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HelloWorld.lf

By convention, LF programs are put in a src directory. This src directory initially contains one program, HelloWorld.lf, which demonstrates:

  1. Reactors - Modular components that encapsulate behavior

    • Greeter: Generates periodic greetings
    • Counter: Receives and counts messages
    • HelloWorld: Main reactor that composes the system

  2. Timers - Periodic and one-shot events

    • Startup reaction (executes once at program start)
    • Periodic timer (500 msec intervals)
    • Shutdown reaction (executes once at program end)

  3. Ports - Communication between reactors

    • output greeting: char* - Output port in Greeter
    • input message: char* - Input port in Counter
    • Connection: greeter.greeting -> counter.message

  4. State Variables - Persistent data across reactions

    • state count: int = 0 - Tracks greeting count
    • state total: int = 0 - Tracks received messages

  5. Reactions - Event-driven code blocks

    • Triggered by: startup, timer, input, shutdown
    • C code embedded between {= =} delimiters

  6. Timeout - Program execution duration

    • timeout: 2 sec - Program runs for 2 seconds

How to Compile and Run

You need the Lingua Franca compiler (lfc) for command-line compilation and/or the Visual Studio Code extension for compiling and running within VSCode or Cursor. See https://www.lf-lang.org/ for installation instructions.

Compile on the Command Line

lfc src/HelloWorld.lf

This will:

  • Generate C code in src-gen/HelloWorld/
  • Compile the generated code
  • Create an executable in bin/HelloWorld

Run on the Command Line

./bin/HelloWorld

Compile and Run in VSCode or Cursor

In the icon bar, click on the "run" triangle. Or from the command palette (Command-Shift P), select "Lingua Franca: Build and Run".

Expected Output

% bin/HelloWorld
---- System clock resolution: 1000 nsec
---- Start execution on Tue Jan 13 10:09:58 2026 ---- plus 977859000 nanoseconds
=== Greeter started ===

╔════════════════════════════════════════════╗
║  Lingua Franca Hello World Demo            ║
║  Demonstrating reactive programming        ║
╚════════════════════════════════════════════╝

[Message #1 at 0 ms] Hello, World! Welcome to Lingua Franca!
[Message #2 at 500 ms] First periodic greeting!
[Message #3 at 1000 ms] Second periodic greeting!
[Message #4 at 1500 ms] Greetings from Lingua Franca!
[Message #5 at 2000 ms] Greetings from Lingua Franca!
=== Greeter shutting down after 4 greetings ===
=== Counter received 5 messages total ===
---- Elapsed logical time (in nsec): 2,000,000,000
---- Elapsed physical time (in nsec): 2,003,074,000

Key Concepts

Logical Time

Lingua Franca uses logical time - a deterministic notion of time that ensures reproducible execution. Unless the target property fast is set to true, events are handled only when physical time advances to their logical time, giving real-time behavior.

Determinism

The output is data deterministic. Running the program multiple times will produce identical results with the same logical timestamps.

Zero-Delay Communication

Connections between reactors (like greeter.greeting -> counter.message) have zero logical delay by default, meaning the downstream reaction executes at the same logical time.

Next Steps

  • Try modifying the timer period
  • Add more reactors to the composition
  • Experiment with logical actions and delays using after
  • Explore physical actions for non-deterministic inputs
  • Try changing the program to a federated one

For many more examples, see the src directories within the ../context directory.