thus always be false. In conclusion, the player will never be able to finish the mission!

How do we fix this? The solution is the mission/event memory model. In particular, all mission / event instances have a table that gets set called mem. This table has the particular property of being persistent, i.e., even if the player saves and loads the game, the contents will not change! We can then use this table and insert values to avoid issues with saving and loading games. Let us update the previous code to work as expected with saving and loading.

function create ()
   mem.dest = spob.get("Caladan")

   -- ...

   hook.land( "land" )
end

function land ()
   if spob.cur() == mem.dest then
      -- ...
   end
end

We can see the changes are minimal. We no longer declare the dest variable, and instead of setting and accessing dest, we use mem.dest, which is the dest field of the mem persistent memory table. With these changes, the mission is now robust to saving and loading!

It is important to note that almost everything can be stored in the mem table, and this includes other tables. However, make sure to not create loops, or it will hang the saving of the games.

The most common use of the persistent memory table mem is variables that keep track of the mission progress, such as if the player has delivered cargo or has talked to a certain NPC.