Spanda

Chapter 3 — Anatomy of a robot program

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The robot block

Everything autonomous lives inside robot Name { ... }:

robot Rover {
  // sensors — how it sees
  // actuators — how it moves
  // safety — what it must never violate
  // behaviors / tasks — what it does
}

Order inside the block is flexible, but this mental model helps.

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Sensors — how it sees

sensor lidar: Lidar on "/scan";

Piece	Meaning
lidar	Your name for this sensor
Lidar	Type (defines .read(), distances, etc.)
on "/scan"	Where data comes from (ROS-style topic)

Read it in code:

let scan = lidar.read();
if scan.nearest_distance < 1.0 m {
  wheels.stop();
}

Tip: Distances use units — 1.0 m, not bare 1.0. The compiler catches unit mistakes.

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Actuators — how it moves

actuator wheels: DifferentialDrive;

Common commands:

wheels.drive(linear: 0.3 m/s, angular: 0.0 rad/s);
wheels.stop();
wheels.execute(safe_action);   // only SafeAction, not raw AI output

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Safety — the reflexes

safety {
  max_speed = 1.0 m/s;
  stop_if lidar.nearest_distance < 0.5 m;
}

These run before motion reaches hardware. Not comments. Not optional at runtime.

You can also define zones (keep-out regions) — see examples/patrol_with_zones.sd.

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Behaviors — what it does

One-shot or looping logic:

behavior patrol() {
  loop every 100ms {
    // read sensors, decide, actuate
  }
}

loop every 100ms = “run this block ten times per second.” Good for control loops.

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Tasks — background chores

task watchdog every 50ms {
  let scan = lidar.read();
  let _ = scan;
}

Tasks are scheduled separately from behaviors — useful for monitoring, logging, or parallel work. Priorities: critical, high, low.

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Events and triggers — “when X, do Y”

event ObstacleSeen;

on ObstacleSeen {
  wheels.stop();
}

Emit from anywhere: emit ObstacleSeen;

More trigger types (timers, conditions, topics): see triggers.md or examples/triggers_demo.sd.

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Hardware and deploy — “will it run on my board?”

hardware RoverV1 {
  memory: 4 GB;
  sensors [ Lidar ];
  actuators [ DifferentialDrive ];
}

deploy Rover to RoverV1;

Then:

spanda verify rover.sd --target RoverV1

Verification answers: “Does this program match this hardware?” before you flash an SD card.

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Copy-paste starter template

robot MyRover {
  sensor lidar: Lidar on "/scan";
  actuator wheels: DifferentialDrive;

  safety {
    max_speed = 1.0 m/s;
    stop_if lidar.nearest_distance < 0.5 m;
  }

  behavior patrol() {
    loop every 100ms {
      let scan = lidar.read();
      if scan.nearest_distance < 1.0 m {
        wheels.stop();
      } else {
        wheels.drive(linear: 0.3 m/s, angular: 0.0 rad/s);
      }
    }
  }
}

Save as my_rover.sd, then spanda check my_rover.sd and spanda run my_rover.sd.

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Next: AI without the scary parts

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