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\begin{document}
\chapter{Introduction}%
\label{chp:introduction}

\begin{chapterabstract}
	The sheer number of connected devices around us is mind boggling and seems increases exponentially for many years.
	In 2022, there is an estimated number of 13.4 billion of devices connected that sense, act or otherwise interact with people and the physical world surrounding us\footnote{\url{https://transformainsights.com/research/tam/market}, accessed on: \formatdate{2022}{10}{13}}.
	These devices, together with all the scaffolding and integration such as the various networks providing the communication, (cloud) computers realising the back end or administration and the devices in our pockets providing us with a view on the system are called the \gls{IOT}.
	\Gls{IOT} systems can be seen as layered systems, where each layer is powered by different types of computers; programming languages and even programming paradigms.
	This thesis shows a novel way of orchestrating these brobdingnagian systems using the \gls{TOP} paradigm.
	It does so by giving a proof-of-concept implementation for a \gls{TOP} system specifically designed for the \gls{IOT}: \gls{MTASK}.
	At the core of the \gls{MTASK} system is a \gls{DSL}, embedded in the general purpose \gls{TOP} system \gls{ITASK}.
	Using the \gls{MTASK} system, all layers of an \gls{IOT} system can be programmed from a single declarative specification.

	This chapter provides the required background material, an overview of the concrete contributions and a reading guide.
\end{chapterabstract}
\todo{Introduction in the abstract doen zoals nu?}

\section{Internet of Things}
The \gls{IOT} is growing rapidly and it is changing the way people and machines interact with the world.
While the term \gls{IOT} briefly gained interest around 1999 to describe the communication of \gls{RFID} devices~\citep{ashton_internet_1999,ashton_that_2009}, it probably already popped up halfway the eigthies in a speech by \citet{peter_t_lewis_speech_1985}:

\begin{quote}
	\emph{The \acrlong{IOT}, or \acrshort{IOT}, is the integration of people, processes and technology with connectable devices and sensors to enable remote monitoring, status, manipulation and evaluation of trends of such devices.}
\end{quote}

CISCO states that the \gls{IOT} only started when there where as many connected devices as there were people on the globe, i.e.\ around 2008~\citep{evans_internet_2011}.
Today, the \gls{IOT} is the term for a system of devices that sense the environment, act upon it and communicate with each other and the world.
These connected devices are already in households all around us in the form of smart electricity meters, fridges, phones, watches, home automation, \etc.

When describing \gls{IOT} systems, a tiered---or layered---architecture is often used to compartmentalize the technology.
The number of tiers heavily depends on the required complexity of the model but for the intents and purposes of the thesis, the four layer architecture shown in \cref{fig:iot-layers} is used.

\begin{figure}[ht]
	\centering
	\includestandalone{iot-layers}
	\caption{A four-layer \gls{IOT} architecture.}%
	\label{fig:iot-layers}
\end{figure}

Closest to the end-user is the presentation layer, it provides the interface between the user and the \gls{IOT} application.
In home automation this may be a web interface or a app used on a phone or mounted tablet to interact with the edge devices and view the sensor data.

The application layer provides the \glspl{API}, interfaces and data storage.
A cloud service or local server provides this layer in a typical home automation application.

All layers are connected using the network layer.
In many applications this is implemented using conventional networking techniques such as WiFi or Ethernet.
However, networks or layers on top of it tailored to the needs of \gls{IOT} applications have been increasingly popular such as \gls{BLE}, LoRa, ZigBee, LTE-M, or \gls{MQTT}.

The perception layer---also called edge layer---collects the data and interacts with the environment.
It consists of edge devices such as microprocessors equipped with various sensors and actuators.
In home automation this layer consists of all the devices hosting the sensors and actuators such as in a smart lightbulb, an actuator to open a door or a temperature and humidity sensor.

Across the layers, the devices are a large heterogeneous collection of different platforms, protocols, paradigms, and programming languages often resulting in impedance problems or semantic friction between layers when programming \citep{ireland_classification_2009}.
Even more so, perception layer specifically often is a heterogeneous collections of microprocessors in itself as well, each having their own peculiarities, language of choice and hardware interfaces.
As the edge hardware needs to be cheap, small-scale, and energy efficient, the microprocessors used to power these devices do not have a lot of computational power, only a soup\c{c}on of memory, and little communication bandwidth.
Typically the devices do not run a full fledged \gls{OS} but a compiled firmware.
This firmware is often written in an imperative language that needs to be flashed to the program memory.
Program memory typically is flash based and only lasts a couple of thousand writes before it wears out.
While devices are getting a bit faster, smaller, and cheaper, they keep these properties to an extent, greatly reducing the flexibility for dynamic systems where tasks are created on the fly, executed on demand, or require parallel execution.
These problems can be mitigated by dynamically sending code to be interpreted to the microprocessor.
With interpretation, a specialized interpreter is flashed in the program memory once that receives the program code to execute at runtime.

%weiser_computer_1991
\section{\texorpdfstring{\Acrlongpl{DSL}}{Domain-specific languages}}
% General
Programming languages can be divided up into two categories: \glspl{DSL}\footnote{Historically this has been called DSEL as well.} and \glspl{GPL}~\citep{fowler_domain_2010}.
Where \glspl{GPL} are not made with a demarcated area in mind, \glspl{DSL} are tailor-made for a specific domain.
Writing idiomatic domain-specific code in an \gls{DSL} is easy but this may come at the cost of the \gls{DSL} being less expressive to an extent that it may not even be Turing complete.
\Glspl{DSL} come in two main flavours: standalone and embedded\footnote{Also called external and internal respectively.} of which \glspl{EDSL} can again be classified into heterogeneous and homogeneous languages (see \cref{fig:hyponymy_of_dsls} for this hyponymy).

\begin{figure}[ht]
	\centering
	\includestandalone{hyponymy_of_dsls}
	\caption{Hyponymy of \glspl{DSL} (adapted from \citet[pg.\ 2]{mernik_extensible_2013})}%
	\label{fig:hyponymy_of_dsls}
\end{figure}

\subsection{Standalone and embedded}
\glspl{DSL} where historically created as standalone languages, meaning all the machinery is developed solely for the language.
The advantage of this approach is that the language designer is free to define the syntax and type system of the language as they wish, not being restricted by any constraint.
Unfortunately it also means that they need to develop a compiler or interpreter for the language to be usable making standalone \glspl{DSL} costly to create.
Examples of standalone \glspl{DSL} are regular expressions, make, yacc, XML, SQL, \etc.

A dichotomous approach is embedding the \gls{DSL} in a host language, i.e.\ \glspl{EDSL}~\citep{hudak_modular_1998}.
By defining the language as constructs in the host language, much of the machinery is inherited and the cost of creating embedded languages is very low.
There is more linguistic reuse~\cite{krishnamurthi_linguistic_2001}.
There are however two sides to the this coin.
If the syntax of the host language is not very flexible, the syntax of the \gls{DSL} may become clumsy.
Furthermore, errors shown to the programmer may be larded with host language errors, making it difficult for a non-expert of the host language to work with the \gls{DSL}.

\subsection{Heterogeneity and homogeneity}
\Citet{tratt_domain_2008} applied a notion from metaprogramming~\citep{sheard_accomplishments_2001} to \glspl{EDSL} to define homogeneity and heterogeneity of \glspl{EDSL} as follows:

\begin{quote}
	A homogeneous system is one where all the components are specifically designed to work with each other, whereas in heterogeneous systems at least one of the components is largely, or completely, ignorant of the existence of the other parts of the system.
\end{quote}

Homogeneous \glspl{EDSL} are therefore languages that are solely defined as an extension to their host language.
They often restrict features of the host language to provide a safer interface or capture an idiomatic pattern in the host language for reuse.
The difference between a library and a homogeneous \glspl{EDSL} is not always clear.
Examples of homogeneous \glspl{EDSL} are libraries such as ones for sets, \glspl{GUI} creation, LISP's macro system, \etc.

On the other hand, heterogeneous \glspl{EDSL} are languages that are not executed in the host language.
For example, \citep{elliott_compiling_2003} describe the language Pan, for which the final representation in the host language is a compiler that will, when executed, generate code for a completely different target platform.
In fact, \gls{ITASK} and \gls{MTASK} are both heterogeneous \glspl{EDSL} and \gls{MTASK} specifically is a compiling \gls{DSL}.

\section{\texorpdfstring{\Glsxtrlong{TOP}}{Task-oriented programming}}
\Gls{TOP} is a declarative programming paradigm designed to model interactive systems \citep{plasmeijer_task-oriented_2012}.
Instead of dividing problems into layers or tiers, as is done in \gls{IOT} architectures as well, it deals with separation of concerns in a novel way.
From the data types, utilising various \emph{type-parametrised} concepts, all other aspects are handled automatically (see \cref{fig:tosd}).
This approach to software development is called \gls{TOSD}~\citep{wang_maintaining_2018}.

\begin{figure}[ht]
	\centering
	\begin{subfigure}[t]{.5\textwidth}
		\centering
		\includestandalone{traditional}
		\caption{Traditional layered approach.}
	\end{subfigure}%
	\begin{subfigure}[t]{.5\textwidth}
		\centering
		\includestandalone{tosd}
		\caption{\Gls{TOSD} approach.}
	\end{subfigure}
	\caption{Separation of concerns in a traditional setting and in \gls{TOSD} (adapted from~\cite[pg.\ 20]{wang_maintaining_2018}).}%
	\label{fig:tosd}
\end{figure}

\begin{description}
	\item[\Glsxtrshort{UI} (presentation layer):]
		The \gls{UI} of the system is automatically generated from the representation of the type.
%		For instance, \gls{TOP} languages implemented in an \gls{FP} language often use generic programming or template metaprogramming to automatically achieve this.
%		\Gls{TOP} languages embedded in imperative programming languages may use introspection\todo{Do I want this sentence here?}.
		Even though the \gls{UI} is generated from the structure of the datatypes, in practical \gls{TOP} systems it can be tweaked afterwards to suit the specific needs of the application.
	\item[Tasks (business layer):]
		A task is an abstract representation of a piece of work that needs to be done.
		It provides an intuitive abstraction over work in the real world.
		Just as with real-life tasks and workflow, tasks can be combined in various ways such as in parallel or in sequence.
		Furthermore, a task is observable which means it is possible to observe a---partial---result during execution and act upon it by for example starting new tasks.
		Examples of tasks are filling in a form, sending an email, reading a sensor or even doing a physical task.
	\item[\Glsxtrshortpl{SDS} (resource access):]
		Tasks can communicate using task values but this imposes a problem in many collaboration patterns where tasks that are not necessarily related need to share data.
		Tasks can also share data using \glspl{SDS}, an abstraction over any data.
		An \gls{SDS} can represent typed data stored in a file, a chunk of memory, a database \etc.
		\Glspl{SDS} can also represent external impure data such as the time, random numbers or sensory data.
		Similar to tasks, transformation and combination of \glspl{SDS} is possible.
	\item[Programming language (\glsxtrshort{UOD}):]
		The \gls{UOD} from the business layer is explicitly and separately modelled by the relations that exist in the functions of the host language.
\end{description}

The concept of \gls{TOP} originated from the \gls{ITASK} framework, a declarative workflow language for defining multi-user distributed web applications implemented as an \gls{EDSL} in the lazy pure \gls{FP} language \gls{CLEAN}~\citep{plasmeijer_itasks:_2007,plasmeijer_task-oriented_2012}.
While \gls{ITASK} conceived \gls{TOP}, it is not the only \gls{TOP} language.
Some \gls{TOP} languages arose from Master's and Bachelor's thesis projects (e.g.\ \textmu{}Task \citep{piers_task-oriented_2016} and LTasks \citep{van_gemert_task_2022}) or were created to solve a practical problem (e.g.\ Toppyt \citep{lijnse_toppyt_2022} and hTask \citep{lubbers_htask_2022}).

Furthermore, \gls{TOPHAT} is a fully formally specified \gls{TOP} language designed to capture the essence of \gls{TOP} formally~\citep{steenvoorden_tophat_2019}.
 created \textmu{}Task, a \gls{TOP} language for specifying non-interruptible embedded systems implemented as an \gls{EDSL} in \gls{HASKELL}.
\citet{van_gemert_task_2022} created LTasks, a \gls{TOP} language for interactive terminal applications implemented in LUA, a dynamically typed imperative language.
\citet{lijnse_toppyt_2022} created Toppyt, a \gls{TOP} language based on \gls{ITASK}, implemented in \gls{PYTHON}, but designed to be simpler and smaller.
Finally there is \gls{MTASK}, \gls{TOP} language designed for defining workflow for \gls{IOT} devices~\cite{koopman_task-based_2018}.
It is written in \gls{CLEAN} as an \gls{EDSL} fully integrated with \gls{ITASK} and allows the programmer to define all layers of an \gls{IOT} system from a single source.

\section{Outline}
\todo[inline]{uitbreiden}
On Wikipedia, a rhapsody is defined as follows~\citep{wikipedia_contributors_rhapsody_2022}:
\begin{quote}
	A \textbf{rhapsody} in music is a one-movement work that is episodic yet integrated, free-flowing in structure, featuring a range of highly contrasted moods, colour, and tonality. An air of spontaneous inspiration and a sense of improvisation make it freer in form than a set of variations.
\end{quote}
This thesis follows the tradition and consists of three movements that are episodic yet integrated, a purely functional rhapsody.
\Cref{prt:dsl} is about \gls{EDSL} techniques, \cref{prt:top} elaborates on \gls{TOP} for the \gls{IOT} and \cref{prt:tvt} compares traditional tiered \gls{IOT} architectures to a tierless architectures such as \gls{TOP}.
The movements are readable independently if the reader is familiarised with the background material provided in \cref{chp:introduction}.
The thesis wraps up with \cref{chp:conclusion} that provides a conclusion and an outlook on future work.

\subsection*{\nameref{prt:dsl}}
This movement is a cumulative---paper-based---movement that focusses on techniques for embedding \glspl{DSL} in \gls{FP} languages.
After reading the first chapter, subsequent chapters in this movement are readable independently.

\subsubsection*{\fullref{chp:dsl_embedding_techniques}}
This chapter shows the basic \gls{DSL} embedding techniques and compares the properties of several embedding methods.
This chapter is not based on a paper and written as a extra background material for the subsequent chapters in the movement.

\subsubsection*{\fullref{chp:classy_deep_embedding}}
This chapter is based on the paper: \bibentry{lubbers_deep_2022}\todo{change in-press when published}.

While supervising \citeauthor{amazonas_cabral_de_andrade_developing_2018}'s \citeyear{amazonas_cabral_de_andrade_developing_2018} Master's thesis, focussing on an early version of \gls{MTASK}, a seed was planted for a novel deep embedding technique for \glspl{DSL} where the resulting language is extendible both in constructs and in interpretation using type classes and existential data types.
Slowly the ideas organically grew to form the technique shown in the paper.

The research from this paper and writing the paper was solely performed by me.
\Cref{sec:classy_reprise} was added after publication and contains a (yet) unpublished extension of the embedding technique.

\subsubsection*{\fullref{chp:first-class_datatypes}}
This chapter is based on the paper: \bibentry{lubbers_first-class_2022}\todo{change when accepted}.

It shows how to inherit data types from the host language in \glspl{EDSL} using metaprogramming.

The research in this paper and writing the paper was performed by me, though there were weekly meetings with Pieter Koopman and Rinus Plasmeijer in which we discussed and refined the ideas.

\subsection*{\nameref{prt:top}}
This part is a monograph focussing on \glspl{TOP} for the \gls{IOT} and hence are the chapters best read in order.
The monograph is compiled from the following papers and revised lecture notes.

\begin{itemize}
	\item \citeentry{koopman_task-based_2018}

		This was the initial \gls{TOP}/\gls{MTASK} paper.
		Pieter Koopman wrote it, I helped with the software and research.
	\item \citeentry{lubbers_task_2018}
		
		This paper was an extension of my Master's thesis~\citep{lubbers_task_2017}.
		It shows how a simple imperative variant of \gls{MTASK} was integrated with \gls{ITASK}.
		While the language was a lot different than later versions, the integration mechanism is still used in \gls{MTASK} today.
		The research in this paper and writing the paper was performed by me, though there were weekly meetings with Pieter Koopman and Rinus Plasmeijer in which we discussed and refined the ideas.
	\item \citeentry{lubbers_multitasking_2019}\footnote{%
		This work acknowledges the support of the ERASMUS+ project ``Focusing Education on Composability, Comprehensibility and Correctness of Working Software'', no. 2017--1--SK01--KA203--035402
		}

		This paper was a short paper on the multitasking capabilities of \gls{MTASK} in contrast to traditional multitasking methods for \gls{ARDUINO}.
		The research in this paper and writing the paper was performed by me, though there were weekly meetings with Pieter Koopman and Rinus Plasmeijer.
	\item \citeentry{koopman_simulation_2018}\footnotemark[\value{footnote}]\todo{change when published}

		These revised lecture notes are from a course on the \gls{MTASK} simulator was provided at the 2018 \gls{CEFP}/\gls{3COWS} winter school in Ko\v{s}ice, Slovakia.

		Pieter Koopman wrote and taught it, I helped with the software and research.
	\item \citeentry{lubbers_writing_2019}\footnotemark[\value{footnote}]\todo{change when published}

		These revised lecture notes are from a course on programming in \gls{MTASK} provided at the 2019 \gls{CEFP}/\gls{3COWS} summer school in Budapest, Hungary.

		Pieter Koopman prepared and taught half of the lecture and supervised the practical session.
		I taught the other half of the lecture, wrote the lecture notes, made the assignments and supervised the practical session.
	\item \citeentry{lubbers_interpreting_2019}

		This paper shows an implementation for \gls{MTASK} for microcontrollers in the form of a compilation scheme and informal semantics description.

		The research in this paper and writing the paper was performed by me, though there were weekly meetings with Pieter Koopman and Rinus Plasmeijer.
	\item \citeentry{crooijmans_reducing_2022}\todo{change when published}

		This paper shows how to create a scheduler so that devices running \gls{MTASK} tasks can go to sleep more automatically.
		Furthermore, it shows how to integrate hardware interrupts into \gls{MTASK}.
		The research was carried out by \citet{crooijmans_reducing_2021} during his Master's thesis.
		I did the daily supervision and helped with the research, Pieter Koopman was the formal supervisor and wrote most of the paper.
	\item \emph{Green Computing for the Internet of Things}\footnote{
		This work acknowledges the support of the Erasmus+ project ``SusTrainable---Promoting Sustainability as a Fundamental Driver in Software Development Training and Education'', no. 2020--1--PT01--KA203--078646}\todo{change when published}

		These revised lecture notes are from a course on sustainable programming using \gls{MTASK} provided at the 2022 SusTrainable summer school in Rijeka, Croatia.

		Pieter prepared and taught a quarter of the lecture and supervised the practical session.
		I prepared and taught the other three quarters of the lecture, made the assignments and supervised the practical session\todo{writing contribution}.
\end{itemize}

\subsection*{\nameref{prt:tvt}}
This chapter is based on the journal paper: \citeentry{lubbers_could_2022}\footnote{This work is an extension of the conference article: \citeentry{lubbers_tiered_2020}\footnotemark{}}.
\footnotetext{This paper was partly funded by the Radboud-Glasgow Collaboration Fund.}

It compares programming traditional tiered architectures to tierless architectures by showing a qualitative and a quantitative four-way comparison of a smart campus application.

Writing the paper was performed by all authors.
I created the server application, the \gls{CLEAN}/\gls{ITASK}/\gls{MTASK} implementation (\acrshort{CWS}) and the \gls{CLEAN}/\gls{ITASK} implementation (\acrshort{CRS})
Adrian Ramsingh created the \gls{MICROPYTHON} implementation (\acrshort{PWS}), the original \gls{PYTHON} implementation (\acrshort{PRS}) and the server application were created by Jeremy Singer, Dejice Jacob and Kristian Hentschel~\citep{hentschel_supersensors:_2016}.

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