Getting Started

This demonstrates how to build a simple Idris 2 application for the Arduino. This is largely just proof of concept. You will need a few things to get started:

1. A recent version of Idris 2 that includes C with reference counting backend.

2. Arduino Software. The current version was tested against version 1.8.13. You need to untar or install the package for your platform.

3. Some Arduino-compatible hardware

Keep in mind that this repository contains git submodules for the arduino building system called Arduino-Makefile and for the slightly modified Idris 2 RTS with removed pthread imports and concurrency primitives since pthread is not supported for Arduino.

After fetching this repository with submodules, you may need to tune the Makefile in the root directory to select the board you use. This repository contains presets for several boards that lie at aux-makefiles/board-*.mk.

Additionaly you may need to modify Makefile and files in the aux-makefiles directory. There are variables that you may set according to you installation.

Installation-related variables:

• ARDUINO_DIR = where you unpacked the Arduino software
• ARDUINO_PACKAGE_DIR = where your Arduino packages reside (in case you are using some in your project)
• BOARD_TAG and BOARD_SUB = your board name (e.g., uno, mega) and subname (if applicable, e.g. atmega2560 for mega)
• IDRIS_MAIN = idr-file with the entry point (i.e. with the main : IO () function)

Variables that now are defaulted to git submodules or set in board-*.mk example files:

• IDRIS_SUPPORT_SRC_PATH = path to the refc support (i.e. the C RTS) of the Idris2
• ARDUINO_MAKEFILE_PATH = path to the main makefile from Arduino-Makefile building system; usually it points to Arduino.mk but this may depend on the architecture of the board you are using

Variables related to AVR boards:

• AVR_TOOLS_DIR = dir containing bin/avr-gcc unless it is shipped with the Arduino software
• AVRDUDE_CONF = path to avrdude.conf unless avrdude is shipped with the Arduino software

Now you can run make to build the project or make upload to build & upload the code.

How This Works

This process simply uses the Idris C backend to produce a .c file which is then built using Arduino.mk. The standard Arduino.mk rules are augmented via the Idris2.mk file in the aux-makefiles directory. Eventually the Idris2.mk should be installed in globally as all the project specific information should be in the local Makefile.

The standard FFI is used to import Arduino C functions into Idris 2. In the example we are importing standard Arduino functions defined in Arduino.h which are usually available with no additional effort in the *.ino sketches.

Limitations

Arduino Uno only has 2K of RAM -- so even in C it is easy to exceed its limitations.

However, we are currently using recently added Idris 2's RTS with reference counting garbage collector. By the original claim the main goal of this is portability - especially, being able to run Idris programs on unusual or constrained platforms which sounds promising.

Future

Porting Idris to the Arduino is fun for bragging rights -- but the miniscle amount of RAM greatly limits its usefulness. However, there are many other embedded systems which could also be targetted. Two that I am interested in are:

• teensy - $20 - 64K of RAM, 72MHz Cortex-M4. Can use many Arduino libraries.

• Raspberry Pi - $25-35, 512MB of RAM, 700MHz ARM1176JZF-S.

For the Raspberry Pi, it is already rather trivial to generate .c code on another machine, and then compile it on the RaspberryPi -- assuming you are running a Linux based OS on the Raspberry Pi.

However -- I hate waiting for Linux to boot and having to worry about it shutting down properly. So, my goal would be to target the RaspberryPi hardware directly. There are challenges here such as integrating with the built-in bootloader, writing drivers for the hardware with may require binary blobs, etc.

Why?

You might wonder why use Idris for embedded development? But, for me, it seems like the perfect match. Debugging code on embedded systems is extremely annoying. Often times your only output is a single LED, making it slow to narrow down a problem. In many cases the system might be controlling dangerous or expensive equipment which could be damaged or even lead to loss of life.

I first became interested in functional programming over a decade ago when working as a firmware engineer. Frusted with having to debug stupid C code bugs I searched out and found splint (called lc-lint at the time). It's a tool that statically checks C code eliminating many of my coding errors. To perform the extra checks you have to add additional annotations to your code via specially formatted C comments.

splint saved me so much time I wondered why languages didn't have that functionality built-in. This led me to the Clean programming language, and then on to Haskell.

But, it is Idris that seems to really have enough power to make it exciting for embedded development because it can capture the constraints of the system without becoming awkward. For example, I would like to control a string of 50 individually addressable RGB LEDs. I know that my system has exactly 50 LEDS -- and so I can specify that the code must supply a Vect 50 RGB.

I also wish to use Idris to control a propane flame effect with a verified pilot. The system should never open the solenoid and release the propane fuel unless the pilot light can be verified. Using Idris, I can enforce this constraint at compile time and be certain it holds.

Often times I wish to communicate between the embedded system and a host machine. The Idris Protocols library seems like a very attractive solution to handling the over-the-wire-format and ensuring both sides are functioning correctly.