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\chapter{Prelude}%
\label{chp:introduction}
\begin{chapterabstract}
	This chapter introduces the dissertation by providing:
	\begin{itemize}
		\item a general introduction to the topics and research directions;
		\item a reading guide;
		\item background material on the \glsxtrlong{IOT}, \glsxtrlongpl{DSL}, \glsxtrlong{TOP}, \gls{ITASK}, and \gls{MTASK};
		\item and a detailed overview of the scientific contributions of this dissertation.
	\end{itemize}
\end{chapterabstract}

This dissertation is about orchestrating \gls{IOT} systems harmlessly and efficiently.
\todo{beter?}
There are at least 13.4 billion devices connected to the internet at the time of writing \citep{transforma_insights_current_2023}.
Each of these devices sense, act, or otherwise, interact with people, computers, and the environment.
Despite their immense diversity, they are all computers and they all require software to operate.

An increasing number of these connected devices are so-called edge devices that operate in the \gls{IOT}.
Edge devices are the leaves of the \gls{IOT} systems.
They perform the interaction with the physical world.
It is not uncommon for edge devices to be physically embedded in the fabric itself.
Typically, they reside in hard-to-reach places such as light bulbs, clothing, smart electricity meters, buildings, or even farm animals.
The majority of edge devices are powered by microcontrollers.
Microcontrollers are equipped with a lot of connectivity for integrating peripherals such as sensors and actuators.
The connectivity makes them very suitable to interact with their surroundings.
These miniature computers contain integrated circuits that accommodate a microprocessor designed for use in embedded applications.
As a consequence, microcontrollers are cheap; tiny; have little memory; and contain a slow, but energy-efficient processor.

When coordinating an orchestra of edge devices, there is room for little error.
Edge devices come and go, perform their own pieces, or are sometimes instructed to perform a certain piece, they might even operate without a central authority.
In a traditional setting, an \gls{IOT} engineer has to program each device and their interoperation using different programming paradigms, programming languages, and abstraction levels.
This results in semantic friction, which makes programming and maintaining \gls{IOT} systems a complex and error-prone process.

This dissertation describes the research carried out around orchestrating these complex \gls{IOT} systems using \gls{TOP}.
\Gls{TOP} is an innovative tierless programming paradigm for interactive multi-layered systems.
By utilising advanced compiler technologies, much of the internals, communication, and interoperation between the tiers or layers of the applications are automatically generated.
The compiler makes an application controlling all interconnected components from a single declarative specification of the required work.
For example, the \gls{TOP} system \gls{ITASK} is used to program all layers of multi-user distributed web applications from a single source specification.
It is implemented in the general-purpose lazy functional programming language \gls{CLEAN}, and therefore requires relatively powerful hardware.
The inflated hardware requirements are no problem for regular computers but impractical for the average edge device.

This is where an additional \glspl{DSL} must play its part.
\Glspl{DSL} are programming languages tailored to a specific domain.
Consequently, jargon is not expressed in terms of the language itself, but are built-in language features.
Furthermore, the \gls{DSL} can eschew language or system features that are irrelevant for the domain.
Using \glspl{DSL}, hardware requirements can be drastically lowered, even while maintaining a high abstraction level for the specified domain.

To incorporate the plethora of edge devices in the orchestra of a \gls{TOP} system, the \gls{MTASK} system is used.
The \gls{MTASK} language is a novel programming language for programming \gls{IOT} edge devices using \gls{TOP}.
Where \gls{ITASK} abstracts away from the gritty details of multi-tier web applications, \gls{MTASK} has domain-specific abstractions for \gls{IOT} edge devices, maintaining the high abstraction level that \gls{TOP} offers.
As it is integrated with \gls{ITASK}, it allows for all layers of an \gls{IOT} application to be programmed from a single source.

\section{Reading guide}%
\label{lst:reading_guide}
This work is structured as a purely functional rhapsody.
The \citet{wikipedia_contributors_rhapsody_2022} define a musical rhapsody is defined as follows:
\begin{quote}\emph{%
	A \emph{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.}
\end{quote}
This dissertation consists of three episodes.
\Cref{prt:dsl} is a paper-based---otherwise known as cumulative---episode containing chapters that provide insight in advanced \gls{DSL} embedding techniques for \gls{FP} languages.
The chapters can be read independently of each other.
\Cref{prt:top} is a monograph showing \gls{MTASK}, a \gls{TOP} \gls{DSL} for the \gls{IOT}.
Hence, the chapters in this episode are best read in order.
It introduces \gls{IOT} edge device programming, shows the complete \gls{MTASK} language, provides details on how \gls{MTASK} is integrated with \gls{ITASK}, shows how the byte code compiler is implemented, presents a guide for green computing with \gls{MTASK}, and ends with a conclusion and overview of future and related work.
\Cref{prt:tvt} consists of a single chapter that is based on a journal article.
The chapter provides a qualitative and quantitative comparison of traditional tiered \gls{IOT} programming  and tierless programming using a real-world application.
The chapter is readable independently.

The following sections in this prelude provide background material on the \gls{IOT}, \glspl{DSL}, and \gls{TOP} after which a detailed overview of the contributions is presented.

\section{Internet of things}%
\label{sec:back_iot}
The \gls{IOT} is growing rapidly, and it is changing the way people and machines interact with each other and 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 eighties in a company speech by \citet{lewis_speech_1985}:

\begin{quote}
	\emph{The \glsxtrlong{IOT}, or \glsxtrshort{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}

Much later, CISCO states that the \gls{IOT} started when there were as many connected devices as there were people on the globe, i.e.\ around 2008 \citep{evans_internet_2011}.
Today, \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 they operate in.
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 for compartmentalisation.
The number of tiers depends on the required complexity of the model.
For the intents and purposes of this thesis, the layered architecture as shown in \cref{fig:iot-layers} is used.

\begin{figure}
	\centering
	\includestandalone{iot-layers}
	\caption{A layered \gls{IOT} architecture.}%
	\label{fig:iot-layers}
\end{figure}

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

The application layer is the core of the system.
It provides the \glspl{API}, data interfaces, data storage processing, and data processing of \gls{IOT} systems.
A cloud server or local server provides this layer in a typical home automation application.

The perception layer---also called edge layer---collects the data and interacts with the environment.
It consists of edge devices such as microcontrollers equipped with various sensors and actuators.
In home automation this layer consists of all devices hosting sensors and actuators such as smart light bulbs, actuators to open doors, or temperature and humidity sensors.

All layers are connected using the network layer.
In some applications this is implemented using conventional networking techniques such as \gls{WIFI} or Ethernet.
However, network technology that is tailored to the needs of the specific interconnection between two layers is increasingly popular.
Examples of this are BLE, LoRa, ZigBee, and LTE-M as a communication protocol for connecting the perception layer to the application layer using \gls{IOT} transport protocols such as \gls{MQTT}.
Protocols such as HTTP, AJAX, and WebSocket connecting the presentation layer to the application layer that are designed for the use in web applications.

\todo[inline]{Hier enkele zinnen over maintainability}
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, the perception layer itself is often a heterogeneous collection of microcontrollers in itself, each having their own peculiarities, programming language of choice, and hardware interfaces.
As edge hardware needs to be cheap, small scale, and energy efficient, the microcontrollers used to power them do not have a lot of computational power, only a smidge of memory, and little communication bandwidth.
Typically, these devices are unable to run a full-fledged general-purpose \gls{OS}.
Rather they employ compiled firmware written in imperative languages that combines all tasks on the device in a single program.
While devices are getting a bit faster, smaller, and cheaper, they keep these properties to an extent.
For example, more powerful microcontrollers are capable of running \glspl{RTOS}, but this still requires a lot of resources and fixes the programs at compile time.
As a consequence, the flexibility is greatly reduced for dynamic systems in which tasks are created on the fly, executed on demand, require parallel execution, or have dynamic scheduling behaviour.
As program memory is mostly flash-based and only lasts a couple of thousands of writes before it wears out, it is not suitable for repeated reconfiguring and reprogramming.

Memory wear problems can be mitigated by dynamically sending code to be interpreted to the microcontroller.
With interpretation, a specialised interpreter is flashed in the program memory once it receives the program code to execute at run time.
Therefore, as the programs are not stored in the flash memory, it does not wear out.
It is challenging to create interpreters for small edge devices due to the severe hardware restrictions.
This dissertation describes a \gls{DSL} that includes the high-level programming concepts of \gls{TOP}, while it can be executed on edge devices with very limited hardware requirements.
It does so by compiling the \gls{DSL} to byte code that is executed in a feather-light domain-specific \gls{OS}.

\section{Domain-specific languages}%
\label{sec:back_dsl}
% General
Programming languages can be divided up into two categories: \glspl{DSL} 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 a \gls{DSL} is easier and requires less \gls{GPL} knowledge for a domain expert.
This does come at the cost of the \gls{DSL} being sometimes less expressive to an extent that it may not even be Turing complete.
\Glspl{DSL} come in two main flavours: standalone and embedded (\cref{sec:standalone_embedded})\footnote{Standalone and embedded are also called external and internal respectively.}.
Standalone languages are languages for which the complete toolchain has been developed, just as for any other \gls{GPL}.
Embedded languages piggyback on an existing language, they are defined in terms of their host language.
\Glspl{EDSL} can further be classified into heterogeneous and homogeneous languages (\cref{sec:hetero_homo}).
In homogeneous languages all components are integrated whereas in heterogeneous \glspl{DSL}, some parts are agnostic of the other systems, e.g.\ a \gls{DSL} that generates code for execution on a totally different system.
This hyponymy is shown in \cref{fig:hyponymy_of_dsls}.

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

\subsection{Standalone and embedded}%
\label{sec:standalone_embedded}
\glspl{DSL} were historically created as standalone languages, meaning that all 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, and all the scaffolding for the language, making standalone \glspl{DSL} costly to create.
Examples of standalone \glspl{DSL} are \TeX, make, yacc, XML, SQL, \etc.

The dichotomous approach to standalone \glspl{DSL} 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 \citep{krishnamurthi_linguistic_2001}.
This greatly reduces the cost of creating embedded languages and shields the user from having to learn the host language and toolchain.
However, there are two sides to this coin.
If the syntax of the host language is not very flexible, the syntax of the \gls{DSL} can become clumsy.
Furthermore, \gls{DSL} 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}.
\Gls{FP} languages are especially suitable for hosting embedded \glspl{DSL}.
They offer tooling for building abstraction levels by a strong and versatile type system, minimal but flexible syntax, and referential transparency.

\subsection{Heterogeneity and homogeneity}%
\label{sec:hetero_homo}
\Citet{tratt_domain_2008} applies a notion from metaprogramming \citep{sheard_accomplishments_2001} to \glspl{EDSL} to define homogeneity and heterogeneity of \glspl{EDSL} as follows:

\begin{quote}
	\emph{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 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, regions, but also more complex tasks such as \glspl{GUI}.

On the other hand, heterogeneous \glspl{EDSL} are languages that are not executed in the host language.
For example, \citet{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.

Both \gls{ITASK} and \gls{MTASK} are \glspl{EDSL}.
Programs written in \gls{ITASK} run in the host language, and it is a homogeneous \gls{DSL}.
Tasks written using \gls{MTASK} are dynamically compiled to byte code for an edge device, making it a heterogeneous \gls{DSL}.
The interpreter running on the edge device has no knowledge of the higher level task specification.
It just interprets the byte code it was sent and takes care of the communication.

\section{Task-oriented programming}%
\label{sec:back_top}
\Gls{TOP} is a declarative programming paradigm for modelling interactive systems \citep{plasmeijer_task-oriented_2012}.
Instead of dividing problems into layers, \gls{TOP} deals with separation of concerns in a novel way.
This approach to software development is called \gls{TOSD} \citep{wang_maintaining_2018}.

\begin{figure}
	\centering
	\includestandalone{tosd}
	\caption{Separation of concerns in \gls{TOSD} (adapted from \citep[\citepage{20}]{wang_maintaining_2018}).}%
	\label{fig:tosd}
\end{figure}

\begin{description}
	\item[Types:]
		As can be seen from \cref{fig:tosd}, types are the pivotal component in \gls{TOP}.
		From the data types, utilising various \emph{type-parametrised} concepts, all other aspects are handled automatically.
		Hence, all other components arise from and depend on the types in the program.
	\item[Tasks:]
		In \gls{TOP} languages, tasks are the basic building blocks.
		A task is an abstract representation of a piece of work that needs to be done.
		It provides an abstraction over work in the real world.
		The nature of tasks makes them observable during execution.
		It is possible to observe the current task value act upon it, e.g.\ taking a partial result as good enough, or by starting new tasks.
		Examples of tasks are filling forms, sending emails, reading sensors or even doing physical tasks.
		Just as with real-life tasks, multiple tasks can be combined in various ways such as in parallel or in sequence to form workflows.
		Such combination operators are called task combinators.
	\item[\Glspl{SDS}:]
		Tasks mainly communicate using their observable task values.
		However, some collaboration patterns are more easily expressed by tasks that share common data.
		\Glspl{SDS} fill this gap, they offer a safe 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 sensor data.
		In many \gls{TOP} languages, combinators are available to filter, combine, transform, and focus \glspl{SDS}.
	\item[\Gls{UI}:]
		The \gls{UI} of the system is automatically generated from the structural representation of the types.
		Though, practical \gls{TOP} systems allow tweaking afterwards to suit the specific needs of the application.
	\item[\Gls{UOD}:]
		The \gls{UOD} is explicitly and separately modelled by the data types and relations that exist in the functions of the host language.
\end{description}

\Cref{fig:tosd} differs from the presented \gls{IOT} architecture shown in \cref{fig:iot-layers} because they represent different concepts.
The \gls{IOT} architecture is an execution architecture whereas \gls{TOSD} is a software development model.
E.g.\ from a software development perspective, a task is a task, whether it is executed on a microcontroller, a server, or a client.
Only when a task is executed, the location of the execution becomes important, but this is taken care of by the system.
Some concepts from the \gls{TOSD} model can be mapped upon the \gls{IOT} architecture in two ways.
Firstly, edge devices can be seen as simple resources, thus accessed through \glspl{SDS}.
The second view is that edge devices contain miniature \gls{TOP} systems in itself.
The individual components in the miniature systems, the tasks, the \glspl{SDS}, are, in the eventual execution, connected to the main system.
\todo{hier plaatje uit 6?: nee}

\subsection{The iTask system}
The concept of \gls{TOP} originated from the \gls{ITASK} framework, a declarative language and \gls{TOP} engine for defining interactive multi-user distributed web applications.
The \gls{ITASK} system is implemented as an \gls{EDSL} in the programming language \gls{CLEAN}\footnote{\Cref{chp:clean_for_haskell_programmers} contains a guide for \gls{CLEAN} tailored to \gls{HASKELL} programmers.} \citep{plasmeijer_itasks:_2007,plasmeijer_task-oriented_2012}.
It is under development for over fifteen years and has proven itself through use in industry for some time now as well.
For example, it is the main language of VIIA, an advanced application for monitoring coasts \citep{top_software_viia_2023}.
From the structural properties of the data types and the current status of the work to be done, the entire \gls{UI} is automatically generated.
Browsers are powering \gls{ITASK}'s presentation layer.
The framework is built on top of standard web techniques such as JavaScript, HTML, and {CSS}.
The browser runs the actual \gls{ITASK} code using an interpreter that operates on \gls{CLEAN}'s intermediate language \gls{ABC} \citep{staps_lazy_2019}.

Tasks in \gls{ITASK} have either \emph{no value}, an \emph{unstable} or a \emph{stable} task value.
For example, an editor for filling in a form initially has no value.
Once the user entered a complete value, its value becomes an unstable value, it can still be changed or even reverted to no value by emptying the editor again.
Only when for example a continue button is pressed, a task value becomes stable, fixing its value.
The allowed task value transitions are shown in \cref{fig:taskvalue}.

\begin{figure}
	\centering
	\includestandalone{taskvalue}
	\caption{Transition diagram for task values in \gls{ITASK}.}%
	\label{fig:taskvalue}
\end{figure}

As an example, \cref{lst:todo,fig:todo} show the code and \gls{UI} for an interactive to-do list application.
The user modifies a shared to-do list through an editor directly or using some predefined actions.
Furthermore, in parallel, the length of the list is shown to demonstrate \glspl{SDS}.
Using \gls{ITASK}, complex collaborations of users and tasks can be described on a high level.

From the data type definitions (\cref{lst:todo_dt}), using generic programming (\cref{lst:todo_derive}), the \glspl{UI} for the data types are automatically generated.
Then, using the parallel task combinator (\cleaninline{-\|\|}) the task for updating the to-dos (\cref{lst:todo_update}) and the task for viewing the length are combined (\cref{lst:todo_length}, shown as \emph{Length: 2} in the bottom of the image).
This particular parallel combinator uses the result of the left-hand side task.
Both tasks operate on the to-do \gls{SDS} (\cref{lst:todo_sds}).
The task for updating the to-do list is an editor (\cref{lst:todo_editor}) combined using a step combinator (\crefrange{lst:todo_contfro}{lst:todo_contto}).
The actions either change the value, sorting or clearing it, or terminate the task by returning the current value of the \gls{SDS}, visualised as three buttons on the bottom right of the \gls{UI}.
Special combinators (e.g.\ \cleaninline{@>>} at \cref{lst:todo_ui}) are used to tweak the \gls{UI} to display informative labels.
\todo[inline]{Zou je hier niet zeggen dat dit iTask programma de Presentation en Application layer uit Figure 1.1 maakt door dezelfde code in de browser en op de server te runnen en alle netwerkdingen te genereren? Dat is hier wel niet erg nodig. Maar geeft je wel het opstapje om in 1.4.2.\ te zeggen dat de edge devices die wij willen gebruiken dat niet kunnen. Deze paragraaf kan ook in 1.4.2.}

\cleaninputlisting[float=,firstline=6,lastline=22,tabsize=3,numbers=left,caption={The code for a shared to-do list in \gls{ITASK}.},label={lst:todo}]{lst/sharedlist.icl}

\begin{figure}
	\centering
	\includegraphics[width=\linewidth]{todo0g}
	\caption{The \gls{UI} for the shared to-do list in \gls{ITASK}.}%
	\label{fig:todo}
\end{figure}

\subsection{The mTask system}
The work for \gls{IOT} edge devices can often be succinctly described by \gls{TOP} programs.
Software on microcontrollers is usually composed of smaller basic tasks, are interactive, and share data with other components or the server.
The \gls{ITASK} system seems an obvious candidate for bringing \gls{TOP} to \gls{IOT} edge devices.
However, an \gls{ITASK} application contains many features that are not needed on \emph{edge devices} such as higher-order tasks, support for a distributed architecture, or a multi-user web server.
\todo[inline]{Dat is op zich toch niet zo'n probleem. Een iTAsk server heeft ook geen GUI, maar wel alle iTask code om die te maken.
Het feit dat dit niet hoeft op een edge device geeft je wel een van de handvatten om mTask in minder memory te runnen}
Furthermore, \gls{IOT} edge devices are in general not powerful enough to run or interpret \gls{CLEAN}\slash\gls{ABC} code, they just lack the processor speed and memory.
To bridge this gap, \gls{MTASK} is developed, a domain-specific \gls{TOP} system for \gls{IOT} edge devices that is integrated in \gls{ITASK} \citep{koopman_task-based_2018}.
The \gls{ITASK} language abstracts away from details such as user interfaces, data storage, client-side platforms, and persistent workflows.
On the other hand, \gls{MTASK} offers abstractions for edge layer-specific details such as the heterogeneity of architectures, platforms, and frameworks; peripheral access; task scheduling; and lowering energy consumption.

The \gls{MTASK} system is seamlessly integrated with \gls{ITASK}.
Tasks in \gls{MTASK} are integrated in such a way that they function as regular \gls{ITASK} tasks.
Furthermore, \glspl{SDS} on the device can proxy \gls{ITASK} \glspl{SDS}.
Using \gls{MTASK}, the programmer can define all layers of an \gls{IOT} system as a single declarative specification.
The \gls{MTASK} language is written in \gls{CLEAN} as a multi-view \gls{EDSL} and hence there are multiple interpretations possible.
This thesis mostly discusses the byte code compiler.
From an \gls{MTASK} task constructed at run time, a compact binary representation of the work that needs to be done is compiled.
And while the byte code for \gls{MTASK} is generated at run time, the type system of the host language \gls{CLEAN} prevents type errors in the generated code.
This byte code is then sent to a device that running the \gls{MTASK} \gls{RTS}.
This feather-light domain-specific \gls{OS} is written in portable \gls{C} with a minimal device specific interface and it executes the tasks using interpretation and rewriting.

To illustrate \imtask{}, an example application is shown.
The application is an interactive application for blinking \pgls{LED} on the microcontroller at a certain frequency that can be set and updated at run time.
\Cref{lst:intro_blink,fig:intro_blink} show the \gls{ITASK} part of the code and a screenshot.
Using \cleaninline{enterInformation}, the connection specification of the \gls{TCP} device is queried through a web editor (\cref{lst:intro_enterDevice,fig:intro_blink_dev}).
\Cref{lst:intro_withshared} defines \pgls{SDS} to communicate the blinking interval between the server and the edge device.
The \gls{MTASK} device is connected using \cleaninline{withDevice} at \cref{lst:intro_withdevice}.
Once connected, the \cleaninline{intBlink} task is sent to the device (\cref{lst:intro_liftmtask}) and, in parallel, a web editor is shown that updates the value of the interval \gls{SDS} (\cref{lst:intro_editor,fig:intro_blink_int}).
To allow terminating of the task, the \gls{ITASK} task ends with a sequential operation that returns a constant value when the button is pressed, making the task stable.
\todo[inline]{foto device+led?}
\todo[inline]{enterDevice nieuwe regel, globale func?}

\cleaninputlisting[float={!ht},firstline=10,lastline=18,numbers=left,caption={The \gls{ITASK} code for the interactive blinking application.},label={lst:intro_blink}]{lst/blink.icl}

\begin{figure}
	\centering
	\begin{subfigure}{.5\textwidth}
		\centering
		\includegraphics[width=.9\linewidth]{blink1g}
		\caption{Device selection.}\label{fig:intro_blink_dev}
	\end{subfigure}%
	\begin{subfigure}{.5\textwidth}
		\centering
		\includegraphics[width=.9\linewidth]{blink2g}
		\caption{Changing the interval.}\label{fig:intro_blink_int}
	\end{subfigure}
	\caption{The \gls{UI} for the interactive blink application in \gls{MTASK}.}%
	\label{fig:intro_blink}
\end{figure}

The \cleaninline{intBlink} task (\cref{lst:intro_blink_mtask}) is the \gls{MTASK} part of the application.
It blinks \pgls{LED} on the edge device with the delay that is dynamically adjustable by the user via an \gls{ITASK} editor in the browser.
It has its own tasks, \glspl{SDS}, and \gls{UOD}.
This task first defines \gls{GPIO} pin 13 to be of the output type (\cref{lst:intro:declarePin}).
Then the \gls{ITASK} \gls{SDS} is lifted to an \gls{MTASK} \gls{SDS} (\cref{lst:intro:liftsds}), enabling the machinery to keep the \gls{SDS} in sync both on the device and the server.
The main expression of the program calls the \cleaninline{blink} function with an initial state.
This function on \crefrange{lst:intro:blink_fro}{lst:intro:blink_to} first reads the interval \gls{SDS}, waits the specified delay, writes the state to the \gls{GPIO} pin, and calls itself recursively using the inverse of the state in order to run continuously.
The \cleaninline{>>|.} operator denotes the sequencing of tasks in \gls{MTASK}.

\cleaninputlisting[float={!ht},linerange={23-,25-33},numbers=left,caption={The \gls{MTASK} code for the interactive blinking application.},label={lst:intro_blink_mtask}]{lst/blink.icl} %chktex 8
\todo[inline]{Meer wit rondom =jes, doornummeren?}

\subsection{Other TOP languages}
While \gls{ITASK} conceived \gls{TOP}, it is no longer the only \gls{TOP} system.
Some \gls{TOP} languages were created to fill a gap encountered in practise.
Toppyt \citep{lijnse_toppyt_2022} is a general purpose \gls{TOP} language written in \gls{PYTHON} used to host frameworks for modelling command \& control systems.
The hTask system is a \gls{TOP} system written in \gls{HASKELL} used as a vessel for experimenting with asynchronous \glspl{SDS} \citep{lubbers_htask_2022}.
Furthermore, some \gls{TOP} systems arose from Master's and Bachelor's thesis projects.
For example, \textmu{}Task \citep{piers_task-oriented_2016}, a \gls{TOP} language for modelling non-interruptible embedded systems in \gls{HASKELL}, and LTasks \citep{van_gemert_task_2022}, a \gls{TOP} language written in the dynamically typed programming language Lua.
Finally, there are \gls{TOP} languages with strong academic foundations.
\Gls{TOPHAT} is a fully formally specified \gls{TOP} language designed to capture the essence of \gls{TOP} \citep{steenvoorden_tophat_2019}.
Such a formal specification allows for symbolic execution, hint generation, but also the translation to \gls{ITASK} for actually performing the work \citep{steenvoorden_tophat_2022}.
\Citeauthor{steenvoorden_tophat_2022} distinguishes two instruments for \gls{TOP}: \gls{TOP} languages and \gls{TOP} engines.
The language is the \emph{formal} language for specifying interactive systems.
The engine is the software or hardware that executes these specifications as a ready-for-work application.
\todo[inline]{Noemen dat een vergelijkbare semantics voor mTask under construction is met een verwijzing naar de thesis van Elina?}

\section{Contributions}%
\label{sec:contributions}
This section provides a thorough overview of the relation between the scientific publications and the contents of this thesis.

\subsection{\Fullref{prt:dsl}}
The \gls{MTASK} system is an \gls{EDSL} and during the development of it, several novel basal techniques for embedding \glspl{DSL} in \gls{FP} languages have been found.
This paper-based episode contains the following papers:
\todo{papers met bibitem doen? of conferentie noemen.}
\begin{enumerate}
	\item \emph{Deep Embedding with Class} \citep*{lubbers_deep_2022} is the basis for \cref{chp:classy_deep_embedding}.
		It shows a novel deep embedding technique for \glspl{DSL} where the resulting language is extendible both in constructs and in interpretation just using type classes and existential data types.
		The related work section is updated with the research found after publication.
		\Cref{sec:classy_reprise} was added after publication and contains a (yet) unpublished extension of the embedding technique for reducing the required boilerplate at the cost of requiring some advanced type system extensions.
	\item \emph{First-\kern-1ptClass Data Types in Shallow Embedded Domain-Specific Languages} \citep*{lubbers_first-class_2022}\label{enum:first-class} is the basis for \cref{chp:first-class_datatypes}.
		It shows how to inherit data types from the host language in \glspl{EDSL} using metaprogramming by providing a proof-of-concept implementation using \gls{HASKELL}'s metaprogramming system: \glsxtrlong{TH}.
		The chapter also serves as a gentle introduction to, and contains a thorough literature study on \glsxtrlong{TH}.
\end{enumerate}

%\paragraph{In preparation}
%Furthermore, there are some papers either in preparation or under review describing methods for creating \glspl{DSL}.
%They describe techniques found while developing the \gls{MTASK} \gls{DSL} that have not made it (yet) into the system.
%Hence, they are not part of the dissertation.
%
%\begin{itemize}
%	\item \emph{Strongly-Typed Multi-\kern-2ptView Stack-\kern-1.25ptBased Computations} shows how to create type-safe \glspl{EDSL} representing stack-based computations.
%		Instead of encoding the arguments to a function as arguments in the host language, stack-based approaches use a run time stack that contains the arguments.
%		By encoding the required contents of the stack in the types, such systems can be made type safe.
%
%	\item \emph{Template Metaprogramming using Two-Stage Generic Functions} shows how a sufficiently rich generic programming system can achieve much of the same functionality as template metaprogramming.
%		The generic programming functionality of \gls{Clean} is built into the compiler.
%		As a result, metadata of the generic types is added to the generic structure.
%		From this metadata, we can destill not only type and constructor names but also arities, fixity, kinds, types, \etc{}.
%		This allows us, by 
%\end{itemize}

\paragraph{Contribution:}
The papers are written by me, there were weekly meetings with co-authors in which we discussed and refined the ideas.

\subsection{\crtCref{prt:top}: \hspace{8.28992pt}\nameref{prt:top}}
This episode is a monograph that shows the design, properties, implementation and usage of the \gls{MTASK} system and \gls{TOP} for the \gls{IOT}.
It is compiled from the following publications:

\begin{enumerate}[resume]
	\item \emph{A Task-\kern-1.25ptBased \glsxtrshort{DSL} for Microcomputers} \citep*{koopman_task-based_2018}
		is the initial \gls{TOP}\slash{}\gls{MTASK} paper.
		It provides an overview of the initial \gls{TOP} \gls{MTASK} language and shows first versions of some interpretations.
	\item \emph{Task Oriented Programming for the Internet of Things} \citep*{lubbers_task_2018}\footnote{This work is an extension of my Master's thesis \citep{lubbers_task_2017}.}
		shows how a simple imperative variant of \gls{MTASK} was integrated with \gls{ITASK}.
		While the language differs a lot from the current version, the integration mechanism is still used.
%		\paragraph{Contribution}
%		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 \emph{Multitasking on Microcontrollers using Task Oriented Programming} \citep*{lubbers_multitasking_2019}\footnote{This work acknowledges the support of the \erasmusplus{} project ``Focusing Education on Composability, Comprehensibility and Correctness of Working Software'', no.\ 2017--1--SK01--KA203--035402.}
		is a short paper on the multitasking capabilities of \gls{MTASK} comparing it to traditional multitasking methods for \gls{ARDUINO}.
%		\paragraph{Contribution}
%		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 \emph{Simulation of a Task-\kern-1.25ptBased Embedded Domain Specific Language for the Internet of Things} \citep*{koopman_simulation_2023}\footnotemark[\value{footnote}]
		are the revised lecture notes for a course on the \gls{MTASK} simulator provided at the 2018 \gls{3COWS} winter school in Ko\v{s}ice, Slovakia, January 22--26, 2018.
%		\paragraph{Contribution}
%		Pieter Koopman wrote and taught it, I helped with the software and research.
	\item \emph{Writing Internet of Things Applications with Task Oriented Programming} \citep*{lubbers_writing_2023}\footnotemark[\value{footnote}]
		are the revised lecture notes from a course on programming \gls{IOT} systems using \gls{MTASK} provided at the 2019 \gls{3COWS} summer school in Budapest, Hungary, June 17--21, 2019.
%		\paragraph{Contribution}
%		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 \emph{Interpreting Task Oriented Programs on Tiny Computers} \citep*{lubbers_interpreting_2019}
		shows an implementation of the byte code compiler and \gls{RTS} of \gls{MTASK}.
%		\paragraph{Contribution}
%		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 \emph{Reducing the Power Consumption of IoT with Task-Oriented Programming} \citep*{crooijmans_reducing_2022}
		shows how to create a scheduler so that devices running \gls{MTASK} tasks can go to sleep more automatically and how interrupts are incorporated in the language.
%		\paragraph{Contribution}
%		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} \citep*{lubbers_green_2022}\footnote{This work acknowledges the support of the \erasmusplus{} project ``SusTrainable---Promoting Sustainability as a Fundamental Driver in Software Development Training and Education'', no.\ 2020--1--PT01--KA203--078646.}
		are the revised lecture notes from a course on sustainable \gls{IOT} programming with \gls{MTASK} provided at the 2022 SusTrainable summer school in Rijeka, Croatia, July 4--8, 2022.

%		\paragraph{Contribution}
%		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
\end{enumerate}

\paragraph{Contribution:}
The original \gls{MTASK} language, and their initial interpretations were developed by Pieter Koopman and Rinus Plasmeijer.
I extended the language, developed the byte code interpreter, the integration with \gls{ITASK}, and the \gls{RTS}.
The papers of which I am first author are solely written by me, there were weekly meetings with the co-authors in which we discussed and refined the ideas.

\subsection{\Fullref{prt:tvt}}
\Cref{prt:tvt} is based on a journal paper that quantitatively and qualitatively compares traditional \gls{IOT} architectures with \gls{TOP} \gls{IOT} architectures.
\begin{enumerate}[resume]
	\item \emph{Could Tierless Programming Reduce IoT Development Grief?} \citep*{lubbers_could_2022}
		is an extended version of paper~\ref{enum:iot20}.
		It compares programming traditional tiered architectures to tierless architectures by illustrating a qualitative and a quantitative four-way comparison of a smart-campus application.
	\item \emph{Tiered versus Tierless \glsxtrshort{IOT} Stacks: Comparing Smart Campus Software Architectures} \citep*{lubbers_tiered_2020}\footnote{This work was partly funded by the 2019 Radboud-Glasgow Collaboration Fund.}\label{enum:iot20} compares traditional tiered programming to tierless architectures by comparing two implementations of a smart-campus application.
\end{enumerate}

\paragraph{Contribution:}
Writing the paper was performed by all authors.
I created the server application, the \cimtask{} implementation (\glsxtrshort{CWS}), and the \citask{} implementation (\glsxtrshort{CRS});
Adrian Ramsingh created the \gls{MICROPYTHON} implementation (\glsxtrshort{PWS}); the original \gls{PYTHON} implementation (\glsxtrshort{PRS}) and the server application were created by \citet{hentschel_supersensors:_2016}.

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