\chapter{\texorpdfstring{\glsentrytext{CLEAN}}{Clean} for \texorpdfstring{\glsentrytext{HASKELL}}{Haskell} Programmers}%
\label{chp:clean_for_haskell_programmers}
-This appendix is meant to give people who are familiar with the \gls{FP} language \gls{HASKELL} a consise overview of the \gls{CLEAN} language elements and how they differ from \gls{HASKELL}.
+This appendix is meant give people who are familiar with the \gls{FP} language \gls{HASKELL} a consise overview of the programming language \gls{CLEAN} and how it differs from \gls{HASKELL}.
The goal is to support the reader when reading \gls{CLEAN} code.
\Cref{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 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.
-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 \glsxtrlong{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}.
+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 concurrency 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.
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 \citep{van_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.
+In the past, a \emph{double-edged} fronted even existed that allowed \gls{CLEAN} to be extended with \gls{HASKELL98} syntax and vice versa \citep{van_groningen_exchanging_2010}, however this frontend is no longer maintained.
+This chapter gives only 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}.
+Much 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.} 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 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.
+In \gls{HASKELL}, in patterns, strictness is enforced using \haskellinline{!}\requiresGHCmod{BangPatterns}.
+Within functions, the strict let (\cleaninline{#!}) is 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 \citep{barendsen_uniqueness_1996}.
+Types in \gls{CLEAN} may be \emph{unique}, which means that they cannot 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 \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 \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.
+An \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.
-Finally, using \cleaninline{.} (a dot), it is possible to state that several variables are equally unique.
+Finally, using \cleaninline{.} (a full stop), it is possible to state that several variables are equally unique.
Uniqueness is propagated automatically in function types but must be marked manually in data types.
Examples can be seen in \cref{lst:unique_examples}.
\subsection{Expressions}
Patterns in \gls{CLEAN} can be used as predicates as well \citep[\citesection{3.4.3}]{plasmeijer_clean_2021}.
-Using the \cleaninline{=:} operator, a value can be tested against a pattern.
+Using the \cleaninline{=:} operator, a value is 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.}]
\subsection{Generics}
Polytypic functions \citep{jeuring_polytypic_1996}---also known as generic or kind-indexed fuctions---are built into \gls{CLEAN} \citep[\citesection{7.1}]{plasmeijer_clean_2021}\citep{alimarine_generic_2005} whereas in \gls{HASKELL} they are implemented as a library \citep[\citesection{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}.
+The syntax of the built-in generics of \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}.
-\cleaninputlisting[firstline=4,label={lst:generic_eq},caption={Generic equality function in \gls{CLEAN}.}.]{lst/generic_eq.icl}
+\cleaninputlisting[firstline=4,label={lst:generic_eq},caption={Generic equality function}]{lst/generic_eq.icl}
-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})}.
-\cleaninputlisting[language=Clean,firstline=4,label={lst:generic_print},caption={Generic print function in \gls{CLEAN}.}]{lst/generic_print.icl}
+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 (see \cref{chp:first-class_datatypes,sec:ccodegen}).
+\cleaninputlisting[language=Clean,firstline=4,label={lst:generic_print},caption={Generic print function}]{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 \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 \citep[\citesection{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}.
+To illustrate this, \cref{lst:gadt_haskell} shows a \gls{GADT} in \gls{HASKELL}\requiresGHCmod{GADTs} that can be implemented as in \cref{lst:gadt_clean}.
-\cleaninputlisting[firstline=4,lastline=24,label={lst:gadt_clean},caption={Expression \gls{GADT} using equivalence types.}]{lst/expr_gadt.icl}
\haskellinputlisting[firstline=4,label={lst:gadt_haskell},caption={Expression \gls{GADT}.}]{lst/expr_gadt.hs}
+\cleaninputlisting[firstline=4,lastline=24,label={lst:gadt_clean},caption={Expression \gls{GADT} using equivalence types.}]{lst/expr_gadt.icl}
\clearpage
\section{Syntax}
\midrule
\cleaninline{import Mod => qualified f1, :: t} & \haskellinline{import qualified Mod (f1, t)}\\
& \haskellinline{import Mod hiding (f1, t)}\\
- \cleaninline{/* multi line /* nested */ */} & \haskellinline{\{- multi line \{- nested -\} \}}\\
-
\midrule
\multicolumn{2}{c}{Basic types}\\
\midrule
\cleaninline{True :: Bool} & \haskellinline{True :: Bool}\\
\cleaninline{toInteger 42 :: Integer} & \haskellinline{42 :: Integer}\\
\cleaninline{38.0 :: Real} & \haskellinline{38.0 :: Float -- or Double}\\
- \cleaninline{\"Hello\" +++ \"World\" :: String}\footnote{Strings are represented as unboxed character arrays.}
- & \haskellinline{\"Hello\" ++ \"World\" :: String}\footnote{Strings are represented as lists of characters by default but may be overloaded as well if \GHCmod{OverloadedStrings} is enabled.}\\
+ \cleaninline{\"Hello\" +++ \"World\" :: String}\footnote{Strings are unboxed character arrays.}
+ & \haskellinline{\"Hello\" ++ \"World\" :: String}\footnote{Strings are lists of characters by default but may be overloaded as well if \GHCmod{OverloadedStrings} is enabled.}\\
\cleaninline{['Hello'] :: [Char]} & \haskellinline{\"Hello\" :: String}\\
\cleaninline{?t} & \haskellinline{Maybe t}\\
\cleaninline{(?None, ?Just e)} & \haskellinline{(Nothing, Just e)}\\
\multicolumn{2}{c}{Type definitions}\\
\midrule
\cleaninline{:: T a0 ... :== t} & \haskellinline{type T a0 ... = t}\\
- \cleaninline{:: T a0 ... } & \haskellinline{data T a1 ...}\\
+ \cleaninline{:: T a0 ... } & \haskellinline{data T a0 ...}\\
\quad\cleaninline{= C1 f0 ... fn \| ... \| Cn f0 ... fn} & \quad\haskellinline{= C1 f0 ... fn \| ... \| Cn f0 ... fn}\\
\cleaninline{:: T a0 ...} & \haskellinline{data T a0 ...}\\
\quad\cleaninline{= \{ f0 :: t0, ..., fn :: tn \} } & \quad\haskellinline{= T \{ f0 :: t0, ..., fn :: tn \} }\\
\quad\cleaninline{\| otherwise = t} or \cleaninline{= t} & \quad\haskellinline{\| otherwise = t}\\
\midrule
+ \pagebreak
\multicolumn{2}{c}{Record expressions}\\
\midrule
\cleaninline{:: R = \{ f :: t \}} & \haskellinline{data R = R \{ f :: t \}}\\
\midrule
\multicolumn{2}{c}{Record patterns}\\
\midrule
- \cleaninline{:: R0 = \{ f0 :: R1 \}} & \haskellinline{data R0 = R0 \{ f0 :: R1 \}}\\
- \cleaninline{:: R1 = \{ f1 :: t \}} & \haskellinline{data R1 = R1 \{ f1 :: t \}}\\
+ \cleaninline{:: R0 = \{ f0 :: R1 Int \}} & \haskellinline{data R0 = R0 \{ f0 :: R1 Int \}}\\
+ \cleaninline{:: R1 t = \{ f1 :: t \}} & \haskellinline{data R1 t = R1 \{ f1 :: t \}}\\
\cleaninline{g \{ f0 \} = e f0} & \haskellinline{g (R0 \{f0=x\}) = e x}\\
& or \haskellinline{g (R0 \{f0\}) = e f0}\requiresGHCmod{RecordPuns}\\
\cleaninline{g \{ f0 = \{f1\} \} = e f1} & \haskellinline{g (R0 \{f0=R1 \{f1=x\}\}) = e x}\\
\cleaninline{a = \{v0, ..., vn\}} & \haskellinline{a = array (0, n+1)}\\
& \quad\haskellinline{[(0, v0), ..., (n, vn)]}\\
\cleaninline{a = \{e \\\\ p <-: a\}} & \haskellinline{a = array (0, length a-1)}\\
- & \quad\haskellinline{[e \| (i, a) <- [0..] `zip` a]}\\
+ & \quad\haskellinline{[e \| (i, a) <- zip [0..] a]}\\
\cleaninline{a.[i]} & \haskellinline{a!i}\\
\cleaninline{a![i]}\footnote{This operator allows for field selection from unique arrays.} & \haskellinline{(\\v->(v!i, v)) a}\\
\cleaninline{\{ a \& [i] = e\}} & \haskellinline{a//[(i, e)]}\\
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