\begin{document}
\ifSubfilesClassLoaded{
- \author{%
- Mart Lubbers\\
- \texttt{mart@cs.ru.nl}
- \and
- Peter Achten\\
- \texttt{peter@cs.ru.nl}
- }
- \title{Clean for Haskell Programmers}
- \date{\today}
-
- \stopthumb{}%
- \setcounter{chapter}{1}
-
\pagenumbering{arabic}
- \maketitle
- \tableofcontents
}{
- \chapter{\glsentrytext{CLEAN} for \glsentrytext{HASKELL} Programmers}%
- \label{chp:clean_for_haskell_programmers}
}
+\myappendix{chp:clean_for_haskell_programmers}{\texorpdfstring{\glsentrytext{CLEAN}}{Clean} for \texorpdfstring{\glsentrytext{HASKELL}}{Haskell} Programmers}%
+
This note is meant to give people who are familiar with the functional programming language \gls{HASKELL} a consise overview of \gls{CLEAN} language elements and how they differ from \gls{HASKELL}.
The goal is to support the reader when reading \gls{CLEAN} code.
Table~\ref{tbl:syn_clean_haskell} shows frequently occuring \gls{CLEAN} language elements on the left side and their \gls{HASKELL} equivalent on the right side.
Obviously, this summary is not exhaustive.
-Some \gls{CLEAN} language elements that are not easily translatable to \gls{HASKELL} and thus do not occur in the summary follow below.
+Some \gls{CLEAN} language elements that are not easily translatable to \gls{HASKELL} and thus do not occur in the summary following below.
We hope you enjoy these notes and that it aids you in reading \gls{CLEAN} programs.
While \gls{CLEAN} and \gls{HASKELL} were both conceived around 1987 and have similar syntax, there are some subtle differences in syntax and functionality.
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existed that allowed \gls{CLEAN} to be extended with \gls{HASKELL98} syntax and vice versa, however this frontend is no longer maintained~\cite{groningen_exchanging_2010}.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}.
-When \gls{HASKELL} is mentioned we actually mean \gls{GHC}'s \gls{HASKELL} and by \gls{CLEAN} we mean \gls{CLEAN} 3.1's \gls{ITASK} compiler.
+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}
\subsection{Modules}
\subsection{Strictness}
In \gls{CLEAN}, by default, all expressions are evaluated lazily.
-Types can be annotated with a strictness attribute (\cleaninline{!}), resulting in the values being evaluated to head-normal form before the function is entered.
+Types can be annotated with a strictness attributes (\cleaninline{!}), resulting in the values being evaluated to head-normal form before the function is entered.
In \gls{HASKELL}, in patterns, strictness can be enforced using \haskellinline{!}\requiresGHCmod{BangPatterns}.
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\todo{cite}.
+Types in \gls{CLEAN} may be \emph{unique}, which means that they may not be shared~\cite{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.
-\Gls{CLEAN} uses the \emph{world-as-value} paradigm where \cleaninline{World} represents the external environment and is always unique.
+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}.
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.
+In \gls{HASKELL}, interaction with the world is done using the \haskellinline{IO} monad~\cite{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.
Uniqueness is propagated automatically in function types but must be marked manually in data types.
Examples can be seen in \cref{lst:unique_examples}.
-\begin{lstClean}[label={lst:unique_examples},caption={Examples of uniqueness annotations in \gls{CLEAN}.}]
+\begin{lstClean}[label={lst:unique_examples},caption={Examples of uniqueness annotations.}]
f :: *a -> *a // f works on unique values only
f :: .a -> .a // f works on unique and non-unique values
f :: v:a u:b -> u:b, [v<=u] // f works when a is less unique than b
%:: T = T (Int -> *(*World -> *World)) // Writing :: T = T (Int *World -> *World) won't work
\subsection{Expressions}
-\todo[inline]{Matches pattern expression \texttt{=:}}
+Patterns in \gls{CLEAN} can be used as predicates as well~\cite[Chp.~3.4.3]{plasmeijer_clean_2021}.
+Using the \cleaninline{=:} operator, a value can be tested against a pattern.
+Variable names are not allowed but wildcard patterns \cleaninline{\_} are.
+
+\begin{lstClean}[label={lst:matches_pattern_expression},caption={Examples of \emph{matches pattern} expressions.}]
+isNil :: [a] -> Bool
+isNil l = l=:[]
+
+:: T = A Int | B Bool
-\todo[inline]{Let before}
+ifAB :: T a a -> a
+ifAB x ifa ifb = if (x =: (A _)) ifa ifb
+\end{lstClean}
+
+Due to the nature of uniqueness typing, many functions in \gls{CLEAN} are state transition functions with possibly unique states.
+The \emph{let before} construct allows the programmer to specify sequential actions without having to invent unique names for the different versions of the state.
+\Cref{lst:let_before} shows an example of the usage of the \emph{let before} construct (adapted from~\cite[Chp.~3.5.4]{plasmeijer_clean_2021}).
+
+\begin{lstClean}[label={lst:let_before},caption={Let before expression example.}]
+readChars :: *File -> ([Char], *File)
+readChars file
+# (ok, char, file) = freadc file
+| not ok = ([], file)
+# (chars, file) = readChars file
+= ([char:chars], file)
+\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}.
Metadata about the types is available using the \cleaninline{of} syntax that gives the function access to metadata records, as can be seen in \cref{lst:generic_print} showing a generic print function. This abundance of metadata allows for very complex generic functions that near the expression level of template metaprogramming\ifSubfilesClassLoaded{}{ (See \cref{chp:first-class_datatypes})}.
\lstinputlisting[language=Clean,firstline=4,label={lst:generic_print},caption={Generic print function in \gls{CLEAN}.}]{lst/generic_print.icl}
-\subsection{\glsentrytext{GADT}s}
-GADTs are enriched data types that allow the type instantiation of the constructor to be explicitly defined~\cite{cheney_first-class_2003,hinze_fun_2003}.
+\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}.
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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