Development¶

Second Loop keeps its sources in normal Java files and combines them into one file for OnBot Java, so the robot only needs a single upload.

Layout¶

Functions live in Builtins, MathBuiltins, StringBuiltins and CollBuiltins; helpers written in the REPL language itself live in Prelude.java. Each file's top-level types become static nested classes of SecondLoop in the combined file.

Build¶

$ ./build.sh

The script amalgamates, compiles against the FTC stand-ins, runs every test, and then compile-checks the single file. It needs Python 3 and a JDK, and if there is no javac binary it uses the JDK's compiler module instead.

The tests pair a REPL form with its expected printed output:

t.check("6.0", "(+ 1 2 3)");

FTC modules have their own test classes, which run against the stand-ins. Any new feature or fix gets a check there or in ReplTest.

Constraints¶

• Java 8 only. The Control Hub runs Android 7.1, which rules out lambdas, method references, streams, String.join, var and List.of. Write anonymous inner classes and loops.
• Only the FTC SDK and the standard library. No third-party dependencies.
• Thread safety. The REPL sessions, futures and the OpMode thread run at the same time, and FTC objects such as Telemetry are not thread-safe, so the OpMode thread does the rendering (see TelemetryModule.render).
• Stopping must be prompt. Loops call Interp.checkStop(), catch never swallows Interrupted, and Future.cancel() waits for the thread to end.

Telemetry¶

FTC's Telemetry forgets its lines after every update(), so TelemetryModule holds them instead: the functions only edit a list, and the OpMode thread draws it on every status tick, every 100 ms (STATUS_MILLIS in src/SecondLoop.java). Nothing in the language calls telemetry.update(), and lines stay until (telemetry/clear) runs. It works during INIT, before START is pressed.

Only the newest 50 lines are kept (MAX_LINES), so a loop that adds a line every round cannot fill memory. In a loop, clear and add the lines each round, so the display shows the current state rather than a history:

(telemetry/clear)
(telemetry/add-line "power" power)
(telemetry/add-line (format "heading %.1f" heading))

showStatus adds one line above these, such as Ready at 192.168.43.1:9001 (1 client), so the address a laptop needs is on the Driver Hub from INIT. When the robot has several addresses, ReplServer.preferredAddress picks 192.168.43.1 or 192.168.49.1 if either is there.

LocalRepl¶

LocalRepl runs the same interpreter on a laptop, with no gamepad, motor, servo, battery, IMU or telemetry functions:

$ java -cp build/classes secondloop.LocalRepl --stdin  # type directly
$ java -cp build/classes secondloop.LocalRepl          # serve on port 9001
$ nc localhost 9001                                    # in another terminal

It has the interpreter and the prelude, but not gamepad, motor/*, servo/*, crservo/*, battery/voltage, imu/* or telemetry/*, which need the FTC SDK.

Documentation¶

The docs are Markdown in docs/, with site settings in zensical.toml and a single Python dependency in pyproject.toml. With uv installed:

$ uv run zensical serve          # preview at http://127.0.0.1:8000
$ uv run zensical build --clean  # write the site to site/

CI runs uv run zensical build --clean on every push and pull request, and deploys site/ to GitHub Pages on main.