appx/c4hp.tex

   1 \documentclass[../thesis.tex]{subfiles}
   2
   3 \input{subfilepreamble}
   4
   5 \begin{document}
   6 \input{subfileprefix}
   7 \ifSubfilesClassLoaded{\appendix}{}
   8 \chapter{\texorpdfstring{\glsentrytext{CLEAN}}{Clean} for \texorpdfstring{\glsentrytext{HASKELL}}{Haskell} Programmers}%
   9 \label{chp:clean_for_haskell_programmers}
  10
  11 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}.
  12 The goal is to support the reader when reading \gls{CLEAN} code.
  13 \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.
  14 Obviously, this summary is not exhaustive.
  15 Some \gls{CLEAN} language elements that are not easily translatable to \gls{HASKELL} and thus do not occur in the summary following below.
  16 We hope you enjoy these notes and that it aids you in reading \gls{CLEAN} programs.
  17
  18 While \gls{CLEAN} and \gls{HASKELL} were both conceived around 1987 and have similar syntax, there are some subtle differences in syntax and functionality.
  19 This section describes some of the history of \gls{CLEAN} and provides a crash course in \gls{CLEAN} pecularities written for \gls{HASKELL} programmers.
  20 It is based on the
  21
  22 \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}.
  23 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.
  24 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}.
  25
  26 Initially, when it was used mostly as an intermediate language, it had a fairly spartan syntax.
  27 However, over the years, the syntax got friendlier and it currently it looks a lot like \gls{HASKELL}.
  28 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existed that allowed \gls{CLEAN} to be extended with \gls{HASKELL98} syntax and vice versa, however this frontend is no longer maintained \citep{van_groningen_exchanging_2010}.
  29 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}.
  30 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}.
  31 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.
  32
  33 \section{Features}
  34 \subsection{Modules}
  35 \Gls{CLEAN} has separate implementation and definition modules.
  36 The definition module contains the class definitions, instances, function types and type definitions (possibly abstract).
  37 Implementation modules contain the function implementations as well.
  38 This means that only what is defined in the definition module is exported in \gls{CLEAN}.
  39 This differs greatly from \gls{HASKELL}, as there is only a module file there.
  40 Choosing what is exported in \gls{HASKELL} is done using the \haskellinline{module Mod(...)} syntax.
  41
  42 \subsection{Strictness}
  43 In \gls{CLEAN}, by default, all expressions are evaluated lazily.
  44 Types can be annotated with a strictness attributes (\cleaninline{!}), resulting in the values being evaluated to head-normal form before the function is entered.
  45 In \gls{HASKELL}, in patterns, strictness can be enforced using \haskellinline{!}\requiresGHCmod{BangPatterns}.
  46 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.
  47
  48 \subsection{Uniqueness typing}
  49 Types in \gls{CLEAN} may be \emph{unique}, which means that they may not be shared \citep{barendsen_uniqueness_1996}.
  50 The uniqueness type system allows the compiler to generate efficient code because unique data structures can be destructively updated.
  51 Furthermore, uniqueness typing serves as a model for side effects as well \citep{achten_high_1993,achten_ins_1995}.
  52 \Gls{CLEAN} uses the \emph{world-as-value} paradigm where \cleaninline{World} represents the external environment and is always unique \citep{backus_introduction_1990}.
  53 A program with side effects is characterised by a \cleaninline{Start :: *World -> *World} start function.
  54 In \gls{HASKELL}, interaction with the world is done using the \haskellinline{IO} monad \citep{peyton_jones_imperative_1993}.
  55 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.
  56 Besides marking types as unique, it is also possible to mark them with uniqueness attributes variables \cleaninline{u:} and define constraints on them.
  57 For example, to make sure that an argument of a function is at least as unique as another argument.
  58 Finally, using \cleaninline{.} (a dot), it is possible to state that several variables are equally unique.
  59 Uniqueness is propagated automatically in function types but must be marked manually in data types.
  60 Examples can be seen in \cref{lst:unique_examples}.
  61
  62 \begin{lstClean}[label={lst:unique_examples},caption={Examples of uniqueness annotations.}]
  63 f :: *a -> *a                // f works on unique values only
  64 f :: .a -> .a                // f works on unique and non-unique values
  65 f :: v:a u:b -> u:b, [v<=u]  // f works when a is less unique than b
  66 \end{lstClean}
  67 %f :: (Int, *World) -> *World // The uniqueness is propagated automatically (i.e. *(Int, *World)))
  68 %:: T = T *(Int, *World)      // Writing :: T = T (Int, *World) won't work
  69 %:: T = T (Int -> *(*World -> *World)) // Writing :: T = T (Int *World -> *World) won't work
  70
  71 \subsection{Expressions}
  72 Patterns in \gls{CLEAN} can be used as predicates as well \citep[\citesection{3.4.3}]{plasmeijer_clean_2021}.
  73 Using the \cleaninline{=:} operator, a value can be tested against a pattern.
  74 Variable names are not allowed but wildcard patterns \cleaninline{\_} are.
  75
  76 \begin{lstClean}[label={lst:matches_pattern_expression},caption={Examples of \emph{matches pattern} expressions.}]
  77 isNil :: [a] -> Bool
  78 isNil l = l=:[]
  79
  80 :: T = A Int | B Bool
  81
  82 ifAB :: T a a -> a
  83 ifAB x ifa ifb = if (x =: (A _)) ifa ifb
  84 \end{lstClean}
  85
  86 Due to the nature of uniqueness typing, many functions in \gls{CLEAN} are state transition functions with possibly unique states.
  87 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.
  88 \Cref{lst:let_before} shows an example of the usage of the \emph{let before} construct (adapted from \citep[\citesection{3.5.4}]{plasmeijer_clean_2021}).
  89
  90 \begin{lstClean}[label={lst:let_before},caption={Let before expression example.}]
  91 readChars :: *File -> ([Char], *File)
  92 readChars file
  93 # (ok, char, file) = freadc file
  94 | not ok           = ([], file)
  95 # (chars, file)    = readChars file
  96 = ([char:chars], file)
  97 \end{lstClean}
  98
  99 \subsection{Generics}
 100 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}.
 101 The implementation of generics in \gls{CLEAN} is very similar to that of Generic H$\forall$skell \citep{hinze_generic_2003}.
 102 %When calling a generic function, the kind must always be specified and depending on the kind, the function may require more arguments.
 103
 104 For example, defining a generic equality is done as in \cref{lst:generic_eq}.
 105 \cleaninputlisting[firstline=4,label={lst:generic_eq},caption={Generic equality function in \gls{CLEAN}.}.]{lst/generic_eq.icl}
 106
 107 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})}.
 108 \cleaninputlisting[language=Clean,firstline=4,label={lst:generic_print},caption={Generic print function in \gls{CLEAN}.}]{lst/generic_print.icl}
 109
 110 \subsection{\texorpdfstring{\glsentrytext{GADT}}{GADT}s}
 111 \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}.
 112 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}.
 113 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}.
 114
 115 \cleaninputlisting[firstline=4,lastline=24,label={lst:gadt_clean},caption={Expression \gls{GADT} using equivalence types.}]{lst/expr_gadt.icl}
 116 \haskellinputlisting[firstline=4,label={lst:gadt_haskell},caption={Expression \gls{GADT}.}]{lst/expr_gadt.hs}
 117
 118 \clearpage
 119 \section{Syntax}
 120 \lstset{basicstyle=\tt\footnotesize}
 121 \begin{longtable}{p{.45\linewidth}p{.5\linewidth}}
 122         \caption[]{Syntactical differences between \gls{CLEAN} and \gls{HASKELL}.}%
 123         \label{tbl:syn_clean_haskell}\\
 124         \toprule
 125         \gls{CLEAN} & \gls{HASKELL}\\
 126         \midrule
 127         \endfirsthead%
 128         \caption[]{(continued)}\\
 129         \toprule
 130         \gls{CLEAN} & \gls{HASKELL}\\
 131         \midrule
 132         \endhead%
 133
 134         \midrule
 135         \multicolumn{2}{c}{Comments}\\
 136         \midrule
 137         \cleaninline{// single line} & \haskellinline{-- single line}\\
 138         \cleaninline{/* multi line /* nested */ */} & \haskellinline{\{- multi line \{- nested -\} \}}\\
 139
 140         \midrule
 141         \multicolumn{2}{c}{Imports}\\
 142         \midrule
 143         \cleaninline{import Mod => qualified f1, :: t} & \haskellinline{import qualified Mod (f1, t)}\\
 144                                                        & \haskellinline{import Mod hiding (f1, t)}\\
 145         \cleaninline{/* multi line /* nested */ */} & \haskellinline{\{- multi line \{- nested -\} \}}\\
 146
 147         \midrule
 148         \multicolumn{2}{c}{Basic types}\\
 149         \midrule
 150         \cleaninline{42 :: Int} & \haskellinline{42 :: Int}\\
 151         \cleaninline{True :: Bool} & \haskellinline{True :: Bool}\\
 152         \cleaninline{toInteger 42 :: Integer} & \haskellinline{42 :: Integer}\\
 153         \cleaninline{38.0 :: Real} & \haskellinline{38.0 :: Float -- or Double}\\
 154         \cleaninline{\"Hello\" +++ \"World\" :: String}\footnote{Strings are represented as unboxed character arrays.}
 155                 & \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.}\\
 156         \cleaninline{['Hello'] :: [Char]} & \haskellinline{\"Hello\" :: String}\\
 157         \cleaninline{?t} & \haskellinline{Maybe t}\\
 158         \cleaninline{(?None, ?Just e)} & \haskellinline{(Nothing, Just e)}\\
 159
 160         \midrule
 161         \multicolumn{2}{c}{Type definitions}\\
 162         \midrule
 163         \cleaninline{:: T a0 ... :== t} & \haskellinline{type T a0 ... = t}\\
 164         \cleaninline{:: T a0 ... } & \haskellinline{data T a1 ...}\\
 165         \quad\cleaninline{= C1 f0 ... fn \| ... \| Cn f0 ... fn} & \quad\haskellinline{= C1 f0 ... fn \| ... \| Cn f0 ... fn}\\
 166         \cleaninline{:: T a0 ...} & \haskellinline{data T a0 ...}\\
 167         \quad\cleaninline{= \{ f0 :: t0, ..., fn :: tn \} } & \quad\haskellinline{= T \{ f0 :: t0, ..., fn :: tn \} }\\
 168         \cleaninline{:: T a0 ... =: t} & \haskellinline{newtype T a0 ... = t}\\
 169         \cleaninline{:: T = E.t: Box t \& C t} & \haskellinline{data T = forall t.C t => Box t}\requiresGHCmod{ExistentialQuantification}\\
 170
 171         \midrule
 172         \multicolumn{2}{c}{Function types}\\
 173         \midrule
 174         \cleaninline{f0 :: a0 a1 ... -> t}
 175                 & \haskellinline{f0 :: (c0 v0, c1 v1, c2 v2) =>}\\
 176         \quad\cleaninline{\| c0 v0 \& c1, c2 v1}
 177                 & \quad\haskellinline{a0 -> a1 ... -> t}\\
 178         \cleaninline{(+) infixl 6 :: Int Int -> Int} & \haskellinline{infixl 6 +}\\
 179                 & \haskellinline{(+) :: Int -> Int -> Int}\\
 180         \cleaninline{qid :: (A.a: a -> a) -> (Bool, Int)}
 181                 & \haskellinline{qid :: (forall a: a -> a) -> (Bool, Int)}\requiresGHCmod{RankNTypes}\\
 182         \cleaninline{qid id = (id True, id 42)} &
 183                 \haskellinline{qid id = (id True, id 42)}\\
 184
 185         \midrule
 186         \multicolumn{2}{c}{Type classes}\\
 187         \midrule
 188         \cleaninline{class f a :: t} & \haskellinline{class f a where f :: t}\\
 189         \cleaninline{class C a \| C0, ... , Cn a}\footnote{In contrast to the \gls{HASKELL} variant, this does not require an instance.} & \haskellinline{class (C0 a, ..., Cn, a) => C a}\\
 190         \cleaninline{class C s ~m where ...} & \haskellinline{class C s m \| m -> s where ...}\requiresGHCmod{MultiParamTypeClasses}\\
 191         \cleaninline{instance C t \| C0, ..., Cn a} & \haskellinline{instance (C0 a, ..., Cn a) => C t}\\
 192         \quad\cleaninline{where ...} & \quad\haskellinline{where ...}\\
 193
 194         \midrule
 195         \multicolumn{2}{c}{As pattern}\\
 196         \midrule
 197         \cleaninline{x=:p} & \haskellinline{x@p}\\
 198
 199         \midrule
 200         \multicolumn{2}{c}{Lists}\\
 201         \midrule
 202         \cleaninline{[1,2,3]} & \haskellinline{[1,2,3]}\\
 203         \cleaninline{[x:xs]} & \haskellinline{x:xs}\\
 204         \cleaninline{[e \\\\ e <- xs \| p e]} & \haskellinline{[e \| e <- xs, p e]}\\
 205         \cleaninline{[l \\\\ l <- xs, r <- ys]} & \haskellinline{[l \| l <- xs, r <- ys]}\\
 206         \cleaninline{[(l, r) \\\\ l <- xs \& r <- ys]} & \haskellinline{[(l, r) \| (l, r) <- zip xs ys]}\\
 207                                                        & or \haskellinline{[(l, r) \| l <- xs \| r <- ys]}\requiresGHCmod{ParallelListComp}\\
 208
 209         \midrule
 210         \multicolumn{2}{c}{Lambda expressions}\\
 211         \midrule
 212         \cleaninline{\\a0 a1 ...->e} or \cleaninline{\\....e} or \cleaninline{\\...=e} & \haskellinline{\\a0 a1 ...->e}\\
 213
 214         \midrule
 215         \multicolumn{2}{c}{Case distinction}\\
 216         \midrule
 217         \cleaninline{if p e0 e1} & \haskellinline{if p then e0 else e1}\\
 218         \cleaninline{case e of p0 -> e0; ...} & \haskellinline{case e of p0 -> e0; ...}\\
 219         \quad or \cleaninline{case e of p0 = e0; ...}\\
 220         \cleaninline{f p0 ... pn} & \haskellinline{f p0 ... pn}\\
 221         \quad\cleaninline{\| c = t} & \quad\haskellinline{\| c = t}\\
 222         \quad\cleaninline{\| otherwise = t} or \cleaninline{= t} & \quad\haskellinline{\| otherwise = t}\\
 223
 224         \midrule
 225         \multicolumn{2}{c}{Record expressions}\\
 226         \midrule
 227         \cleaninline{:: R = \{ f :: t \}} & \haskellinline{data R = R \{ f :: t \}}\\
 228         \cleaninline{r = \{ f = e \}} & \haskellinline{r = R \{ f = e \}}\\
 229         \cleaninline{r.f} & \haskellinline{f r}\\
 230                                           & \quad or \haskellinline{r.f}\requiresGHCmod[Requires \gls{GHC} version 9.2.0 or higher]{OverloadedRecordDot}\\
 231         \cleaninline{r!f}\footnote{This operator allows for field selection from unique records.} & \haskellinline{(\\v->(f v, v)) r}\\
 232         \cleaninline{\{r \& f = e \}} & \haskellinline{r \{ f = e \}}\\
 233
 234         \midrule
 235         \multicolumn{2}{c}{Record patterns}\\
 236         \midrule
 237         \cleaninline{:: R0 = \{ f0 :: R1 \}} & \haskellinline{data R0 = R0 \{ f0 :: R1 \}}\\
 238         \cleaninline{:: R1 = \{ f1 :: t \}} & \haskellinline{data R1 = R1 \{ f1 :: t \}}\\
 239         \cleaninline{g \{ f0 \} = e f0} & \haskellinline{g (R0 \{f0=x\}) = e x}\\
 240                                         & or \haskellinline{g (R0 \{f0\}) = e f0}\requiresGHCmod{RecordPuns}\\
 241         \cleaninline{g \{ f0 = \{f1\} \} = e f1} & \haskellinline{g (R0 \{f0=R1 \{f1=x\}\}) = e x}\\
 242
 243         \midrule
 244         \multicolumn{2}{c}{Arrays}\\
 245         \midrule
 246         \cleaninline{:: A :== \{t\}} & \haskellinline{type A = Array Int t}\\
 247         \cleaninline{a = \{v0, ..., vn\}} & \haskellinline{a = array (0, n+1)}\\
 248                                           & \quad\haskellinline{[(0, v0), ..., (n, vn)]}\\
 249         \cleaninline{a = \{e \\\\ p <-: a\}} & \haskellinline{a = array (0, length a-1)}\\
 250                                              & \quad\haskellinline{[e \| (i, a) <- [0..] `zip` a]}\\
 251         \cleaninline{a.[i]} & \haskellinline{a!i}\\
 252         \cleaninline{a![i]}\footnote{This operator allows for field selection from unique arrays.} & \haskellinline{(\\v->(v!i, v)) a}\\
 253         \cleaninline{\{ a \& [i] = e\}} & \haskellinline{a//[(i, e)]}\\
 254
 255         \midrule
 256         \multicolumn{2}{c}{Dynamics}\\
 257         \midrule
 258         \cleaninline{f :: a -> Dynamic \| TC a} & \haskellinline{f :: Typeable a => a -> Dynamic}\\
 259         \cleaninline{f e = dynamic e} & \haskellinline{f e = toDyn (e)}\\
 260         \cleaninline{g :: Dynamic -> t} & \haskellinline{g :: Dynamic -> t}\\
 261         \cleaninline{g (e :: t) = e0} & \haskellinline{g d = case fromDynamic d}\\
 262         \cleaninline{g e = e1} & \quad\haskellinline{Just e -> e0}\\
 263                                & \quad\haskellinline{Nothing -> e1}\\
 264
 265         \bottomrule
 266 \end{longtable}
 267
 268 \input{subfilepostamble}
 269 \end{document}