/
capstone.tex
5710 lines (4155 loc) · 451 KB
/
capstone.tex
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\documentclass{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[letterpaper]{geometry}
\usepackage{setspace}
\newcommand{\packageGraphicx}{\usepackage{graphicx}}
\newcommand{\packageHyperref}{\usepackage{hyperref}}
\newcommand{\renewrmdefault}{\renewcommand{\rmdefault}{ptm}}
\newcommand{\packageRelsize}{\usepackage{relsize}}
\newcommand{\packageMathabx}{\usepackage{mathabx}}
% Avoid conflicts between "mathabx" and "wasysym":
\newcommand{\packageWasysym}{
\let\leftmoon\relax \let\rightmoon\relax \let\fullmoon\relax \let\newmoon\relax \let\diameter\relax
\usepackage{wasysym}}
\newcommand{\packageTextcomp}{\usepackage{textcomp}}
\newcommand{\packageFramed}{\usepackage{framed}}
\newcommand{\packageHyphenat}{\usepackage[htt]{hyphenat}}
\newcommand{\packageColor}{\usepackage[usenames,dvipsnames]{color}}
\newcommand{\doHypersetup}{\hypersetup{bookmarks=true,bookmarksopen=true,bookmarksnumbered=true}}
\newcommand{\packageTocstyle}{\IfFileExists{tocstyle.sty}{\usepackage{tocstyle}\usetocstyle{standard}}{}}
\newcommand{\packageCJK}{\IfFileExists{CJK.sty}{\usepackage{CJK}}{}}
% this bit intentionally left blank
% This is the default style configuration for Scribble-generated Latex
\packageGraphicx
\packageHyperref
\renewrmdefault
\packageRelsize
\packageMathabx
\packageWasysym
\packageTextcomp
\packageFramed
\packageHyphenat
\packageColor
\doHypersetup
\packageTocstyle
\packageCJK
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Configuration that is especially meant to be overridden:
% Inserted before every ``chapter'', useful for starting each one on a new page:
\newcommand{\sectionNewpage}{}
% Inserted before every book ``part''
\newcommand{\partNewpage}{\sectionNewpage}
% Hooks for actions within the `document' environment:
\newcommand{\preDoc}{}
\newcommand{\postDoc}{}
% Generated by `secref'; first arg is section number, second is section title:
\newcommand{\BookRef}[2]{\emph{#2}}
\newcommand{\ChapRef}[2]{\SecRef{#1}{#2}}
\newcommand{\SecRef}[2]{section~#1}
\newcommand{\PartRef}[2]{part~#1}
% Generated by `Secref':
\newcommand{\BookRefUC}[2]{\BookRef{#1}{#2}}
\newcommand{\ChapRefUC}[2]{\SecRefUC{#1}{#2}}
\newcommand{\SecRefUC}[2]{Section~#1}
\newcommand{\PartRefUC}[2]{Part~#1}
% Variants of the above with a label for an internal reference:
\newcommand{\BookRefLocal}[3]{\hyperref[#1]{\BookRef{#2}{#3}}}
\newcommand{\ChapRefLocal}[3]{\hyperref[#1]{\ChapRef{#2}{#3}}}
\newcommand{\SecRefLocal}[3]{\hyperref[#1]{\SecRef{#2}{#3}}}
\newcommand{\PartRefLocal}[3]{\hyperref[#1]{\PartRef{#2}{#3}}}
\newcommand{\BookRefLocalUC}[3]{\hyperref[#1]{\BookRefUC{#2}{#3}}}
\newcommand{\ChapRefLocalUC}[3]{\hyperref[#1]{\ChapRefUC{#2}{#3}}}
\newcommand{\SecRefLocalUC}[3]{\hyperref[#1]{\SecRefUC{#2}{#3}}}
\newcommand{\PartRefLocalUC}[3]{\hyperref[#1]{\PartRefUC{#2}{#3}}}
% Variants of the above with a section number is empty (i.e., UnNumbered):
\newcommand{\BookRefUN}[1]{\BookRef{}{#1}}
\newcommand{\ChapRefUN}[1]{\SecRefUN{#1}}
\newcommand{\SecRefUN}[1]{``#1''}
\newcommand{\PartRefUN}[1]{\SecRefUN{#1}}
\newcommand{\BookRefUCUN}[1]{\BookRefUN{#1}}
\newcommand{\ChapRefUCUN}[1]{\ChapRefUN{#1}}
\newcommand{\SecRefUCUN}[1]{\SecRefUN{#1}}
\newcommand{\PartRefUCUN}[1]{\PartRefUN{#1}}
\newcommand{\BookRefLocalUN}[2]{\hyperref[#1]{\BookRefUN{#2}}}
\newcommand{\ChapRefLocalUN}[2]{\SecRefLocalUN{#1}{#2}}
\newcommand{\SecRefLocalUN}[2]{\hyperref[#1]{\SecRefUN{#2}}}
\newcommand{\PartRefLocalUN}[2]{\SecRefLocalUN{#1}{#2}}
\newcommand{\BookRefLocalUCUN}[2]{\BookRefLocalUN{#1}{#2}}
\newcommand{\ChapRefLocalUCUN}[2]{\ChapRefLocalUN{#1}{#2}}
\newcommand{\SecRefLocalUCUN}[2]{\SecRefLocalUN{#1}{#2}}
\newcommand{\PartRefLocalUCUN}[2]{\PartRefLocalUN{#1}{#2}}
\newcommand{\SectionNumberLink}[2]{\hyperref[#1]{#2}}
% Enabled with a 'enable-index-merge part style property. This default
% implementation isn't good enough, because the argument is a
% comma-separated sequence of labels:
\newcommand{\Smanypageref}[1]{\pageref{#1}}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Fonts
% Font commands used by generated text:
\newcommand{\Scribtexttt}[1]{{\texttt{#1}}}
\newcommand{\textsub}[1]{$_{\hbox{\textsmaller{#1}}}$}
\newcommand{\textsuper}[1]{$^{\hbox{\textsmaller{#1}}}$}
\newcommand{\intextcolor}[2]{\textcolor{#1}{#2}}
\newcommand{\intextrgbcolor}[2]{\textcolor[rgb]{#1}{#2}}
\newcommand{\incolorbox}[2]{{\fboxrule=0pt\fboxsep=0pt\colorbox{#1}{#2}}}
\newcommand{\inrgbcolorbox}[2]{{\fboxrule=0pt\fboxsep=0pt\colorbox[rgb]{#1}{#2}}}
\newcommand{\plainlink}[1]{#1}
\newcommand{\techoutside}[1]{#1}
\newcommand{\techinside}[1]{#1}
\newcommand{\badlink}[1]{#1}
\newcommand{\indexlink}[1]{#1}
\newcommand{\noborder}[1]{#1}
\newcommand{\Smaller}[1]{\textsmaller{#1}}
\newcommand{\Larger}[1]{\textlarger{#1}}
\newcommand{\planetName}[1]{PLane\hspace{-0.1ex}T}
\newcommand{\slant}[1]{{\textsl{#1}}}
% Used for <, >, and | in tt mode. For some fonts and installations,
% there seems to be an encoding issue, so pick T1 explicitly:
\newcommand{\Stttextmore}{{\fontencoding{T1}\selectfont>}}
\newcommand{\Stttextless}{{\fontencoding{T1}\selectfont<}}
\newcommand{\Stttextbar}{{\fontencoding{T1}\selectfont|}}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Tables
% The `stabular' environment seems to be the lesser of evils among
% page-breaking table environments (and we've made a copy as ``pltstabular'
% to make sure that it doesn't change).
\makeatletter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\message{pltstabular is a modification of stabular}
%% A renamed vsetion of:
%% stabular.sty
%% Copyright 1998 Sigitas Tolu\v sis
%% VTeX Ltd., Akademijos 4, Vilnius, Lithuania
%% e-mail sigitas@vtex.lt
%% http://www.vtex.lt/tex/download/macros/
%%
% This program can redistributed and/or modified under the terms
% of the LaTeX Project Public License Distributed from CTAN
% archives in directory macros/latex/base/lppl.txt; either
% version 1 of the License, or (at your option) any later version.
%
% PURPOSE: Improve tabular environment.
%
% SHORT DESCRIPTION:
%
% Changed internal commands: \@mkpream, \@addamp, \@xhline
%
% Provides new commands in tabular (used after command \\):
% \emptyrow[#1]
% -------------
% Adds empty row, #1 - height of the row
%
% \tabrow{#1}[#2]
% ---------------
% Adds row of natural height: #1\\[#2]
%
% Provides new environments: pltstabular and pltstabular*
% -------- ---------
% One more multi-page version of tabular
%
%
\def\empty@finalstrut#1{%
\unskip\ifhmode\nobreak\fi\vrule\@width\z@\@height\z@\@depth\z@}
\def\no@strut{\global\setbox\@arstrutbox\hbox{%
\vrule \@height\z@
\@depth\z@
\@width\z@}%
\gdef\@endpbox{\empty@finalstrut\@arstrutbox\par\egroup\hfil}%
}%
\def\yes@strut{\global\setbox\@arstrutbox\hbox{%
\vrule \@height\arraystretch \ht\strutbox
\@depth\arraystretch \dp\strutbox
\@width\z@}%
\gdef\@endpbox{\@finalstrut\@arstrutbox\par\egroup\hfil}%
}%
\def\@mkpream#1{\@firstamptrue\@lastchclass6
\let\@preamble\@empty\def\empty@preamble{\add@ins}%
\let\protect\@unexpandable@protect
\let\@sharp\relax\let\add@ins\relax
\let\@startpbox\relax\let\@endpbox\relax
\@expast{#1}%
\expandafter\@tfor \expandafter
\@nextchar \expandafter:\expandafter=\reserved@a\do
{\@testpach\@nextchar
\ifcase \@chclass \@classz \or \@classi \or \@classii \or \@classiii
\or \@classiv \or\@classv \fi\@lastchclass\@chclass}%
\ifcase \@lastchclass \@acol
\or \or \@preamerr \@ne\or \@preamerr \tw@\or \or \@acol \fi}
\def\@addamp{%
\if@firstamp
\@firstampfalse
\edef\empty@preamble{\add@ins}%
\else
\edef\@preamble{\@preamble &}%
\edef\empty@preamble{\expandafter\noexpand\empty@preamble &\add@ins}%
\fi}
\newif\iftw@hlines \tw@hlinesfalse
\def\@xhline{\ifx\reserved@a\hline
\tw@hlinestrue
\else\ifx\reserved@a\Hline
\tw@hlinestrue
\else
\tw@hlinesfalse
\fi\fi
\iftw@hlines
\aftergroup\do@after
\fi
\ifnum0=`{\fi}%
}
\def\do@after{\emptyrow[\the\doublerulesep]}
\def\emptyrow{\noalign\bgroup\@ifnextchar[\@emptyrow{\@emptyrow[\z@]}}
\def\@emptyrow[#1]{\no@strut\gdef\add@ins{\vrule \@height\z@ \@depth#1 \@width\z@}\egroup%
\empty@preamble\\
\noalign{\yes@strut\gdef\add@ins{\vrule \@height\z@ \@depth\z@ \@width\z@}}%
}
\def\tabrow#1{\noalign\bgroup\@ifnextchar[{\@tabrow{#1}}{\@tabrow{#1}[]}}
\def\@tabrow#1[#2]{\no@strut\egroup#1\ifx.#2.\\\else\\[#2]\fi\noalign{\yes@strut}}
%
\def\endpltstabular{\crcr\egroup\egroup \egroup}
\expandafter \let \csname endpltstabular*\endcsname = \endpltstabular
\def\pltstabular{\let\@halignto\@empty\@pltstabular}
\@namedef{pltstabular*}#1{\def\@halignto{to#1}\@pltstabular}
\def\@pltstabular{\leavevmode \bgroup \let\@acol\@tabacol
\let\@classz\@tabclassz
\let\@classiv\@tabclassiv \let\\\@tabularcr\@stabarray}
\def\@stabarray{\m@th\@ifnextchar[\@sarray{\@sarray[c]}}
\def\@sarray[#1]#2{%
\bgroup
\setbox\@arstrutbox\hbox{%
\vrule \@height\arraystretch\ht\strutbox
\@depth\arraystretch \dp\strutbox
\@width\z@}%
\@mkpream{#2}%
\edef\@preamble{%
\ialign \noexpand\@halignto
\bgroup \@arstrut \@preamble \tabskip\z@skip \cr}%
\let\@startpbox\@@startpbox \let\@endpbox\@@endpbox
\let\tabularnewline\\%
% \let\par\@empty
\let\@sharp##%
\set@typeset@protect
\lineskip\z@skip\baselineskip\z@skip
\@preamble}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\makeatother
\newenvironment{bigtabular}{\begin{pltstabular}}{\end{pltstabular}}
% For the 'boxed table style:
\newcommand{\SBoxedLeft}{\textcolor[rgb]{0.6,0.6,1.0}{\vrule width 3pt\hspace{3pt}}}
% Formerly used to keep the horizontal line for a definition on the same page:
\newcommand{\SEndFirstHead}[0]{ \nopagebreak \\ }
% Corrects weirdness when a table is the first thing in
% an itemization:
\newcommand{\bigtableinlinecorrect}[0]{~
\vspace{-\baselineskip}\vspace{\parskip}}
% Used to indent the table correctly in an itemization, since that's
% one of the things stabular gets wrong:
\newlength{\stabLeft}
\newcommand{\bigtableleftpad}{\hspace{\stabLeft}}
\newcommand{\atItemizeStart}[0]{\addtolength{\stabLeft}{\labelsep}
\addtolength{\stabLeft}{\labelwidth}}
% For a single-column table in simple environments, it's better to
% use the `list' environment instead of `stabular'.
\newenvironment{SingleColumn}{\begin{list}{}{\topsep=0pt\partopsep=0pt%
\listparindent=0pt\itemindent=0pt\labelwidth=0pt\leftmargin=0pt\rightmargin=0pt%
\itemsep=0pt\parsep=0pt}\item}{\end{list}}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Etc.
% ._ and .__
\newcommand{\Sendabbrev}[1]{#1\@}
\newcommand{\Sendsentence}[1]{\@#1}
% Default style for a nested flow:
\newenvironment{Subflow}{\begin{list}{}{\topsep=0pt\partopsep=0pt%
\listparindent=0pt\itemindent=0pt\labelwidth=0pt\leftmargin=0pt\rightmargin=0pt%
\itemsep=0pt}\item}{\end{list}}
% For the 'inset nested-flow style:
\newenvironment{SInsetFlow}{\begin{quote}}{\end{quote}}
% Indent a 'code-inset nested flow:
\newcommand{\SCodePreSkip}{\vskip\abovedisplayskip}
\newcommand{\SCodePostSkip}{\vskip\belowdisplayskip}
\newenvironment{SCodeFlow}{\SCodePreSkip\begin{list}{}{\topsep=0pt\partopsep=0pt%
\listparindent=0pt\itemindent=0pt\labelwidth=0pt\leftmargin=2ex\rightmargin=2ex%
\itemsep=0pt\parsep=0pt\onehalfspacing}\item}{\end{list}\SCodePostSkip}
\newcommand{\SCodeInsetBox}[1]{\setbox1=\hbox{\hbox{\hspace{2ex}#1\hspace{2ex}}}\vbox{\SCodePreSkip\vtop{\box1\SCodePostSkip}}}
% Inset a 'vertical-inset nested flow:
\newcommand{\SVInsetPreSkip}{\vskip\abovedisplayskip}
\newcommand{\SVInsetPostSkip}{\vskip\belowdisplayskip}
\newenvironment{SVInsetFlow}{\SVInsetPreSkip\begin{list}{}{\topsep=0pt\partopsep=0pt%
\listparindent=0pt\itemindent=0pt\labelwidth=0pt\leftmargin=0pt\rightmargin=0pt%
\itemsep=0pt\parsep=0pt}\item}{\end{list}\SVInsetPostSkip}
\newcommand{\SVInsetBox}[1]{\setbox1=\hbox{\hbox{#1}}\vbox{\SCodePreSkip\vtop{\box1\SCodePostSkip}}}
% The 'compact itemization style:
\newenvironment{compact}{\begin{itemize}}{\end{itemize}}
\newcommand{\compactItem}[1]{\item #1}
% The nested-flow style for `centerline':
\newenvironment{SCentered}{\begin{trivlist}\item \centering}{\end{trivlist}}
% The \refpara command corresponds to `margin-note'. The
% refcolumn and refcontent environments also wrap the note,
% because they simplify the CSS side.
\newcommand{\refpara}[1]{\normalmarginpar\marginpar{\raggedright \footnotesize #1}}
\newcommand{\refelem}[1]{\refpara{#1}}
\newenvironment{refcolumn}{}{}
\newenvironment{refcontent}{}{}
\newcommand{\refparaleft}[1]{\reversemarginpar\marginpar{\raggedright \footnotesize #1}}
\newcommand{\refelemleft}[1]{\refparaleft{#1}}
\newenvironment{refcolumnleft}{}{}
% Macros used by `title' and `author':
\newcommand{\titleAndVersionAndAuthors}[3]{\title{#1\\{\normalsize \SVersionBefore{}#2}}\author{#3}\maketitle}
\newcommand{\titleAndVersionAndEmptyAuthors}[3]{\title{#1\\{\normalsize \SVersionBefore{}#2}}#3\maketitle}
\newcommand{\titleAndEmptyVersionAndAuthors}[3]{\title{#1}\author{#3}\maketitle}
\newcommand{\titleAndEmptyVersionAndEmptyAuthors}[3]{\title{#1}\maketitle}
\newcommand{\titleAndVersionAndAuthorsAndShort}[4]{\title[#4]{#1\\{\normalsize \SVersionBefore{}#2}}\author{#3}\maketitle}
\newcommand{\titleAndVersionAndEmptyAuthorsAndShort}[4]{\title[#4]{#1\\{\normalsize \SVersionBefore{}#2}}#3\maketitle}
\newcommand{\titleAndEmptyVersionAndAuthorsAndShort}[4]{\title[#4]{#1}\author{#3}\maketitle}
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\begin{document}
\preDoc
\titleAndEmptyVersionAndAuthors{Programming Language Implementation}{}{\SNumberOfAuthors{1}\SAuthor{Charles Saternos}}
\label{t:x28part_x22Programmingx5fLanguagex5fImplementationx22x29}
\newpage \tableofcontents \newpage
\sectionNewpage
\Ssection{Introduction}{Introduction}\label{t:x28part_x22Introductionx22x29}
From the outside, programming languages seem like mystical entities that
somehow translate arcane instructions to tangible outputs. The inner
workings of these magical beasts are best left to those who have years of
research under their belt and advanced knowledge of computers.
While programming language implementations do draw on many areas of
computer science knowledge, simple programming language can easily be
developed by any hacker with moderate interest and some free time. In
fact, a simple meta{-}circular interpreter developed in Racket
(the language used for implementations in this book), could be written in
one sitting.
This book is meant to be an introduction to some of the concepts and
techniques used to build programming languages. We{'}ll be looking at simple
implementations, with runnable example code throughout the book. Most code
examples can be run on their own, and the reader is encouraged to run
them, particularly when they are confused. Tinkering with example code is
one of the best ways to learn new programming techniques.
For larger code examples that require multiple chunks of code, the book is
written using literate programming. Literate programming provides
a textual version of the code, meant for reading, as well as the option to
stitch the code chunks together into a runnable source file. This allows
the reader to check out the exact code they read in the chapter, change
it, and run it for themselves.
\Ssubsection{Book source}{Book source}\label{t:x28part_x22Bookx5fsourcex22x29}
The source for this whole book is available on GitHub
\href{https://github.com/charles-l/capstone}{\Snolinkurl{https://github.com/charles-l/capstone}}.
The reader is encouraged to download the source and run it for themself.
\Ssubsection{Why bother with programming language implementation?}{Why bother with programming language implementation?}\label{t:x28part_x22Whyx5fbotherx5fwithx5fprogrammingx5flanguagex5fimplementationx5fx22x29}
There are many reasons for learning how programming languages work under
the hood, but some of the most useful are gaining a deeper understanding
of what programming languages do, how to implement your own domain
specific languages, and building your own general purpose programming
languages for fun.
Gaining a comprehensive knowledge of compilers and interpreters gives you
the ability to reason about error messages, and tune the performance of
programs written in your your favorite programming language. For instance,
knowledge of how the C compiler works, gives a programmer the ability to
determine the difference between compile time and link time errors. When
they get a linker error, they{'}ll realize they forgot to include a library
or object file in their compilation command.
Performance tuning is also easier once a programmer knows how a programing
language is optimizing. For instance, in Lua, strings are interned
{-} they{'}re inserted into a large hash{-}table, so string comparisons are
cheap. This means the programmer doesn{'}t have to intern the strings in
a hash table by hand. Or in Python, knowing that the bytecode compiler
generates an opcode to create a dictionary directly for an inline
declaration (i.e. \Scribtexttt{{\char`\{}{\char`\}}}), and the \Scribtexttt{dict()} function does a function
call generates the opcode, meaning an inline \Scribtexttt{{\char`\{}{\char`\}}} is faster than
calling \Scribtexttt{dict()}.
\sectionNewpage
\Ssection{Background}{Background}\label{t:x28part_x22Backgroundx22x29}
This chapter will introduce the basics of the Racket programming language,
and will discuss some theory that will be used later during
implementation.
\Ssubsection{Racket}{Racket}\label{t:x28part_x22Racketx22x29}
\includegraphics[scale=0.5]{rkttmp98239c067a6552b13196269e9adf471433a234a1.png}
We{'}ll use the Racket programming language for the implementations
throughout the book. Racket is a descendent of Lisp, the second high{-}level
programming language created (after Fortran). Despite its age, Lisp
dialects are still popular today.
Racket is designed to aid programming language developers in prototyping
and experimenting with new programming language paradigms. It is a test
bed, with many tools and libraries that make implementations simple and
concise. Instead of tracking memory usage in C, or writing dozens of
classes in a complicated hierarchy in Java, we will focus on developing
compilers and interpreters using domain{-}specific languages provided by
Racket.
Even without the libraries and extra resources, Racket is still an
excellent choice for developing programming languages. With built{-}in
functional pattern matching, idiomatic use of recursion, which is useful
for tree{-}navigation, and a simple, easy{-}to{-}implement kernel
(which makes building Lisp interpreters trivial), Racket is a great choice
for language development.
Racket is a simple language that you can learned quickly. You{'}re
encouraged to enter the simple programs throughout this book to see the
results yourself. Fidget with them in a REPL
(the interactive prompt for Racket programs). Tweak
them and observe what changes in the output. You{'}re encouraged to copy the
code throughout the book, but it{'}s especially vital to understand this
chapter since concepts we introduce here are referenced throughout the book.
Lisp{'}s\NoteBox{\NoteContent{We{'}ll talk generally about Lisp and Scheme since the features
are relevant to Racket}} syntax is extremely minimal. Expressions are
written in prefix notation, with parentheses to denote function calls.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{println}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{"Hello}\mbox{\hphantom{\Scribtexttt{x}}}\RktVal{world"}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktOut{"Hello world"
}\end{SingleColumn}\end{SCodeFlow}
Literal values (i.e. strings, numbers, or booleans) and variables are
written without parentheses.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktVal{"reeeee"}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{"reeeee"}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktVal{2}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{2}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{define}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{the{-}number{-}three}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{3}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktSym{the{-}number{-}three}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{3}\end{SingleColumn}\end{SCodeFlow}
The number of parentheses is significant in Lisp, unlike other languages
where they{'}re used for redefining order of operations. It{'}s nonsense to
write extra parentheses in many contexts.
For instance, the variable we defined in the previous example in another
pair of parentheses will attempt to execute the bound value (\Scribtexttt{3}) as
a function. This doesn{'}t make sense, so it{'}s important to match
parentheses carefully (a good editor is invaluable!).
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{the{-}number{-}three}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktErr{the{-}number{-}three: undefined;
cannot reference an identifier before its definition
in module: {\textquotesingle}anonymous{-}module
}\end{SingleColumn}\end{SCodeFlow}
The DrRacet IDE that ships with Racket [\hyperref[t:x28cite_x22DrSchemex22x29]{DrScheme}] is a common choice
for newcomers to the language. DrRacket features a REPL
(read{-}eval{-}print{-}loop {-} a prompt for interactive program development),
a graphical debugger, and macro expander. It highlights and indents Racket
code properly, and generally makes life easier for users who aren{'}t
familiar with Lisp syntax and notation.
Lisp doesn{'}t (by default) have infix notation. Standard numeric operations
are written in prefix form. For example, 3 * (4 + 2) is written:
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{*}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{3}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{+}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{4}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{2}\RktPn{)}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{18}\end{SingleColumn}\end{SCodeFlow}
This format seems awkward at first, but has the advantage of simple syntax
that is straightforward to parse which makes user defined macros possible
(which we{'}ll look at later).
Racket has standard data types (strings, numbers, and characters):
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktVal{"reeeee"}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{"reeeee"}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktVal{42}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{42}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktVal{\#{\char`\\}z}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{\#{\char`\\}z}\end{SingleColumn}\end{SCodeFlow}
And it has built{-}in data{-}types that are typically seen in Lisps. Namely
symbols, \Scribtexttt{cons} cells, lists, and vectors.
Symbols are similar to strings. The only distinction is the symbol{'}s value
is "interned" (i.e. inserted into an internal hash table), to make lookups
fast. This means symbols are cheap to compare, so they can be used in
contexts where enums would be used in other languages.
Symbols are created with the quote keyword:
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{quote}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{symbols{-}are{-}cooler{-}than{-}strings}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}symbols{-}are{-}cooler{-}than{-}strings}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{println}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{quote}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{just{-}a{-}symbol}\RktPn{)}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktOut{{\textquotesingle}just{-}a{-}symbol
}\end{SingleColumn}\end{SCodeFlow}
Since it{'}s awkward to keep write \Scribtexttt{quote} every time we create a symbol,
we can use the shorthand \Scribtexttt{}\Scribtexttt{{'}}\Scribtexttt{}.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{symbols{-}are{-}cooler{-}than{-}strings}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}symbols{-}are{-}cooler{-}than{-}strings}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{println}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{just{-}a{-}symbol}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktOut{{\textquotesingle}just{-}a{-}symbol
}\end{SingleColumn}\end{SCodeFlow}
Admittedly, the quote syntax is shocking when seen by programmers who use
languages that use single quotes to denote characters or strings. But
quote shorthand is terser than the \Scribtexttt{quote} keyword, so we will use it.
Lisp{'}s primary container structure is the cons cell. A cons cell is a pair
of two values. The first and second elements of the pair are accessed with
the \Scribtexttt{car} and \Scribtexttt{cdr} functions respectively.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{b}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(a {\hbox{\texttt{.}}} b)}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{2}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{"cool"}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(2 {\hbox{\texttt{.}}} "cool")}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{\#{\char`\\}m}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{\#{\char`\\}e}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(\#{\char`\\}m {\hbox{\texttt{.}}} \#{\char`\\}e)}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{car}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{b}\RktPn{)}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}a}
\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{cdr}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{my{-}other{-}car{-}is}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{a{-}cdr}\RktPn{)}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}a{-}cdr}\end{SingleColumn}\end{SCodeFlow}
Cons cells can hold other cons cells in them, to become a linked list of
values.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{little}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{dotted}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{list}\RktPn{)}\RktPn{)}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(a little dotted {\hbox{\texttt{.}}} list)}\end{SingleColumn}\end{SCodeFlow}
Rather than storing a value in the last cons cell, it{'}s standard to store
the null list, \Scribtexttt{}\Scribtexttt{{'}}\Scribtexttt{()}, which helps simplify list navigation.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{proper}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{cons}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{list}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktPn{(}\RktPn{)}\RktPn{)}\RktPn{)}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(a proper list)}\end{SingleColumn}\end{SCodeFlow}
Since \Scribtexttt{cons}ing a bunch of \Scribtexttt{cons} cells together is tedious, the
helper function, \Scribtexttt{list} will create a linked{-}list of cons cells
(terminated by the empty list).
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{list}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{b}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{{\textquotesingle}}\RktSym{c}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(a b c)}\end{SingleColumn}\end{SCodeFlow}
Furthermore, quoting each individual element can become an annoyance, so
we can use the quote keyword to build a list of symbols.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktPn{(}\RktSym{quote}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktPn{(}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{b}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{c}\RktPn{)}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(a b c)}\end{SingleColumn}\end{SCodeFlow}
Or using the shorthand \Scribtexttt{}\Scribtexttt{{'}}\Scribtexttt{}:
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktSym{{\textquotesingle}}\RktPn{(}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{b}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{c}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}(a b c)}\end{SingleColumn}\end{SCodeFlow}
The quote symbol can construct lists, sublists, and literal values.
\begin{SCodeFlow}\begin{SingleColumn}\begin{bigtabular}{@{\bigtableleftpad}l@{}l@{}l@{}}
\hbox{\Scribtexttt{{\Stttextmore}}} &
\hbox{\mbox{\hphantom{\Scribtexttt{x}}}} &
\begin{RktBlk}\begin{tabular}[t]{@{}l@{}}
\hbox{\RktMeta{}\RktSym{{\textquotesingle}}\RktPn{(}\RktPn{(}\RktSym{a}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{b}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{c}\RktPn{)}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{d}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{e}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktSym{f}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{1}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{2}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{3}\RktMeta{}\mbox{\hphantom{\Scribtexttt{x}}}\RktMeta{}\RktVal{"strings}\mbox{\hphantom{\Scribtexttt{x}}}\RktVal{too{\hbox{\texttt{!}}}"}\RktPn{)}\RktMeta{}}\end{tabular}\end{RktBlk}\end{bigtabular}
\RktRes{{\textquotesingle}((a b c) d e f 1 2 3 "strings too{\hbox{\texttt{!}}}")}