appx/c4hp.tex

   1 \documentclass[../thesis.tex]{subfiles}
   2
   3 \include{subfilepreamble}
   4
   5 \begin{document}
   6 \chapter{\texorpdfstring{\glsentrytext{CLEAN}}{Clean} for \texorpdfstring{\glsentrytext{HASKELL}}{Haskell} Programmers}%
   7 \label{chp:clean_for_haskell_programmers}
   8
   9 This note is meant to give people who are familiar with the \gls{FP} language \gls{HASKELL} a consise overview of \gls{CLEAN} language elements and how they differ from \gls{HASKELL}.
  10 The goal is to support the reader when reading \gls{CLEAN} code.
  11 \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.
  12 Obviously, this summary is not exhaustive.
  13 Some \gls{CLEAN} language elements that are not easily translatable to \gls{HASKELL} and thus do not occur in the summary following below.
  14 We hope you enjoy these notes and that it aids you in reading \gls{CLEAN} programs.
  15
  16 While \gls{CLEAN} and \gls{HASKELL} were both conceived around 1987 and have similar syntax, there are some subtle differences in syntax and functionality.
  17 This section describes some of the history of \gls{CLEAN} and provides a crash course in \gls{CLEAN} pecularities written for \gls{HASKELL} programmers.
  18 It is based on the
  19
  20 \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}.
  21 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.
  22 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}.
  23
  24 Initially, when it was used mostly as an intermediate language, it had a fairly spartan syntax.
  25 However, over the years, the syntax got friendlier and it currently it looks a lot like \gls{HASKELL}.
  26 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existed that allowed \gls{CLEAN} to be extended with \gls{HASKELL98} syntax and vice versa, however this frontend is no longer maintained~\citep{groningen_exchanging_2010}.
  27 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}.
  28 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}.
  29 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.
  30
  31 \section{Features}
  32 \subsection{Modules}
  33 \Gls{CLEAN} has separate implementation and definition modules.
  34 The definition module contains the class definitions, instances, function types and type definitions (possibly abstract).
  35 Implementation modules contain the function implementations as well.
  36 This means that only what is defined in the definition module is exported in \gls{CLEAN}.
  37 This differs greatly from \gls{HASKELL}, as there is only a module file there.
  38 Choosing what is exported in \gls{HASKELL} is done using the \haskellinline{module Mod(...)} syntax.
  39
  40 \subsection{Strictness}
  41 In \gls{CLEAN}, by default, all expressions are evaluated lazily.
  42 Types can be annotated with a strictness attributes (\cleaninline{!}), resulting in the values being evaluated to head-normal form before the function is entered.
  43 In \gls{HASKELL}, in patterns, strictness can be enforced using \haskellinline{!}\requiresGHCmod{BangPatterns}.
  44 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.
  45
  46 \subsection{Uniqueness typing}
  47 Types in \gls{CLEAN} may be \emph{unique}, which means that they may not be shared~\citep{barendsen_uniqueness_1996}.
  48 The uniqueness type system allows the compiler to generate efficient code because unique data structures can be destructively updated.
  49 Furthermore, uniqueness typing serves as a model for side effects as well~\citep{achten_high_1993,achten_ins_1995}.
  50 \Gls{CLEAN} uses the \emph{world-as-value} paradigm where \cleaninline{World} represents the external environment and is always unique~\citep{backus_introduction_1990}.
  51 A program with side effects is characterised by a \cleaninline{Start :: *World -> *World} start function.
  52 In \gls{HASKELL}, interaction with the world is done using the \haskellinline{IO} monad~\citep{peyton_jones_imperative_1993}.
  53 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.
  54 Besides marking types as unique, it is also possible to mark them with uniqueness attributes variables \cleaninline{u:} and define constraints on them.
  55 For example, to make sure that an argument of a function is at least as unique as another argument.
  56 Finally, using \cleaninline{.} (a dot), it is possible to state that several variables are equally unique.
  57 Uniqueness is propagated automatically in function types but must be marked manually in data types.
  58 Examples can be seen in \cref{lst:unique_examples}.
  59
  60 \begin{lstClean}[label={lst:unique_examples},caption={Examples of uniqueness annotations.}]
  61 f :: *a -> *a                // f works on unique values only
  62 f :: .a -> .a                // f works on unique and non-unique values
  63 f :: v:a u:b -> u:b, [v<=u]  // f works when a is less unique than b
  64 \end{lstClean}
  65 %f :: (Int, *World) -> *World // The uniqueness is propagated automatically (i.e. *(Int, *World)))
  66 %:: T = T *(Int, *World)      // Writing :: T = T (Int, *World) won't work
  67 %:: T = T (Int -> *(*World -> *World)) // Writing :: T = T (Int *World -> *World) won't work
  68
  69 \subsection{Expressions}
  70 Patterns in \gls{CLEAN} can be used as predicates as well~\citep[\citesection{3.4.3}]{plasmeijer_clean_2021}.
  71 Using the \cleaninline{=:} operator, a value can be tested against a pattern.
  72 Variable names are not allowed but wildcard patterns \cleaninline{\_} are.
  73
  74 \begin{lstClean}[label={lst:matches_pattern_expression},caption={Examples of \emph{matches pattern} expressions.}]
  75 isNil :: [a] -> Bool
  76 isNil l = l=:[]
  77
  78 :: T = A Int | B Bool
  79
  80 ifAB :: T a a -> a
  81 ifAB x ifa ifb = if (x =: (A _)) ifa ifb
  82 \end{lstClean}
  83
  84 Due to the nature of uniqueness typing, many functions in \gls{CLEAN} are state transition functions with possibly unique states.
  85 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.
  86 \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}).
  87
  88 \begin{lstClean}[label={lst:let_before},caption={Let before expression example.}]
  89 readChars :: *File -> ([Char], *File)
  90 readChars file
  91 # (ok, char, file) = freadc file
  92 | not ok           = ([], file)
  93 # (chars, file)    = readChars file
  94 = ([char:chars], file)
  95 \end{lstClean}
  96
  97 \subsection{Generics}
  98 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}.
  99 The implementation of generics in \gls{CLEAN} is very similar to that of Generic H$\forall$skell~\citep{hinze_generic_2003}.
 100 %When calling a generic function, the kind must always be specified and depending on the kind, the function may require more arguments.
 101
 102 For example, defining a generic equality is done as in \cref{lst:generic_eq}.
 103 \cleaninputlisting[firstline=4,label={lst:generic_eq},caption={Generic equality function in \gls{CLEAN}.}.]{lst/generic_eq.icl}
 104
 105 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})}.
 106 \cleaninputlisting[language=Clean,firstline=4,label={lst:generic_print},caption={Generic print function in \gls{CLEAN}.}]{lst/generic_print.icl}
 107
 108 \subsection{\texorpdfstring{\glsentrytext{GADT}}{GADT}s}
 109 \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}.
 110 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}.
 111 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}.
 112
 113 \cleaninputlisting[firstline=4,lastline=24,label={lst:gadt_clean},caption={Expression \gls{GADT} using equivalence types.}]{lst/expr_gadt.icl}
 114 \haskellinputlisting[firstline=4,label={lst:gadt_haskell},caption={Expression \gls{GADT}.}]{lst/expr_gadt.hs}
 115
 116 \clearpage
 117 \section{Syntax}
 118 \lstset{basicstyle=\tt\footnotesize}
 119 \begin{longtable}{p{.45\linewidth}p{.5\linewidth}}
 120         \caption[]{Syntactical differences between \gls{CLEAN} and \gls{HASKELL}.}%
 121         \label{tbl:syn_clean_haskell}\\
 122         \toprule
 123         \gls{CLEAN} & \gls{HASKELL}\\
 124         \midrule
 125         \endfirsthead%
 126         \caption[]{(continued)}\\
 127         \toprule
 128         \gls{CLEAN} & \gls{HASKELL}\\
 129         \midrule
 130         \endhead%
 131
 132         \midrule
 133         \multicolumn{2}{c}{Comments}\\
 134         \midrule
 135         \cleaninline{// single line} & \haskellinline{-- single line}\\
 136         \cleaninline{/* multi line /* nested */ */} & \haskellinline{\{- multi line \{- nested -\} \}}\\
 137
 138         \midrule
 139         \multicolumn{2}{c}{Imports}\\
 140         \midrule
 141         \cleaninline{import Mod => qualified f1, :: t} & \haskellinline{import qualified Mod (f1, t)}\\
 142                                                        & \haskellinline{import Mod hiding (f1, t)}\\
 143         \cleaninline{/* multi line /* nested */ */} & \haskellinline{\{- multi line \{- nested -\} \}}\\
 144
 145         \midrule
 146         \multicolumn{2}{c}{Basic types}\\
 147         \midrule
 148         \cleaninline{42 :: Int} & \haskellinline{42 :: Int}\\
 149         \cleaninline{True :: Bool} & \haskellinline{True :: Bool}\\
 150         \cleaninline{toInteger 42 :: Integer} & \haskellinline{42 :: Integer}\\
 151         \cleaninline{38.0 :: Real} & \haskellinline{38.0 :: Float -- or Double}\\
 152         \cleaninline{\"Hello\" +++ \"World\" :: String}\footnote{Strings are represented as unboxed character arrays.}
 153                 & \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.}\\
 154         \cleaninline{['Hello'] :: [Char]} & \haskellinline{\"Hello\" :: String}\\
 155         \cleaninline{?t} & \haskellinline{Maybe t}\\
 156         \cleaninline{(?None, ?Just e)} & \haskellinline{(Nothing, Just e)}\\
 157
 158         \midrule
 159         \multicolumn{2}{c}{Type definitions}\\
 160         \midrule
 161         \cleaninline{:: T a0 ... :== t} & \haskellinline{type T a0 ... = t}\\
 162         \cleaninline{:: T a0 ... } & \haskellinline{data T a1 ...}\\
 163         \quad\cleaninline{= C1 f0 ... fn \| ... \| Cn f0 ... fn} & \quad\haskellinline{= C1 f0 ... fn \| ... \| Cn f0 ... fn}\\
 164         \cleaninline{:: T a0 ...} & \haskellinline{data T a0 ...}\\
 165         \quad\cleaninline{= \{ f0 :: t0, ..., fn :: tn \} } & \quad\haskellinline{= T \{ f0 :: t0, ..., fn :: tn \} }\\
 166         \cleaninline{:: T a0 ... =: t} & \haskellinline{newtype T a0 ... = t}\\
 167         \cleaninline{:: T = E.t: Box t \& C t} & \haskellinline{data T = forall t.C t => Box t}\requiresGHCmod{ExistentialQuantification}\\
 168
 169         \midrule
 170         \multicolumn{2}{c}{Function types}\\
 171         \midrule
 172         \cleaninline{f0 :: a0 a1 ... -> t}
 173                 & \haskellinline{f0 :: (c0 v0, c1 v1, c2 v2) =>}\\
 174         \quad\cleaninline{\| c0 v0 \& c1, c2 v1}
 175                 & \quad\haskellinline{a0 -> a1 ... -> t}\\
 176         \cleaninline{(+) infixl 6 :: Int Int -> Int} & \haskellinline{infixl 6 +}\\
 177                 & \haskellinline{(+) :: Int -> Int -> Int}\\
 178         \cleaninline{qid :: (A.a: a -> a) -> (Bool, Int)}
 179                 & \haskellinline{qid :: (forall a: a -> a) -> (Bool, Int)}\requiresGHCmod{RankNTypes}\\
 180         \cleaninline{qid id = (id True, id 42)} &
 181                 \haskellinline{qid id = (id True, id 42)}\\
 182
 183         \midrule
 184         \multicolumn{2}{c}{Type classes}\\
 185         \midrule
 186         \cleaninline{class f a :: t} & \haskellinline{class f a where f :: t}\\
 187         \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}\\
 188         \cleaninline{class C s ~m where ...} & \haskellinline{class C s m \| m -> s where ...}\requiresGHCmod{MultiParamTypeClasses}\\
 189         \cleaninline{instance C t \| C0, ..., Cn a} & \haskellinline{instance (C0 a, ..., Cn a) => C t}\\
 190         \quad\cleaninline{where ...} & \quad\haskellinline{where ...}\\
 191
 192         \midrule
 193         \multicolumn{2}{c}{As pattern}\\
 194         \midrule
 195         \cleaninline{x=:p} & \haskellinline{x@p}\\
 196
 197         \midrule
 198         \multicolumn{2}{c}{Lists}\\
 199         \midrule
 200         \cleaninline{[1,2,3]} & \haskellinline{[1,2,3]}\\
 201         \cleaninline{[x:xs]} & \haskellinline{x:xs}\\
 202         \cleaninline{[e \\\\ e <- xs \| p e]} & \haskellinline{[e \| e <- xs, p e]}\\
 203         \cleaninline{[l \\\\ l <- xs, r <- ys]} & \haskellinline{[l \| l <- xs, r <- ys]}\\
 204         \cleaninline{[(l, r) \\\\ l <- xs \& r <- ys]} & \haskellinline{[(l, r) \| (l, r) <- zip xs ys]}\\
 205                                                        & or \haskellinline{[(l, r) \| l <- xs \| r <- ys]}\requiresGHCmod{ParallelListComp}\\
 206
 207         \midrule
 208         \multicolumn{2}{c}{Lambda expressions}\\
 209         \midrule
 210         \cleaninline{\\a0 a1 ...->e} or \cleaninline{\\....e} or \cleaninline{\\...=e} & \haskellinline{\\a0 a1 ...->e}\\
 211
 212         \midrule
 213         \multicolumn{2}{c}{Case distinction}\\
 214         \midrule
 215         \cleaninline{if p e0 e1} & \haskellinline{if p then e0 else e1}\\
 216         \cleaninline{case e of p0 -> e0; ...} & \haskellinline{case e of p0 -> e0; ...}\\
 217         \quad or \cleaninline{case e of p0 = e0; ...}\\
 218         \cleaninline{f p0 ... pn} & \haskellinline{f p0 ... pn}\\
 219         \quad\cleaninline{\| c = t} & \quad\haskellinline{\| c = t}\\
 220         \quad\cleaninline{\| otherwise = t} or \cleaninline{= t} & \quad\haskellinline{\| otherwise = t}\\
 221
 222         \midrule
 223         \multicolumn{2}{c}{Record expressions}\\
 224         \midrule
 225         \cleaninline{:: R = \{ f :: t \}} & \haskellinline{data R = R \{ f :: t \}}\\
 226         \cleaninline{r = \{ f = e \}} & \haskellinline{r = R \{ f = e \}}\\
 227         \cleaninline{r.f} & \haskellinline{f r}\\
 228                                           & \quad or \haskellinline{r.f}\requiresGHCmod[Requires \gls{GHC} version 9.2.0 or higher]{OverloadedRecordDot}\\
 229         \cleaninline{r!f}\footnote{This operator allows for field selection from unique records.} & \haskellinline{(\\v->(f v, v)) r}\\
 230         \cleaninline{\{r \& f = e \}} & \haskellinline{r \{ f = e \}}\\
 231
 232         \midrule
 233         \multicolumn{2}{c}{Record patterns}\\
 234         \midrule
 235         \cleaninline{:: R0 = \{ f0 :: R1 \}} & \haskellinline{data R0 = R0 \{ f0 :: R1 \}}\\
 236         \cleaninline{:: R1 = \{ f1 :: t \}} & \haskellinline{data R1 = R1 \{ f1 :: t \}}\\
 237         \cleaninline{g \{ f0 \} = e f0} & \haskellinline{g (R0 \{f0=x\}) = e x}\\
 238                                         & or \haskellinline{g (R0 \{f0\}) = e f0}\requiresGHCmod{RecordPuns}\\
 239         \cleaninline{g \{ f0 = \{f1\} \} = e f1} & \haskellinline{g (R0 \{f0=R1 \{f1=x\}\}) = e x}\\
 240
 241         \midrule
 242         \multicolumn{2}{c}{Arrays}\\
 243         \midrule
 244         \cleaninline{:: A :== \{t\}} & \haskellinline{type A = Array Int t}\\
 245         \cleaninline{a = \{v0, ..., vn\}} & \haskellinline{a = array (0, n+1)}\\
 246                                           & \quad\haskellinline{[(0, v0), ..., (n, vn)]}\\
 247         \cleaninline{a = \{e \\\\ p <-: a\}} & \haskellinline{a = array (0, length a-1)}\\
 248                                              & \quad\haskellinline{[e \| (i, a) <- [0..] `zip` a]}\\
 249         \cleaninline{a.[i]} & \haskellinline{a!i}\\
 250         \cleaninline{a![i]}\footnote{This operator allows for field selection from unique arrays.} & \haskellinline{(\\v->(v!i, v)) a}\\
 251         \cleaninline{\{ a \& [i] = e\}} & \haskellinline{a//[(i, e)]}\\
 252
 253         \midrule
 254         \multicolumn{2}{c}{Dynamics}\\
 255         \midrule
 256         \cleaninline{f :: a -> Dynamic \| TC a} & \haskellinline{f :: Typeable a => a -> Dynamic}\\
 257         \cleaninline{f e = dynamic e} & \haskellinline{f e = toDyn (e)}\\
 258         \cleaninline{g :: Dynamic -> t} & \haskellinline{g :: Dynamic -> t}\\
 259         \cleaninline{g (e :: t) = e0} & \haskellinline{g d = case fromDynamic d}\\
 260         \cleaninline{g e = e1} & \quad\haskellinline{Just e -> e0}\\
 261                                & \quad\haskellinline{Nothing -> e1}\\
 262
 263         \bottomrule
 264 \end{longtable}
 265
 266 \input{subfilepostamble}
 267 \end{document}