Expressions

An expression is a small piece of code that reads the game and produces a value. Every rule you write is expressions: one to decide whether something should happen, others to decide how much.

They appear anywhere the editor shows a formula box — a condition on a quest, a damage amount, a field's maximum, the body of a hook.

$.stat('might') * 2 + 5

That is a complete expression. It reads a stat and does arithmetic. There is no return, no function wrapper, and no semicolon needed.

Everything hangs off $

$ is the only thing in scope. It carries the whole readable world:

$.subject.name          // who this rule is about
$.subject.health        // their current hit points
$.player.fields.satiety // a declared field on the host player
$.turn                  // which turn it is
$.timed                 // whether this world has a clock at all

Reading something that does not exist gives you nothing back rather than an error — which matters more than it sounds. See Missing values, because it is the single most common way a rule silently never fires.

The two halves

Reading the world splits into paths and questions.

• Reading values — $.subject.health, $.event.minutes, $.stat(), $.field(). Things that give you a number or a name.
• Asking questions — $.hasItem(), $.alive(), $.hasPlugin(). Things that give you true or false.

Where expressions run

In a sandbox with a fuel budget and frozen globals. Three consequences worth knowing:

An infinite loop is killed, not hung. The turn survives; your rule stops.

There is no channel between two expressions. One rule cannot leave a variable lying around for another to pick up. If two rules need to share state, that state is a field.

An expression that throws evaluates to nothing. The caller falls back to its default rather than the turn crashing. This is why a broken rule looks like a rule that does not fire, instead of an error message.

Next

Start with Reading values. If you are writing a hook body rather than a single formula, read Rule bodies first — the rules are different and the differences are where people get stuck.