# Keywords and Reserved Words

> Every FizzBee keyword in one place: flow modifiers, fairness and assertion modifiers, control flow, state and scope, and built-in types.

Canonical: https://fizzbee.io/reference/language/keywords-and-reserved-words/
Source: https://github.com/fizzbee-io/fizzbee/blob/main/examples/references/LANGUAGE_REFERENCE.md#keywords-and-reserved-words


## Core Keywords

- `action` - Defines an action (model checker entry point)
- `func` - Defines a function
- `role` - Defines a role (class-like structure)
- `symmetric` - Modifier for roles or values (enables symmetry reduction)
- `assertion` - Defines a property to check

## Flow Modifiers

- `atomic` - Execute without yielding
- `serial` - Execute with yields between statements (default)
- `parallel` - Execute statements concurrently
- `oneof` - Choose one of multiple branches

## Fairness Modifiers

- `fair` - Weak fairness (same as `fair<weak>`)
- `fair<weak>` - Eventually executes if continuously enabled
- `fair<strong>` - Eventually executes if infinitely often enabled

## Assertion Modifiers

- `always` - Safety property (holds in all states)
- `exists` - Existential property (holds in at least one state)
- `eventually always` - Liveness property (eventually reaches stable state)
- `always eventually` - Progress property (always makes progress)
- `transition` - Property about state transitions

## Temporal Formula Keywords

Used in the `return` of an `assertion` with no prefix (see
[Temporal Formulas](/reference/language/assertions/#temporal-formulas)):

- `always`, `eventually` - Prefix temporal operators
- `until`, `unless` - Strong and weak until
- `leadsto` - `P leadsto Q` is `always (P implies eventually Q)`
- `implies` - Logical implication
- `possibly` - Some path reaches a state (`exists eventually` is another spelling)

`until`, `unless`, `leadsto`, `implies` and `possibly` are keywords only in
an assertion's return, so they can still be used as names elsewhere.

## Control Flow

- `if`, `elif`, `else` - Conditional branching
- `for` - Iteration over collections
- `while` - Conditional iteration
- `break` - Exit loop early
- `continue` - Skip to next iteration
- `return` - Return from function/action
- `pass` - No-op (explicitly enables action)
- `require` - Guard clause (disables action if false)
- `oneof` - Nondeterministic choice (preferred; `any` is deprecated alias)
- `` `checkpoint` `` - Visualization breakpoint (backtick syntax)

## State and Scope

- `self` - Reference to role instance (implicit in role methods)
- `global` - NOT a keyword (state variables are automatically global)
- `@state` - Decorator for role state (durable/ephemeral)

## Built-in Types

- `enum` - Enumeration definition
- `record` - Mutable named structure
- `struct` - Immutable named structure (Starlark)
- `set`, `dict`, `list` - Standard collections
- `genericset`, `genericmap` - Collections for non-hashable types
- `bag` - Multiset (collection with duplicates)
- `symmetric_values` - Creates interchangeable values for symmetry reduction