Tree Definitions Overview

A Tree Definition declares a reusable sub-tree or node contract in Forester. Definitions isolate control logic into modular components that can accept typed parameters, store state on the Blackboard, or wrap child trees.

Forester classifies tree definitions into four main categories:

Categories of Definitions

1. Control Flow Nodes

Control flow nodes govern how child nodes are scheduled and executed:

• sequence: Ticks children sequentially until one returns Failure or Running. Returns Success if all children succeed.
• fallback: Ticks children sequentially until one returns Success or Running. Returns Failure if all children fail (often used for fallbacks and recovery).
• parallel: Ticks children concurrently based on a specified synchronization policy (e.g. success threshold).

Read more in Control Flow Nodes.

2. Root Entry Point (root)

The root definition declares the top-level starting point of the behavior tree. A Forester script must contain at least one root definition:

root main sequence {
    check_preconditions()
    execute_agent_task()
}

3. Decorator Nodes

Decorators are single-child nodes that modify or control the execution behavior of their child:

• retry(N): Automatically re-ticks its child up to N times upon failure.
• inverter: Flips child Success to Failure and vice versa.
• timeout(ms): Limits child execution time.
• repeat(N): Executes child N times sequentially.

Read more in Decorators.

4. Action Nodes (impl)

Actions represent the leaves of the behavior tree where actual work gets executed (e.g. motor movement, REST calls, LLM prompt generation). Action contracts are defined using impl:

impl call_llm_tool(prompt: string, model: string);
impl execute_motor_step(velocity: number);

Read more in Actions.

5. Lambdas & Higher-Order Trees

• Higher-Order Trees (HOT): Definitions that accept other sub-trees as parameters (sub_tree: tree).
• Lambdas: Anonymous inline sub-tree definitions created and executed at the point of invocation.

Read more in Higher-Order Trees & Lambdas.