Pararules is the first RETE-based rules engine made for games, having broad applicability to other kinds of programs as well. Rules engines have been around since the 70s, but for some reason, they haven't found their way into games yet. With pararules, you can store the entire state of your game and express the logic as a simple series of independent rules.

On the surface, this bears a resemblence to ECSs (entity component systems), but a rules engine can be much more powerful. Rules are fundamentally reactive -- they fire when their data is updated, they can create derived facts that always remain up-to-date, and they can trigger other rules in turn.

You can see it in action in the parakeet example game and the other paranim examples. Also watch the FOSDEM 2022 talk.

Table of contents

• Start with the data
• Your first rule
• Updating the session from inside a rule
• Queries
• Rulesets
• Avoiding infinite loops
• Conditions
• Complex types
• Joins
• Generating ids
• Derived facts
• Serializing a session
• Performance
• Tips
• Wrap up
• Comparison to Clara Rules
• Acknowledgements

Start with the data

Your data is stored as (id, attribute, value) tuples. For example, the player's X position might be (Player, X, 100.0). The delta time (which is the number of seconds since the last frame) might be (Global, DeltaTime, 0.0168121).

To do this, you need to first define your id and attribute enums, like this:

type
  Id = enum
    Global, Player,
  Attr = enum
    DeltaTime, TotalTime,
    X, Y,

Then, you use the schema macro, which receives these two enums and defines what type the value will have for each attribute:

schema Fact(Id, Attr):
  DeltaTime: float
  TotalTime: float
  X: float
  Y: float

Underneath, this creates a new type called Fact which in Nim is called an object variant because it can store multiple types inside.

Your first rule

Let's start by just making a rule that prints out the TotalTime whenever it updates:

# create rule
let rule1 =
  rule printTime(Fact):
    what:
      (Global, TotalTime, tt)
    then:
      echo tt

# create session and add rule
var session = initSession(Fact)
session.add(rule1)

The most important part of a rule is the what block, which specifies what tuples must exist for the rule to fire. The key is that you can create a binding in the id or value column by supplying a symbol that starts with a lowercase letter, like tt above. When the rule fires, the then block is executed, which has access to the bindings you created.

In your game loop, you can then insert the time values:

session.insert(Global, DeltaTime, game.deltaTime)
session.insert(Global, TotalTime, game.totalTime)

The nice thing is that, if you insert something whose id + attribute combo already exists, it will simply replace it. So you can safely insert these values in your game loop without worrying that it will fill up with stale data.

Updating the session from inside a rule

Now imagine you want to make the player move to the right every time the frame is redrawn. Your rule might look like this:

let rule1 =
  rule movePlayer(Fact):
    what:
      (Global, TotalTime, tt)
    then:
      session.insert(Player, X, tt)

var session = initSession(Fact)
session.add(rule1)

This may look weird, because session is defined after the rule, but somehow the rule can still use it. This is because then blocks have an implicit session variable, which refers to whatever session the rule is part of. You should always use this when updating the session from within a rule.

Queries

Updating the player's X attribute isn't useful unless we can get the value externally to render it. To do this, make another rule that binds the values you would like to receive:

let rule2 =
  rule getPlayer(Fact):
    what:
      (Player, X, x)
      (Player, Y, y)

session.add(rule2)

As you can see, rules don't need a then block if you're only using them to query from the outside. In this case, we'll query it in our game loop and we'll get back a tuple whose fields have the names you created as bindings:

let player = session.query(rule2)
echo player.x, " ", player.y

Be careful with query, though. If there are no results, this will cause an exception. You can use queryAll instead:

let results = session.queryAll(rule2)
if results.len > 0:
  let player = results[0]
  echo player.x, " ", player.y

Rulesets

As a convenience, you can define your rules together like this:

let rules =
  ruleset:
    rule movePlayer(Fact):
      what:
        (Global, TotalTime, tt)
      then:
        session.insert(Player, X, tt)
    rule getPlayer(Fact):
      what:
        (Player, X, x)
        (Player, Y, y)

Then you can add them to a session all at once:

for r in rules.fields:
  session.add(r)

The rules value is a tuple whose fields have the same names as the rules' names. This makes it nice to run queries:

let player = session.query(rules.getPlayer)
echo player.x, " ", player.y

Avoiding infinite loops

Imagine you want to move the player's position based on its current position. So instead of just using the total time, maybe we want to add the delta time to the player's latest X position:

rule movePlayer(Fact):
  what:
    (Global, DeltaTime, dt)
    (Player, X, x)
  then:
    session.insert(Player, X, x + dt)

If you try this, it will raise an expection because it hit the recursion limit. That's because you just created an infinite loop. The rule requires the player's X position and then updates it, which then causes the rule to fire again. The simple solution is to tell pararules that a certain tuple should not cause a rule's then block to re-fire when it updates:

rule movePlayer(Fact):
  what:
    (Global, DeltaTime, dt)
    (Player, X, x, then = false)
  then:
    session.insert(Player, X, x + dt)

Conditions

Rules have nice a way of breaking apart your logic into independent units. If we want to prevent the player from moving off the right side of the screen, we could add a condition inside of the then block of movePlayer, but it's good to get in the habit of making separate rules.

To do so, we need to start storing the window size in the session. First, add the attributes:

type
  Id = enum
    Global, Player,
  Attr = enum
    DeltaTime, TotalTime,
    X, Y,
    WindowWidth, WindowHeight,

schema Fact(Id, Attr):
  DeltaTime: float
  TotalTime: float
  X: float
  Y: float
  WindowWidth: int
  WindowHeight: int

Then, wherever your window resize happens, insert the values:

proc windowResized*(width: int, height: int) =
  session.insert(Global, WindowWidth, width)
  session.insert(Global, WindowHeight, height)

Finally, we make the rule:

rule stopPlayer(Fact):
  what:
    (Player, X, x)
    (Global, WindowWidth, windowWidth)
  then:
    if x >= float(windowWidth):
      session.insert(Player, X, 0.0)

Notice that we don't need then = false this time, because the condition is preventing the rule from re-firing (unless the windowWidth is 0, that is!).

While the above code works, you can also put your condition in a special cond block:

rule stopPlayer(Fact):
  what:
    (Global, WindowWidth, windowWidth)
    (Player, X, x)
  cond:
    x >= float(windowWidth)
  then:
    session.insert(Player, X, 0.0)

You can add as many conditions as you want, and they will implicitly work as if they were combined together with and:

rule stopPlayer(Fact):
  what:
    (Global, WindowWidth, windowWidth)
    (Player, X, x)
  cond:
    x >= float(windowWidth)
    windowWidth > 0
  then:
    session.insert(Player, X, 0.0)

Using a cond block is better because it also affects the results of queryAll -- matches that didn't pass the conditions will not be included. Also, in the future I'll probably be able to create more optimal code because it will let me run those conditions earlier in the network.

Complex types

Pararules is not limited to storing scalar types like float and int — you can use any type you want. For example, to store the currently-pressed keys, you can import the sets module and use a HashSet[int] to store them. However, you need to use a type alias when specifying it in the schema:

type
  Id = enum
    Global, Player,
  Attr = enum
    DeltaTime, TotalTime,
    X, Y,
    WindowWidth, WindowHeight,
    PressedKeys,
  IntSet = HashSet[int]

schema Fact(Id, Attr):
  DeltaTime: float
  TotalTime: float
  X: float
  Y: float
  WindowWidth: int
  WindowHeight: int
  PressedKeys: IntSet

If you put HashSet[int] directly in the schema macro, it will fail, because it wants a simple name to refer to it by. Also note that if you create two type aliases that point to the same type, the schema macro will complain...so don't do that. These are just implementation restrictions right now.

When you initialize your game, you can put an empty set in it:

session.insert(Global, PressedKeys, initHashSet[int]())

Then add a rule to get the keys:

rule getKeys(Fact):
  what:
    (Global, PressedKeys, keys)

Wherever your key up/down events are, you can update the keys:

proc keyPressed*(key: int) =
  var (keys) = session.query(rules.getKeys)
  keys.incl(key)
  session.insert(Global, PressedKeys, keys)

proc keyReleased*(key: int) =
  var (keys) = session.query(rules.getKeys)
  keys.excl(key)
  session.insert(Global, PressedKeys, keys)

You can see we are destructuring the keys out of the tuple, modifying it, and then updating PressedKeys with it. Now we can modify movePlayer to change the player's X position based on the arrow keys:

rule movePlayer(Fact):
  what:
    (Global, DeltaTime, dt)
    (Global, PressedKeys, keys, then = false)
    (Player, X, x, then = false)
  then:
    if keys.contains(263): # left arrow
      session.insert(Player, X, x - 1.0)
    elif keys.contains(262): # right arrow
      session.insert(Player, X, x + 1.0)

Notice that we aren't even using DeltaTime anymore, but we're keeping it in the what block so the rule continues to fire every frame. If all tuples in the what block have then = false, it will never fire. This way, it will move the player exactly one pixel each frame that an arrow key is pressed. Not exactly the best way to do character movement, but you get the idea.

Joins

Instead of the getPlayer rule, we could make a more generic "getter" rule that works for any id:

rule getCharacter(Fact):
  what:
    (id, X, x)
    (id, Y, y)

Now, we're making a binding on the id column, and since we're using the same binding symbol ("id") in both, pararules will ensure that they are equal, much like a join in SQL.

We can query it just as before, but if you want to make sure it returns the data for Player, you can add that as a filter to the query:

let player = session.query(rules.getCharacter, id = Player)
echo player.x, " ", player.y

You can also use queryAll to retrieve all results and filter afterwards:

let chars = session.queryAll(rules.getCharacter)
for c in chars:
  if c.id == Player.ord:
    echo c.x, " ", c.y

Generating ids

In addition to known ids like Player, it is likely that you'll want to generate ids at runtime. For example, if you just want to spawn random enemies, you probably don't want to create a special id in the enum for each one. For this reason, insert allows you to just pass arbitrary integers as ids:

var nextId = Id.high.ord + 1

for _ in 0 ..< 5:
  session.insert(nextId, X, rand(50.0))
  session.insert(nextId, Y, rand(50.0))
  nextId += 1

By starting the ids at Id.high.ord + 1, we begin using the first integer after Player. This is important, because if you use anything lower, it'll overlap with one of the known ids:

# these modify the same id
session.insert(1, X, rand(50.0))
session.insert(Player, X, rand(50.0))

This is also why the following is a compile error:

rule getCharacter(Fact):
  what:
    (id, X, x)
    (id, Y, y)
  cond:
    id != Player # type mismatch: got <int, Id>

You need to write id != Player.ord instead, because pararules transforms all ids to normal integers and gives them back that way.

Derived facts

Sometimes we want to make a rule that receives a collection of facts. This is done by creating facts that are derived from other facts.

If you want to create a fact that contains all characters, one clever way to do it is to run a query in the getCharacter rule, and insert the result as a new fact:

rule getCharacter(Fact):
  what:
    (id, X, x)
    (id, Y, y)
  then:
    let chars = session.queryAll(this)
    session.insert(Derived, AllCharacters, chars)
rule printAllCharacters(Fact):
  what:
    (Derived, AllCharacters, chars)
  then:
    echo "All characters: ", chars

The special this symbol refers to the rule itself, so it's the same as running session.queryAll(rules.getCharacter). Every time any character is updated, the query is run again and the derived fact is updated.

For this to work, we need to add a few things to the types at the top:

type
  # update the enums
  Id = enum
    # ...
    Derived
  Attr = enum
    # ...
    AllCharacters
  # define a type to store the query results
  Characters = seq[tuple[id: int, x: float, y: float]]

# update the schema
schema Fact(Id, Attr):
  # ...
  AllCharacters: Characters

When we insert our random enemies, it seems to work:

var nextId = Id.high.ord + 1

for _ in 0 ..< 5:
  session.insert(nextId, X, rand(50.0))
  session.insert(nextId, Y, rand(50.0))
  nextId += 1
# All characters: @[(id: 3, x: 9.394868845262195, y: 25.43175850160218), (id: 4, x: 11.17467397291547, y: 41.71452255154022), (id: 7, x: 30.03014845607234, y: 48.31479800236611), (id: 5, x: 0.5064720243960097, y: 27.21672010097956), (id: 6, x: 8.553239146114377, y: 9.315216984770858)]

But what happens if we retract one?

session.retract(3, X)
session.retract(3, Y)

It didn't print, which means the AllCharacters fact hasn't been updated! This is because then blocks only run on insertions, not retractions. After all, if facts pertinent to a rule are retracted, the match will be incomplete, and there will be nothing to bind the symbols from the what block to.

The solution is to use thenFinally:

rule getCharacter(Fact):
  what:
    (id, X, x)
    (id, Y, y)
  thenFinally:
    let chars = session.queryAll(this)
    session.insert(Derived, AllCharacters, chars)

A thenFinally block runs when a rule's matches are changed at all, including from retractions. This also means you won't have access to the bindings from the what block, so if you want to run code on each individual match, you need to use a normal then block before it.

One last thing: it is highly recommended that you use reference types like ref seq, rather than value types like seq, unless you are sure they will be small. Many copies of facts are made inside the session, so this could be a significant performance problem if you store collections that copy by value.

The example above could be changed to declare the Characters type like this:

type
  # ...
  Characters = ref seq[tuple[id: int, x: float, y: float]]

And then the rule could be changed like this:

rule getCharacter(Fact):
  what:
    (id, X, x)
    (id, Y, y)
  thenFinally:
    var chars: Characters = nil
    new chars
    chars[] = session.queryAll(this)
    session.insert(Derived, AllCharacters, chars)

Serializing a session

To save a session to the disk or send it over a network, we need to serialize it somehow. It wouldn't be a good idea to directly serialize the session. It would prevent you from updating your rules, or possibly even the version of this library, due to all the implementation details contained in the session after deserializing it.

Instead, it makes more sense to just serialize the facts. There is an overload of queryAll that returns a sequence of all the individual facts that were inserted:

let facts = session.queryAll()
echo facts
# @[(id: 2, attr: 13, value: Fact(...)), (id: 7, attr: 3, value: Fact(...)), (id: 5, attr: 2, value: Fact(...)), (id: 4, attr: 3, value: Fact(...)), (id: 6, attr: 3, value: Fact(...)), (id: 5, attr: 3, value: Fact(...)), (id: 6, attr: 2, value: Fact(...)), (id: 4, attr: 2, value: Fact(...)), (id: 3, attr: 3, value: Fact(...)), (id: 7, attr: 2, value: Fact(...)), (id: 3, attr: 2, value: Fact(...))]

As you can see, it returns the data as (int, int, Fact) tuples. The id and attr have been converted to integers, but you can cast them back to enums:

let fact = facts[0]
echo Id(fact.id)
echo Attr(fact.attr)

The value is of the type we defined with the schema macro, which is an object variant. You can pull out the value within it using unwrap:

for fact in facts:
  case Attr(fact.attr):
    of X, Y:
      echo unwrap(fact.value, float)
    of AllCharacters:
      echo unwrap(fact.value, Characters)
    else:
      discard

Notice that it includes the AllCharacters derived fact that we made before. There is no need to serialize derived facts -- they can be derived later, so it's a waste of space. We can filter them out before serializing:

var primaryFacts: seq[(int, int, Fact)]
for fact in facts:
  if fact.id != Derived.ord:
    primaryFacts.add(fact)

How you go about serializing the facts is up to you. When you deserialize them later, simply insert them into your session again.

Keep in mind that, if you save the ids and attrs as integers, they need to remain stable -- that is, you can't move items around in those enums, because they won't match up when you deserialize your facts later.

Performance

Disabling auto fire

By default, rules are fired after every insert call. This can be inefficient; you should normally only fire the rules once per frame. You can disable this when creating your session:

var session = initSession(Fact, autoFire = false)

Then, you need to explicitly fire the rules:

session.insert(Global, DeltaTime, game.deltaTime)
session.insert(Global, TotalTime, game.totalTime)
session.fireRules()

Using staticRuleset

Additionally, a very significant performance gain can be had by taking advantage of Nim's macro system to define your rules "statically":

let (initSession, rules) =
  staticRuleset(Fact, FactMatch):
    rule getPlayer(Fact):
      what:
        (Player, X, x)
        (Player, Y, y)
    rule movePlayer(Fact):
      what:
        (Global, DeltaTime, dt)
        (Global, PressedKeys, keys, then = false)
        (Player, X, x, then = false)
      then:
        if keys.contains(263): # left arrow
          session.insert(Player, X, x - 1.0)
        elif keys.contains(262): # right arrow
          session.insert(Player, X, x + 1.0)

var session: Session[Fact, FactMatch] = initSession(autoFire = false)

for r in rules.fields:
  session.add(r)

This is not a superficial syntax change; there is a very big internal difference. Since staticRuleset knows all of your session's rules at compile time, it is able to generate a special type to store the matches (in the example above, this type is called FactMatch). Normally, matches are stored in tables, which are significantly slower.

You must call staticRuleset at the top-level of your module because it's creating exported types. As arguments, it receives the base and match type and then the rules. It returns a tuple containing a special initSession proc as well as the rules tuple (similar to what you'd get from ruleset).

There are a few downsides:

1. You will not be able to add other rules later, because it must know all of its rules at compile time.
2. Compile times will slow down as more rules are added.

You may think that an additional downside is that you now must define all your rules in one place, without the flexibility to separate them into different modules. This actually isn't true: you can define your rules separately and use a "wrapper" macro to put them into staticRuleset at compile time. This is a bit advanced, but see the test for an example of this.

Using ref types

As mentioned before, it is highly recommended that you use reference types like ref seq, rather than value types like seq, to store collections in the session. Many copies of facts are made inside the session, so this could be a significant performance problem if you store collections that copy by value.

For example, to store a sequence of tuples as a fact, it might look like this:

type Characters = ref seq[tuple[id: int, x: float, y: float]]
var chars: Characters = nil
new chars
chars[] = session.queryAll(rules.getCharacter)
session.insert(Derived, AllCharacters, chars)

Using derived facts

In any non-trivial project, you'll end up with a lot of rules that share some common tuples in their what blocks, like this:

let rules =
  ruleset:
    rule getCharacter(Fact):
      what:
        (id, X, x)
        (id, Y, y)
    rule moveCharacter(Fact):
      what:
        (Global, DeltaTime, dt)
        (id, X, x, then = false)
        (id, Y, y, then = false)
      then:
        session.insert(id, X, x + dt)
        session.insert(id, Y, y + dt)

Here we have a getCharacter rule whose only purpose is for queries, and a moveCharacter rule that modifies it when the timestamp is updated. In both cases, there is a join on the id binding. Joins are not free -- they have a runtime cost, and in this case, that cost is paid twice.

In theory, this could be solved using a common optimization in rules engines calling node sharing. However, node sharing adds a lot of complexity to the codebase, and if it is automatic, it can be easily lost when subtle changes are made to a rule -- a regression that isn't easy to notice.

It turns out that a feature we've already discussed can solve this: derived facts. The above example can be rewritten like this:

let rules =
  ruleset:
    rule getCharacter(Fact):
      what:
        (id, X, x)
        (id, Y, y)
      then:
        session.insert(id, Character, match)
    rule moveCharacter(Fact):
      what:
        (Global, DeltaTime, dt)
        (id, Character, ch, then = false)
      then:
        session.insert(id, X, ch.x + dt)
        session.insert(id, Y, ch.y + dt)

With match we can get all the bindings in a convenient tuple, such as (id: 1, x: 10.0, y: 5.0). We then insert it as a derived fact, and bring it into the moveCharacter rule (the Character attribute, of course, must be added to the Attr enum and its type must be defined in the schema).

This will be faster because we now are only doing the join once, and all subsequent rules are just using the derived fact. As the number of joined tuples gets larger, the performance difference gets more and more substantial.

Tips

In then blocks, you can use the special match symbol to access all the bound values in a single tuple:

var session = initSession(Fact)

session.add:
  rule getCharacter(Fact):
    what:
      (id, X, x)
      (id, Y, y)
    then:
      echo match # (id: 1, x: 10.0, y: 25.0)

session.insert(Player, X, 10.0)
session.insert(Player, Y, 25.0)

If you're trying to debug a cond block, keep in mind that you can put whatever arbitrary code you want in there. For example, you can make it print out the values by creating a new scope with block, as long as the condition itself is the last thing in that scope:

rule stopPlayer(Fact):
  what:
    (Global, WindowWidth, windowWidth)
    (Player, X, x)
  cond:
    block:
      echo x, " ", windowWidth # this is one way you can debug the condition
      x >= float(windowWidth)
    windowWidth > 0
  then:
    session.insert(Player, X, 0.0)

If you want to reference any external values (such as constants) in the what block, you need to quote them, because by default any symbol with a lowercase letter will be interpreted as a binding:

const playerId = Player

session.add:
  rule getPlayer(Fact):
    what:
      (`playerId`, X, x)
      (`playerId`, Y, y)

Wrap up

In a way, rules engines just give you a fancy if statement. You tell it what data it needs, what conditions the data must meet, and what should happen when it does. The real power is that they let you express your game's logic as independent units, each one explicitly stating what they need in order to run. That lets you reason about them in isolation.

Comparison to Clara Rules

I was inspired a lot by Clara Rules, a rules engine for Clojure. Right now pararules has advantages and disadvantages compared to it.

Advantages compared to Clara:

• Pararules stores data in (id, attribute, value) tuples, whereas Clara uses Clojure records. I think storing each key-value pair as a separate fact leads to a much more flexible system. Technically you could make Clara work this way, but records tend to encourage you to combine data together.
• Pararules has built-in support for updating facts. You don't even need to explicitly do it; simply inserting a fact with an existing id + attribute combo will cause the old fact to be removed. This is only possible because of the aforementioned use of tuples.
• Pararules provides a simple rule macro that returns an object that can be added to any (or even multiple) sessions. Clara's defrule macro creates a global var that is implicitly added to a session. I tried to solve that particular problem with my clarax library but with pararules it's even cleaner because each rule is completely separate and can be added to a session independently.
• Pararules makes no distinction between rules and queries — all rules are also queries. Clara has a separate defquery macro for making queries, which means potential duplication since queries can often be the same as the "left hand side" of a rule.

Disadvantages compared to Clara:

• Clara supports truth maintenance, which can be a very useful feature in some domains. I don't plan on supporting this in pararules because I'm not convinced it's that useful for game dev.
• Clara's sessions are completely immutable data structures. This makes it really simple to hold on to old versions of a session, in order to implement time rewinding or as a useful debugging tool. I want to implement this in pararules eventually.

Acknowledgements

I could not have built this without the 1995 thesis paper from Robert Doorenbos, which describes the RETE algorithm better than I've found anywhere else. I also stole a lot of design ideas from Clara Rules.