Reasoning

A store that only holds the facts you put in it is a database. Argon derives new facts from stored ones, answers questions over both, and changes stored state — all through one atom, the rule, which comes in five modes. The modes form a ladder from pure computation to the single mode that writes:

• fn — pure computation over its arguments, no state at all.
• derive — defines a derived relation by a rule.
• query — reads stored and derived facts, returns a typed value.
• check — observes the state and emits diagnostics, writes nothing.
• mutate — changes stored facts. The only writer.

derive and check are written in the Datalog register — :- reads “if”, a comma reads “and”. fn, query, and mutate use ordinary expression syntax. The reference fixes the exact grammar of all five in the rule atom.