Spanda

Lesson 3 — Control flow and loops

Goal: Use enums, match, and if/else to structure robot decision logic.

Example: examples/basics/03_control_flow.sd

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Enums for robot modes

enum DriveMode {
  Idle,
  Cruise,
  Avoid
}

Enums model discrete states — navigation modes, mission phases, fault levels. Use them instead of string flags when the set of values is fixed.

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Pattern matching with match

match mode {
  Idle => wheels.stop();
  Cruise => wheels.drive(linear: 0.5 m/s, angular: 0.0 rad/s);
  Avoid => wheels.drive(linear: 0.0 m/s, angular: 0.3 rad/s);
};

match is exhaustive: the compiler warns if you miss a variant. Each arm executes one action (or block) for that variant.

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Combining match and if

Real programs mix structured modes with sensor-driven branches:

loop every 50ms {
  let scan = lidar.read();

  match mode {
    Idle => wheels.stop();
    Cruise => wheels.drive(linear: 0.5 m/s, angular: 0.0 rad/s);
    Avoid => wheels.drive(linear: 0.0 m/s, angular: 0.3 rad/s);
  };

  if scan.nearest_distance > 2.0 m {
    wheels.drive(linear: 0.3 m/s, angular: 0.0 rad/s);
  }
}

Order matters: safety rules still apply after your logic runs.

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Try it

spanda check examples/basics/03_control_flow.sd
spanda run examples/basics/03_control_flow.sd

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When to use what

Construct	Use when
if / else	Sensor thresholds, one-off conditions
match	Discrete enum variants, Result/Option (Lesson 4)
loop every Nms	Fixed-rate control or polling
state_machine + enter	Explicit transitions with named states (Lesson 5+)

For full state-machine syntax, see spanda-language.md and examples/basics/10_state_machine.sd.

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Exercise

Extend your patrol robot from Lesson 2:

1. Add enum DriveMode { Stop, Forward, Turn }
2. In the loop, match on mode: Forward drives straight, Turn spins in place, Stop calls wheels.stop()
3. Switch to Turn when scan.nearest_distance < 0.8 m

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Next: Lesson 4 — Types, units, and errors

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