frp-arduino: Arduino programming without the hassle of C.

• Introduction
• The language
  • FRP
    • Transforming
    • Keeping state
    • Filtering
  • EDSL
  • Compiles to C
• Examples
  • Running the examples
  • Example: Blinking led
  • Example: Blinking pair of leds
  • Example: Blinking with variable frequency
  • Example: Writing bytes on UART
  • Example: Displaying text on LCD
• API
• Questions
• Contributing
• Developer Documentation
• Resources
• License
• This document

Introduction

We believe that programming the Arduino can be more fun if we don't have to use the C language to program it. We aim to create a new language that allows us to program the Arduino using higher-level constructs. Our mission:

Arduino programming without the hassle of C

The language

The language we create has the following properties:

• It is based on the functional reactive programming (FRP) paradigm
• It is implemented as a deeply embedded domain specific language (EDSL) in Haskell
• It compiles to C code

Lets explore them in more detail.

FRP

This section introduces FRP and shows how it fits in the domain of programming an Arduino.

The central building block in FRP is a stream. A stream contains values that change over time. Consider an input pin on the Arduino. If we constantly read the value of the pin we will get different values (high or low) over time:

We could take this stream and assign it to an output pin. Whenever there is a new value on the input stream, that value will be sent to the output pin. In this example we have a led connected to the output pin:

So building an Arduino application using FRP involves capturing inputs as streams, doing some interesting calculations (we'll come to that), and assigning streams to outputs.

Transforming

The most common thing we do with streams is to transform the values in some way. This operation is called map (mapS). Let's say we have a stream of numbers:

We can transform this stream to a stream of booleans by mapping a function that converts even numbers to true and odd numbers to false:

We now have a stream that alternates its boolean value at a time interval.

Mapping is always a one-to-one conversion.

Keeping state

Streams can also be used to keep track of state. We achieve that with the fold (foldpS) operation.

A fold is like a map where we also have access to a state and the output is the new state.

Let's say we have a stream of booleans representing if a button is pressed or not. Now we want a stream that keeps track of the number of button presses. We can do that by folding the following function (pseudo code) with an initial clickCount value of 0:

if buttonIsPressed
    clickCount + 1
else
    clickCount

The very first time clickCount is 0. Subsequent values are incremented by one if the boolean value is true, otherwise we just pass the current clickCount along.

Filtering

Sometimes we would like to discard values from a stream. We do that with the filter (filterS) operation.

We can for example keep all even numbers in a stream:

EDSL

Our language is embedded in the Haskell language. That means that when we write programs in our language, we are actually writing Haskell programs.

However, our programs will not look like standard Haskell because they use custom operators that are more suited to the FRP paradigm.

By hosting our language inside Haskell, as opposed to making up our own custom syntax, we gain a few things:

• We don't have to write our own parser
• We can take advantage of Haskell's advanced type system

When we combine our program with the language library, we get an executable that, when run, will produce a C file:

The executable is a compiler from our EDSL to C.

Compiles to C

In order to make our EDSL execute on the Arduino, we compile it to a C source file which we then turn into avr assembly code by using the avr gcc toolchain.

Examples

In this section we will see what our EDSL looks like and what kinds of programs we can write using it.

Running the examples

Command to compile an example:

./make [name of example]

Command to compile and upload an example to a connected Arduino:

./make [name of example] upload

A board name can be specified as an environment variable if using a board other than the Arduino Uno. Currently supported board names include "Uno" (default Arduino Uno) and "Nano" (Arduino Nano).

BOARD=[name of board] ./make [name of example] upload

Before we can run these commands, we need to install a few dependencies:

• The Haskell platform
• Arduino-Makefile

Haskell should be installed system wide, but Arduino-Makefile should just be copied to the root of this repository.

In order to use Arduino-Makefile, we also need standard build tools like make and gcc, and in particular, the gcc toolchain for avr.

On a Fedora system, we can install all dependencies with the following commands:

yum install haskell-platform
yum install arduino-core
git clone https://github.com/sudar/Arduino-Makefile.git

Hspec is required for tests to pass:

cabal update && cabal install hspec

The arduino-core package depends on the following packages:

• avr-gcc
• avr-gcc-c++
• avr-libc
• avrdude

Example: Blinking led

import Arduino.Uno

main = compileProgram $ do

    digitalOutput pin13 =: clock ~> toggle

• Source code: examples/Blink.hs
• Generated C code (no need to understand this): examples/Blink.c
• Compile and upload command: ./make Blink upload

This is the hello world of Arduino programs.

Lets examine this example line by line:

import Arduino.Uno

This imports functions that allow us to define a program in the EDSL.

main = compileProgram $ do

The main function is the standard main function in Haskell. The compileProgram function has the following type:

compileProgram :: Action a -> IO ()

That means that we can define a set of actions in the do-block that we pass to compileProgram. It takes those actions, builds an internal representation of the program, and then generates C code and writes that to a file.

So what action is defined by the last line in the example?

digitalOutput pin13 =: clock ~> toggle

Let's look at the type for the =: operator:

(=:) :: Output a -> Stream a -> Action ()

It takes an output of a specific type and connects it to a stream of values of the same type.

The types of digitalOutput and pin13 reveal that we have an output for bits:

digitalOutput :: GPIO -> Output Bit

pin13 :: GPIO

That means that the stream we define on the right hand side has to be a stream of bits. The stream is created with the following expression:

clock ~> toggle

Let's look at the types of the individual components:

clock :: Stream Word

(~>) :: Stream a -> (Stream a -> Stream b) -> Stream b

toggle :: Stream Word -> Stream Bit

clock is a built in stream that produces incrementing integers at a given time interval.

toggle is a function that converts a stream of words to a stream of bits by mapping the isEven function: Even words are converted to 1 and odd words are converted to 0.

~> is an operator that takes a stream on the left hand side and a function on the right hand side. The result is a stream that we get by applying the function to the stream on the left hand side.

The resulting stream in the example is a stream of bits that toggles between 1 and 0 values at a specific time interval. When we connect that stream to the pin where the led is connect, the led will blink at a specific time interval.

Example: Blinking pair of leds

import Arduino.Uno

main = compileProgram $ do

    let doubleOutput = output2 (digitalOutput pin12) (digitalOutput pin13)

    doubleOutput =: every 5000 ~> flip2TupleStream

flip2TupleStream :: Stream a -> Stream (Bit, Bit)
flip2TupleStream = foldpS (\_ -> flip2Tuple) (pack2 (bitLow, bitHigh))
    where
        flip2Tuple :: Expression (a, b) -> Expression (b, a)
        flip2Tuple tuple = let (aValue, bValue) = unpack2 tuple
                           in pack2 (bValue, aValue)

• Source code: examples/DoubleBlink.hs
• Generated C code (no need to understand this): examples/DoubleBlink.c
• Compile and upload command: ./make DoubleBlink upload

This example shows how to group two values together and output them to two different outputs.

Example: Blinking with variable frequency

import Arduino.Uno
import Data.Tuple (swap)

main = compileProgram $ do

    setupAlternateBlink pin11 pin12 (createVariableTick a0)

setupAlternateBlink :: GPIO -> GPIO -> Stream a -> Action ()
setupAlternateBlink pin1 pin2 triggerStream = do
    output2 (digitalOutput pin1) (digitalOutput pin2) =: alternate triggerStream
    where
        alternate :: Stream a -> Stream (Bit, Bit)
        alternate = foldpS2Tuple (\_ -> swap) (bitLow, bitHigh)

createVariableTick :: AnalogInput -> Stream ()
createVariableTick limitInput = accumulator limitStream timerDelta
    where
        limitStream :: Stream Arduino.Uno.Word
        limitStream = analogRead limitInput ~> mapS analogToLimit
        analogToLimit :: Expression Arduino.Uno.Word -> Expression Arduino.Uno.Word
        analogToLimit analog = 1000 + analog * 20

• Source code: examples/FrequencyBlink.hs
• Generated C code (no need to understand this): examples/FrequencyBlink.c
• Compile and upload command: ./make FrequencyBlink upload

This is like blinking a pair of leds except that the frequency of the blinks in this example depends on an analog input.

Example: Writing bytes on UART

import Arduino.Uno

main = compileProgram $ do

    digitalOutput pin13 =: clock ~> toggle

    uart =: timerDelta ~> mapSMany formatDelta ~> flattenS

formatDelta :: Expression Arduino.Uno.Word -> [Expression [Byte]]
formatDelta delta = [ formatString "delta: "
                    , formatNumber delta
                    , formatString "\r\n"
                    ]

• Source code: examples/UART.hs
• Generated C code (no need to understand this): examples/UART.c
• Compile and upload command: ./make UART upload

This example shows how to write bytes to the UART output.

Example: Displaying text on LCD

import Arduino.Uno
import qualified Arduino.Library.LCD as LCD

main = compileProgram $ do

    tick <- def clock

    digitalOutput pin13 =: tick ~> toggle

    setupLCD [ bootup ~> mapSMany (const introText)
             , timerDelta ~> mapSMany statusText
             ]

introText :: [Expression LCD.Command]
introText = concat
    [ LCD.position 0 0
    , LCD.text "FRP Arduino"
    ]

statusText :: Expression Arduino.Uno.Word -> [Expression LCD.Command]
statusText delta = concat
    [ LCD.position 1 0
    , LCD.text ":-)"
    ]

setupLCD :: [Stream LCD.Command] -> Action ()
setupLCD streams = do
    LCD.output rs d4 d5 d6 d7 enable =: mergeS streams
    where
        rs     = digitalOutput pin3
        d4     = digitalOutput pin5
        d5     = digitalOutput pin6
        d6     = digitalOutput pin7
        d7     = digitalOutput pin8
        enable = digitalOutput pin4

• Source code: examples/LCD.hs
• Generated C code (no need to understand this): examples/LCD.c
• Compile and upload command: ./make LCD upload

This example shows how to display text on an LCD display.

API

The API documentation for the latest version is hosted in the Modules section on Hackage:

http://hackage.haskell.org/package/frp-arduino

Questions

We want to be welcoming to newcomers.

In particular, if there is something you don't understand, please let us know and we'll try to explain it and improve our documentation.

To ask a question, create a new issue and attach the question label.

Contributing

The contributors are listed in AUTHORS (add yourself).

We use the C4.1 (Collective Code Construction Contract) process for contributions. More discussions and explanations of the process can be found in the The ZeroMQ Community, in particular here.

Comments on the process:

A patch MUST compile cleanly and pass project self-tests on at least the principle target platform.

In our case, this means that ./test should run without failure.

Developer Documentation

Below is a collection of information to help developers extend and improve frp-arduino.

• Adding Boards

Resources

• Domain-specific Languages and Code Synthesis Using Haskell
• Tech Mesh 2012 - Making EDSLs fly - Lennart Augustsson
• Representing DSL expressions in Haskell

License

The Haskell library that implements the language and all examples are free software, distributed under the GNU General Public License, version 3. For more information, see COPYING.

This document

This document (README.md) is automatically generated from the sources in the doc folder by running python doc/generate_readme.py.