Monoidally-managed Minetest state modifications
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README.md

Player Monoids

This is a small library for managing global player state, so that changes made from different mods do not result in unexpected behavior. The README gives an introduction to the mod, but you might want to reference API.md along the way. This mod, combined with playereffects, deprecates monoidal_effects.

Global Player State

Players have behavior-affecting state that can be modified through mods. A couple examples are physics overrides and armor groups. If more than one mod tries to change them, it can result in unexpected results.

For example, a player could be under a speed boost effect from a playereffects effect, and then sleep in a bed. The bed sets the player’s speed to 0, and sets it back to 1 when they get out. Because the beds mod had no way of knowing that the player was supposed to have a speed boost, it effectively removed it. One hack to “fix” it would be to save the player’s speed and restore it on wakeup, but this would have its own problems if the effect wears off in bed. The beds mod would restore the boosted speed, which wouldn’t be removed, since the effect already went away. Thus an exploit allowing a permanent (until log out) speed boost is introduced.

Player Monoids manages this by creating layers (monoids) on top of player state, which can keep track of different changes and combine them usefully.

Monoids

Creation

A monoid in Player Monoids is an interface to one piece of player state. For example, you could have one monoid covering physics overrides, and another covering fly privilege. You could define a speed monoids like this:

-- The values in my speed monoid must be speed multipliers (numbers).
mymod.speed_monoid = player_monoids.make_monoid({
	combine = function(speed1, speed2)
		return speed1 * speed2
	end,
	fold = function(tab)
		local res = 1
	     	for _, speed in pairs(tab) do
			res = res * speed
	     	end
	end,
	identity = 1,
	apply = function(speed, player)
		local override = player:get_physics_override()
		override.speed = speed
		player:set_physics_override(override)
	end,
	on_change = function() return end,
})

This says that two speed multipliers can be combined by multiplication, that 1 can be used as a neutral element, and that the “interpretation” of the speed multiplier is to set the player’s speed physics override to that value. It also says that nothing in particular needs to happen when the speed changes, other than applying the new speed multiplier.

Use

To add or remove change through a monoid, you must use the add_change and del_change methods. For example, you could speed the player up temporarily like this:

-- Zoom!
local zoom_id = mymod.speed_monoid:add_change(some_player, 10)

minetest.after(5,function() mymod.speed_monoid:del_change(some_player, zoom_id) end)

You could also specify a string ID to use, instead of the numerical one that is automatically provided:

-- Zoom Mk. II
mymod.speed_monoid:add_change(some_player, 10, "mymod:zoom")

minetest.after(5,function() mymod.speed_monoid:del_change(some_player, "mymod:zoom") end)

Reading Values

You can use monoid:value(player) to read the current value of the monoid, for that player. This could be useful if it doesn’t just represent built-in player state. For example, it could represent gardening skill, and you might use it to calculate the chance of success when harvesting spices.

Nesting Monoids

You may have already noticed one limitation of this design. That is, for each kind of player state, you can only combine state changes in one way. If the standard speed monoid combines speed multipliers by multiplication, you cannot change it to instead choose the highest speed multiplier. Unfortunately, there is currently no way change this - you will have to hope that the given monoid combines in a useful way. However, it is possible to manage a subset of the values in a custom way.

Suppose that a speed monoid (mymod.speed_monoid) already exists, using multiplication, but you want to write a mod with speed boosts, and only apply the strongest boost. Most of it could be done the same way:

-- My speed boosts monoid takes speed multipliers (numbers) that are at least 1.
newmod.speed_boosts = player_monoids.make_monoid({
	combine = function(speed1, speed2)
		return math.max(speed1, speed2)
	end,
	fold = function(tab)
		local res = 1
	     	for _, speed in pairs(tab) do
			res = math.max(res, speed)
	     	end
	end,
	identity = 1,
	apply = ???
	on_change = function() return end,
})

But we cannot just change the player speed in apply, otherwise we will break compatibility with the original speed monoid! The trick here is to use the original monoid as a proxy for our effects.

apply = function(speed, player)
      mymod.speed_monoid:add_change(player, speed, "newmod:speed_boosts")
end

This means the speed boosts we control can be limited to the strongest boost, but the resulting boost will still play nice with speed effects from other mods. You could even add another “nested monoid” just for speed maluses, that takes the worst speed drain and applies it as a multiplier.

Standard Monoids

In the spirit of compatibility, this mod provides some canonical monoids for commonly used player state. They combine values in a way that should allow different mods to affect player state fairly. If you make another monoid handling the same state as one of these, you will break compatibility with any mods using the standard monoid.

Physics Overrides

These monoids set the multiplier of the override they are named after. All three take non-negative numbers as values and combine them with multiplication. They are:

  • player_monoids.speed
  • player_monoids.jump
  • player_monoids.gravity

Privileges

These monoids set privileges that affect the player’s ordinary gameplay. They take booleans as input and combine them with logical or. They are:

  • player_monoids.fly
  • player_monoids.noclip

Other

  • player_monoids.collisionbox - Sets the player’s collisionbox. Values are 3D multiplier vectors, which are combined with component-wise multiplication.
  • player_monoids.visual_size - Sets the player’s collisionbox. Values are 2D multiplier vectors (x and y), which are combined with component-wise multiplication.

Use with playereffects

Player Monoids does not provide anything special for persistent effects with limited lifetime. By using monoids with Wuzzy’s playereffects, you can easily create temporary effects that stack with each other. As an example, an effect that gives the player 2x speed:

local speed = player_monoids.speed

local function apply(player)
  speed:add_change(player, 2, "mymod:2x_speed")
end

local function cancel(player)
  speed:del_change(player, "mymod:2x_speed")
end

local groups = { "mymod:2x_speed" }

playereffects.register_effect_type("mymod:2x_speed", "2x Speed", groups, apply, cancel)

Note that this effect does NOT use the “speed” effect group. As long as other speed effects use the speed monoid, we do not want them to be cancelled, since the goal is to combine the effects together. It does use a singleton group to prevent multiple instances of the same effect. I think that playereffects require effects to belong to at least one group, but I am not sure.

Caveats

  • If the global state managed by a monoid is modified by something other than the monoid, you will have the same problem as when two mods both independently try to modify global state without going through a monoid.
    • This includes playereffects effects that affect global player state without going through a monoid.
  • You will also get problems if you use multiple monoids to manage the same global state.
  • The order that different effects get combined together is based on key order, which may not be predictable. So you should try to make your monoids commutative in addition to associative, or at least not care if the order of two changes is swapped.