git @ Cat's Eye Technologies
immutable-envs-in-match

# Samovar

Version 0.2 (unreleased). Subject to change in backwards-incompatible ways.

Samovar is a DSL for world-modeling using propositions.

A Samovar world is an immutable set of rules which operate on a mutable set of facts. Each rule looks like

``````[A] X [B]
``````

and means "If A holds, then X is a possible action to take, and if you do take it, you must make B hold afterwards."

By "hold" we mean "matches the current set of facts."

As an example,

``````[actor(α),item(β),~holding(α,β)] α picks up the β. [holding(α,β)]
``````

If α is an actor and β is an item and α is not holding β, then one possible action is to write out 'α picks up the β' and assert that α is now holding β.

We can add a complementary rule:

``````[actor(α),item(β),holding(α,β)] α puts down the β. [~holding(α,β)]
``````

And we can package this all into a world-description:

``````rules
[actor(α),item(β),~holding(α,β)]  α picks up the β.   [holding(α,β)]
[actor(α),item(β),holding(α,β)]   α puts down the β.  [~holding(α,β)]
end
situations
[actor(Ignatz),item(brick)]
end
``````

And an implementation of Samovar could take this world-description and use it to, among other things, generate textual descriptions of chains of events like

``````Ignatz picks up the brick. Ignatz puts down the brick.
``````

Of course, this is a very simple example. A more complex example might have more actors, more items, and more rules (for example, that two actors cannot be holding the same item at the same time.)

### Discussion

Samovar could be described as an "assertion-retraction engine", which itself could be thought of as a highly stylized form of Prolog programming plus some syntactic sugar.

Alternately, it could be thought of as assigning preconditions and postconditions, like you would find in Hoare logic, to actions in a world-model. Instead of proving that the action satisfies the conditions, though, we simply assume it does, and use the conditions to chain actions together in a sensible order.

### TODO

• Implement the pattern-matching of propositions using this algorithm
• Maybe allow `∨` - there doesn't seem to be as much call for it, though.
• Allow sentence trees to be given for actions.
• Allow situations to define a termination condition, so that the implementation can generate a scenario where the condition is met (by whatever method).
• Consider what it would take to add a predicate that evaluates to whether a given action has been taken previously or not.