This section describes some of the history of \gls{CLEAN} and provides a crash course in \gls{CLEAN} pecularities written for \gls{HASKELL} programmers.
It is based on the
-\Gls{CLEAN}---acronym for Clean \acrlong{LEAN}~\cite{barendregt_towards_1987}---, was originally designed as a \gls{GRS} core language but quickly served as an intermediate language for other functional languages~\cite{brus_clean_1987}.
-In the early days it has also been called \emph{Concurrent} \gls{CLEAN}~\cite{nocker_concurrent_1991} but these days the language has no support for this anymore.
-Fast forward thirty years, \gls{CLEAN} is now a robust language with state-of-the-art features and is actually used in industry as well as academia---albeit in select areas of the world~\cite{plasmeijer_clean_2021}.
+\Gls{CLEAN}---acronym for Clean \acrlong{LEAN}~\citep{barendregt_towards_1987}---, was originally designed as a \gls{GRS} core language but quickly served as an intermediate language for other functional languages~\citep{brus_clean_1987}.
+In the early days it has also been called \emph{Concurrent} \gls{CLEAN}~\citep{nocker_concurrent_1991} but these days the language has no support for this anymore.
+Fast forward thirty years, \gls{CLEAN} is now a robust language with state-of-the-art features and is actually used in industry as well as academia---albeit in select areas of the world~\citep{plasmeijer_clean_2021}.
Initially, when it was used mostly as an intermediate language, it had a fairly spartan syntax.
However, over the years, the syntax got friendlier and it currently it looks a lot like \gls{HASKELL}.
-In the past, a \emph{double-edged} fronted even existed that allowed \gls{CLEAN} to be extended with \gls{HASKELL98} syntax and vice versa, however this frontend is no longer maintained~\cite{groningen_exchanging_2010}.
-This chapter therefore gives a brief syntactical and functional comparison, a complete specification of the \gls{CLEAN} language can be found in the latest language report~\cite{plasmeijer_clean_2021}.
-Many of this is based on work by Achten although that was based on \gls{CLEAN} 2.1 and \gls{HASKELL98}~\cite{achten_clean_2007}.
+In the past, a \emph{double-edged} fronted even existed that allowed \gls{CLEAN} to be extended with \gls{HASKELL98} syntax and vice versa, however this frontend is no longer maintained~\citep{groningen_exchanging_2010}.
+This chapter therefore gives a brief syntactical and functional comparison, a complete specification of the \gls{CLEAN} language can be found in the latest language report~\citep{plasmeijer_clean_2021}.
+Many of this is based on work by Achten although that was based on \gls{CLEAN} 2.1 and \gls{HASKELL98}~\citep{achten_clean_2007}.
When \gls{HASKELL} is mentioned we actually mean \gls{GHC}'s \gls{HASKELL}\footnote{If an extension is enabled, a footnote is added stating that \GHCmod{SomeExtension} is required.} this is denoted and by \gls{CLEAN} we mean \gls{CLEAN} 3.1's compiler with the \gls{ITASK} extensions.
\section{Features}
Within functions the strict let (\cleaninline{#!}) can be used to force evaluate an expression, in \gls{HASKELL} \haskellinline{seq} or \haskellinline{\$!} is used for this.
\subsection{Uniqueness typing}
-Types in \gls{CLEAN} may be \emph{unique}, which means that they may not be shared~\cite{barendsen_uniqueness_1996}.
+Types in \gls{CLEAN} may be \emph{unique}, which means that they may not be shared~\citep{barendsen_uniqueness_1996}.
The uniqueness type system allows the compiler to generate efficient code because unique data structures can be destructively updated.
-Furthermore, uniqueness typing serves as a model for side effects as well~\cite{achten_high_1993,achten_ins_1995}.
-\Gls{CLEAN} uses the \emph{world-as-value} paradigm where \cleaninline{World} represents the external environment and is always unique~\cite{backus_introduction_1990}.
+Furthermore, uniqueness typing serves as a model for side effects as well~\citep{achten_high_1993,achten_ins_1995}.
+\Gls{CLEAN} uses the \emph{world-as-value} paradigm where \cleaninline{World} represents the external environment and is always unique~\citep{backus_introduction_1990}.
A program with side effects is characterised by a \cleaninline{Start :: *World -> *World} start function.
-In \gls{HASKELL}, interaction with the world is done using the \haskellinline{IO} monad~\cite{peyton_jones_imperative_1993}.
+In \gls{HASKELL}, interaction with the world is done using the \haskellinline{IO} monad~\citep{peyton_jones_imperative_1993}.
The \haskellinline{IO} monad could very well be---and actually is---implemented in \gls{CLEAN} using a state monad with the \cleaninline{World} as a state.
Besides marking types as unique, it is also possible to mark them with uniqueness attributes variables \cleaninline{u:} and define constraints on them.
For example, to make sure that an argument of a function is at least as unique as another argument.
\end{lstClean}
\subsection{Generics}
-Polytypic functions~\cite{jeuring_polytypic_1996}---also known as generic or kind-indexed fuctions---are built into \gls{CLEAN}~\cite[Chp.~7.1]{plasmeijer_clean_2021}\cite{alimarine_generic_2005} whereas in \gls{HASKELL} they are implemented as a library~\cite[Chp.~6.19.1]{ghc_team_ghc_2021}.
-The implementation of generics in \gls{CLEAN} is very similar to that of Generic H$\forall$skell~\cite{hinze_generic_2003}.
+Polytypic functions~\citep{jeuring_polytypic_1996}---also known as generic or kind-indexed fuctions---are built into \gls{CLEAN}~\cite[Chp.~7.1]{plasmeijer_clean_2021}\citep{alimarine_generic_2005} whereas in \gls{HASKELL} they are implemented as a library~\cite[Chp.~6.19.1]{ghc_team_ghc_2021}.
+The implementation of generics in \gls{CLEAN} is very similar to that of Generic H$\forall$skell~\citep{hinze_generic_2003}.
%When calling a generic function, the kind must always be specified and depending on the kind, the function may require more arguments.
For example, defining a generic equality is done as in \cref{lst:generic_eq}.
\lstinputlisting[language=Clean,firstline=4,label={lst:generic_print},caption={Generic print function in \gls{CLEAN}.}]{lst/generic_print.icl}
\subsection{\texorpdfstring{\glsentrytext{GADT}}{GADT}s}
-\Glspl{GADT} are enriched data types that allow the type instantiation of the constructor to be explicitly defined~\cite{cheney_first-class_2003,hinze_fun_2003}.
+\Glspl{GADT} are enriched data types that allow the type instantiation of the constructor to be explicitly defined~\citep{cheney_first-class_2003,hinze_fun_2003}.
While \glspl{GADT} are not natively supported in \gls{CLEAN}, they can be simulated using embedding-projection pairs or equivalence types~\cite[Sec.~2.2]{cheney_lightweight_2002}.
To illustrate this, \cref{lst:gadt_clean} shows an example \gls{GADT} that would be implemented in \gls{HASKELL} as done in \cref{lst:gadt_haskell}\requiresGHCmod{GADTs}.
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