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# Player Monoids
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This is a small library for managing global player state, so that changes made
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from different mods do not result in unexpected behavior. The README gives an
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introduction to the mod, but you might want to reference API.md along the way.
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This mod, combined with playereffects, deprecates monoidal_effects.
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Global Player State
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===================
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Players have behavior-affecting state that can be modified through mods. A couple
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examples are physics overrides and armor groups. If more than one mod tries to
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change them, it can result in unexpected results.
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For example, a player could be
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under a speed boost effect from a playereffects effect, and then sleep in a bed.
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The bed sets the player's speed to 0, and sets it back to 1 when they get out.
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Because the beds mod had no way of knowing that the player was supposed to have
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a speed boost, it effectively removed it. One hack to "fix" it would be to save
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the player's speed and restore it on wakeup, but this would have its own problems
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if the effect wears off in bed. The beds mod would restore the boosted speed,
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which wouldn't be removed, since the effect already went away. Thus an exploit
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allowing a permanent (until log out) speed boost is introduced.
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Player Monoids manages this by creating layers (monoids) on top of player state,
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which can keep track of different changes and combine them usefully.
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Monoids
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=======
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Creation
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--------
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A monoid in Player Monoids is an interface to one piece of player state. For
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example, you could have one monoid covering physics overrides, and another
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covering fly privilege. You could define a speed monoids like this:
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```
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-- The values in my speed monoid must be speed multipliers (numbers).
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mymod.speed_monoid = player_monoids.make_monoid({
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combine = function(speed1, speed2)
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return speed1 * speed2
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end,
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fold = function(tab)
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local res = 1
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for _, speed in pairs(tab) do
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res = res * speed
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end
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end,
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identity = 1,
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apply = function(speed, player)
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local override = player:get_physics_override()
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override.speed = speed
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player:set_physics_override(override)
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end,
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on_change = function() return end,
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})
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```
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This says that two speed multipliers can be combined by multiplication, that
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1 can be used as a neutral element, and that the "interpretation" of the speed
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multiplier is to set the player's speed physics override to that value. It also
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says that nothing in particular needs to happen when the speed changes, other
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than applying the new speed multiplier.
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Use
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---
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To add or remove change through a monoid, you must use the ```add_change```
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and ```del_change``` methods. For example, you could speed the player up
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temporarily like this:
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```
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-- Zoom!
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local zoom_id = mymod.speed_monoid:add_change(some_player, 10)
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minetest.after(5,function() mymod.speed_monoid:del_change(some_player, zoom_id) end)
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```
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You could also specify a string ID to use, instead of the numerical one that
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is automatically provided:
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```
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-- Zoom Mk. II
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mymod.speed_monoid:add_change(some_player, 10, "mymod:zoom")
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minetest.after(5,function() mymod.speed_monoid:del_change(some_player, "mymod:zoom") end)
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```
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Reading Values
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--------------
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You can use ```monoid:value(player)``` to read the current value of the monoid,
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for that player. This could be useful if it doesn't just represent built-in
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player state. For example, it could represent gardening skill, and you might use
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it to calculate the chance of success when harvesting spices.
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Nesting Monoids
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---------------
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You may have already noticed one limitation of this design. That is, for each
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kind of player state, you can only combine state changes in one way. If the
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standard speed monoid combines speed multipliers by multiplication, you cannot
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change it to instead choose the highest speed multiplier. Unfortunately, there
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is currently no way change this - you will have to hope that the given monoid
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combines in a useful way. However, it is possible to manage a subset of the
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values in a custom way.
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Suppose that a speed monoid (```mymod.speed_monoid```) already exists, using
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multiplication, but you want to write a mod with speed boosts, and only apply
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the strongest boost. Most of it could be done the same way:
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```
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-- My speed boosts monoid takes speed multipliers (numbers) that are at least 1.
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newmod.speed_boosts = player_monoids.make_monoid({
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combine = function(speed1, speed2)
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return speed1 * speed2
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end,
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fold = function(tab)
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local res = 1
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for _, speed in pairs(tab) do
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res = res * speed
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end
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end,
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identity = 1,
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apply = ???
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on_change = function() return end,
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})
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```
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But we cannot just change the player speed in ```apply```, otherwise we will
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break compatibility with the original speed monoid! The trick here is to use
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the original monoid as a proxy for our effects.
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```
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apply = function(speed, player)
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mymod.speed_monoid:add_change(player, speed, "newmod:speed_boosts")
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end
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```
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This means the speed boosts we control can be limited to the strongest boost, but
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the resulting boost will still play nice with speed effects from other mods.
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You could even add another "nested monoid" just for speed maluses, that takes
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the worst speed drain and applies it as a multiplier.
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Standard Monoids
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================
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In the spirit of compatibility, this mod provides some canonical monoids for
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commonly used player state. They combine values in a way that should allow
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different mods to affect player state fairly. If you make another monoid handling
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the same state as one of these, you will break compatibility with any mods using
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the standard monoid.
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Physics Overrides
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-----------------
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These monoids set the multiplier of the override they are named after. All three
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take non-negative numbers as values and combine them with multiplication. They
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are:
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* ```player_monoids.speed```
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* ```player_monoids.jump```
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* ```player_monoids.gravity```
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Privileges
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----------
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These monoids set privileges that affect the player's ordinary gameplay. They
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take booleans as input and combine them with logical or. They are:
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* ```player_monoids.fly```
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* ```player_monoids.noclip```
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Other
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-----
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* ```player_monoids.collisionbox``` - Sets the player's collisionbox. Values are
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3D multiplier vectors, which are combined with component-wise multiplication.
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* ```player_monoids.visual_size``` - Sets the player's collisionbox. Values are
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2D multiplier vectors (x and y), which are combined with component-wise
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multiplication.
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Use with playereffects
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======================
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Player Monoids does not provide anything special for persistent effects with
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limited lifetime. By using monoids with Wuzzy's playereffects, you can easily
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create temporary effects that stack with each other. As an example, an effect
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that gives the player 2x speed:
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```
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local speed = player_monoids.speed
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local function apply(player)
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speed:add_change(player, 2, "mymod:2x_speed")
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end
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local function cancel(player)
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speed:del_change(player, "mymod:2x_speed")
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end
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local groups = { "mymod:2x_speed" }
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playereffects.register_effect_type("mymod:2x_speed", "2x Speed", groups, apply, cancel)
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```
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Note that this effect does NOT use the "speed" effect group. As long as other
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speed effects use the speed monoid, we do not want them to be cancelled, since
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the goal is to combine the effects together. It does use a singleton group to
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prevent multiple instances of the same effect. I think that playereffects require
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effects to belong to at least one group, but I am not sure.
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Caveats
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=======
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* If the global state managed by a monoid is modified by something other than
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the monoid, you will have the same problem as when two mods both independently
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try to modify global state without going through a monoid.
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* This includes playereffects effects that affect global player state without
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going through a monoid.
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* You will also get problems if you use multiple monoids to manage the same
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global state.
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* The order that different effects get combined together is based on key order,
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which may not be predictable. So you should try to make your monoids commutative
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in addition to associative, or at least not care if the order of two changes
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is swapped.
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