PETS is not a programming language (which tells a computer how to do something). Nor is it a markup language like HTML or other common sort of computer language. It's a "specification language", which is a fancy term for it lets us say what we mean. That is, two parties who both know the rules of a specification language gain the ability to communicate about that language's topic area with precision.
When I started playing Terraforming Mars, I loved many things about the game, and above all its rich and integrated theme. But I was downright fascinated by the iconographic grammar it uses on cards, milestones, map areas and so forth. I started to see dozens of ways in which it serves very effectively as a specification language, and dozens of ways in which it falls short (none of my discussion of which should ever be taken as critical of the game developers, whom I'm nothing but impressed with).
I realized that a suitable specification language could exist -- and if it did, then both the iconographic depiction and a reasonable pass at the textual explanation on each card (etc.) could be derived from it. Moreover, an actual running game engine would in theory not need to be programmed how to handle each card individually; it could simply read the specification and know what to do.
Then I discovered that I was already hard at work on it! For example, the many hours I sunk into Terraforming Mars: The Spreadsheet were an effort to "regularize" the cards and create a uniform way of looking at them that adhered to a few rules.
Now (December 2022) I've been working on it for over two years. It's been my main pursuit outside of work. Some of the effort has been writing actual computer programs to test that the language is suitable in the ways I want it to be. But most of it has been simply grappling with the design of the thing -- so far it's been a generous fountain of absolutely wicked design puzzles!
The topic of this language is queries and manipulations of a Game World. For Terraforming Mars, a Game World represents the state of a game in progress, so we'd better start by understanding what that means.
I have a long answer to that, but the short version is: it's everything you need to know about a game-in-progress in order to fully reconstruct a functionally equivalent game-in-progress at another time or place. It's like a photograph of the game, but also needs to include invisible state like whose turn it is.
PETS's conception of a Terraforming Mars Game World goes like this:
What makes up a Game World is a bunch of things. Countable things, called Components.
Anything that has no effect on the game, like a greenery/city tile waiting in a bowl to be used, simply does not exist. To play a tile is create it.
Examples, in order of ascending weirdness:
- An OceanTile sitting on that two-card spot on the main board
- Each Microbe sitting on your Psychrophiles card
- Each unit of TerraformRating or plant production you have
- Each step the temperature track has been raised (where we say "it's -20 degrees out", PETS says "five TemperatureStep components")
- Each map Area on the board (even though it never changes); there's even an Area called "FloatingInSpace" for use by Stanford Torus
- Each Adjacency between two tiles on the board
- Each VictoryPoint doled out in the endgame
- A ColonyTile in play (playing the first animal card of the game creates the Miranda component)
- Each Tag you have in play, but also, each distinct type of tag you have in play (because two cards, one milestone, and one global event need 'em to be)
- When a UNMI or Pristar player gains a TR, they also gain an invisible "I have raised my TR this generation" component if they don't already have it
- A card is two separate components: a CardBack and a CardFront, where both can't exist at the same time (the play-a-card mechanism converts one to the other)
- You, yourself, are a component
(They do get a bit weirder than this, too.)
Much of the power of the language comes from the way it treats everything in such a homogenized way -- and the idea came straight from the existing icon grammar, which depicts "raise the temperature 1 step" and "gain a plant" in entirely similar ways.
A Game World is nothing but a set of Components -- actually a "multiset", since there can be multiple indistinguishable elements, like your Steel resources.
A change to that state is nothing but adding components, removing components, or transmuting some components into an equal number of other components.
(Well, this is almost true. There are also task queues, which keep track of what the game is waiting on each player to do. But this is a topic for later.)
(insert more stuff here)
There are several basic elements to the language.
- Instructions (
-2 Plantmeans "lose two plants") - Requirements (
MAX 4 TemperatureStepmeans "is the temperature at most -22 C?") - Metrics (
OwnedTile<!Owner>means "owned tiles belonging to opponents") - Effects (
EventCard: 3means "when you play an event card, gain 3 MC") - Actions (
Steel -> 5means "spend 1 steel to gain 5 MC") - Component expressions (
CityTile<LandArea(HAS 2 Neighbor<CityTile<Anyone>>)>means "a city tile on a land area next to >=2 city tiles", no matter which of the above element kinds it appears in) - ChangeEvents, aka game log entries (
469: +OxygenStep BY Player2 VIA GreeneryTile<Player2, Tharsis_5_5> BECAUSE 448) - Class declarations (
CLASS TerraformRating { ProductionPhase: 1; End: VictoryPoint })
See a syntax reference