c01-whatis.sil

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\chapter{What is SILE?}
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SILE is a typesetting system. Its job is to produce beautiful printed
documents from raw content. The best way to understand what SILE is and what it
does is to compare it to other systems which you may have heard of.

\section{SILE versus Word}

When most people produce printed documents using a computer, they usually use
desktop oriented word processing software such as Microsoft Word, Apple Pages,
or LibreOffice Writer. SILE is not a word processor; it is a typesetting
system. There are several important differences.

The job of a word processor is to produce a document that looks exactly like
what you type on the screen. By contrast, the job of a typesetting system is to
take raw content and produce a document that looks as good as possible. The
input for SILE is a text document that includes instructions about how the
content should be laid out on a page. In order to obtain the typeset result,
the input file must be \em{processed} to render the desired output.

Word processors often describe themselves as WYSIWYG: What You See Is What You
Get. SILE is cheerfully \em{not} WYSIWYG. In fact, you don’t see what you get
until you get it. Rather, SILE documents are prepared initially in a \em{text
editor}—a piece of software which focuses on the text itself and not what it
looks like—and then run through SILE in order to produce a PDF document.

For instance, most word processors are built roughly around the concept of a
page with a central content area into which you type and style your content.
The overall page layout is controlled by the page size and margins and more
fine tuning is done by styling the content itself. You typically type
continuously and when you hit the right margin, your cursor will automatically
jump to the next line. In this way, the user interface shows you where the lines
on the printed page will break.

In SILE the overall page layout is defined with a paper size and a series of
one or more content frames. These frame descriptions provide the containers
where content will later be typeset, including information about how it might
flow from one frame to the next. Content is written separately, and SILE works
out automatically how it best flows from frame to frame and from page to page.
So when you are preparing content for SILE, you don’t know where the lines
will break until after it has been processed. You may use your text editor to
type and type and type as long a line as you like, and when SILE comes to
process your instructions, it will consider your input several times
over in order to work out how to best to break the lines to form a paragraph.
For example, if after one pass it finds that it has ended two successive lines
with a hyphenated word, it will go back and try again and see if it can find
better layout.

The same idea applies to page breaks. When you type into a word processor, at
some point you will spill over onto a new page. When preparing content for
SILE, you keep typing, because the page breaks are determined after considering
the layout of the whole document.

In other words, SILE is a \em{language} for describing what you want to happen,
and an interpreter that will make certain formatting decisions about the best
way for those instructions to be turned into print.

\section{SILE versus TeX}

“Ah,” some people will say, “that sounds very much like TeX!”\footnote{Except
that, being TeX users, they will say “Ah, that sounds very much like
T\glue[width=-.1667em]\lower[height=0.5ex]{E}\glue[width=-.125em]X!”} If you
don’t know much about TeX or don’t care, you can probably skip this section.

And it’s true. SILE owes an awful lot of its heritage to TeX. It would be
terribly immodest to claim that a little project like SILE was a worthy
successor to the ancient and venerable creation of the Professor of the Art of
Computer Programming, but… really, SILE is basically a modern rewrite of TeX.

TeX was one of the earliest typesetting systems, and had to make a lot of
design decisions somewhat in a vacuum. Some of those design decisions have
stood the test of time—TeX is still an extremely well-used typesetting system
more than thirty years after its inception, which is a testament to its design
and performance—but many others have not. In fact, most of the development of
TeX since Knuth’s era has involved removing his early decisions and replacing
them with technologies which have become the industry standard: we use
TrueType fonts, not METAFONTs (\tt{xetex}); PDFs, not DVIs (\tt{pstex},
\tt{pdftex}); Unicode, not 7-bit ASCII (\tt{xetex} again); markup languages
and embedded programming languages, not macro languages (\tt{xmltex},
\tt{luatex}). At this point, the parts of TeX that people actually \em{use}
are (1) the box-and-glue model, (2) the hyphenation algorithm, and (3) the
line-breaking algorithm.

SILE follows exactly in TeX’s footsteps for each of these three areas that have
stood the test of time; it contains a slavish port of the TeX line-breaking
algorithm which has been tested to produce exactly the same output as TeX given
equivalent input. But as SILE is itself written in an interpreted
language,\footnote{And if the phrase \code{TeX capacity exceeded} is familiar
to you, you should already be getting excited.} it is very easy to extend or
alter the behaviour of the SILE typesetter.

For instance, one of the things that TeX can’t do particularly well is
typesetting on a grid. This a must-have feature for anyone
typesetting bibles and other documents to be printed on thin paper.
Typesetting on a grid means that each line of text will
line up between the front and back of each piece of paper producing much less
visual bleed-through when printed on thin paper. This is virtually impossible
to accomplish in TeX. There are various hacks to try to make it happen, but
they’re all horrible. In SILE, you can alter the behaviour of the typesetter
and include a very short add-on package to enable grid typesetting.

In fact, nobody uses plain TeX—they all use LaTeX equivalents plus a huge
repository of packages available from the The Comprehensive TeX Archive Network
(CTAN) archive. SILE does not benefit from the
large ecosystem and community that has grown up around TeX; in that sense, TeX
will remain ahead of SILE for some time to come. But in terms of
\em{core capabilities}, SILE is already equivalent to TeX—if not
somewhat more advanced.

\section{SILE versus InDesign}

The other tool that people reach for when designing printed material on
a computer is Adobe InDesign (or similar desktop publishing software, such as
Scribus).

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DTP software is similar to word processing software in that they are
both graphical and largely WYSIWYG, but the paradigm is different. The focus is
usually less on preparing the content than on laying it out on the
page—you click and drag to move areas of text and images around the screen.

InDesign is a complex, expensive, commercial publishing tool. SILE is a free,
open source typesetting tool which is entirely text-based; you enter commands
in a separate editing tool, save those commands into a file, and hand it to
SILE for typesetting. And yet the two systems do have a number of common
features.

In InDesign, text is flowed into \em{frames} on the page. On the left, you can
see an example of what a fairly typical InDesign layout might look like.
SILE also uses the concept of frames to determine where text should appear on
the page, and so it’s possible to use SILE to generate page layouts which are
more flexible and more complex than that afforded by TeX.

\smallskip

Another thing which people use InDesign for is to turn structured data in XML
format—catalogues, directories and the like—into print. The way you do this in
InDesign is to declare what styling should apply to each XML element, and as
the data is read in, InDesign formats the content according to the rules that
you have declared.

You can do the same thing in SILE, except you have a lot more control
over how the XML elements get styled, because you can run any SILE command you
like for a given element, including calling out to Lua code to style a piece of
XML. Since SILE is a command-line filter, armed with appropriate styling
instructions you can go from an XML file to a PDF in one shot.

In the final chapters of this book, we’ll look at some extended examples of
creating a \em{class file} for styling a complex XML document into a PDF with
SILE.

\section{Conclusion}

SILE\footnote{In case you’re wondering, the author pronounces it
\font[family=Gentium Plus]{/saɪəl/}, to rhyme with “trial”.} {}takes some textual
instructions and turns them into PDF output. It has
features inspired by TeX and InDesign, but seeks to be more flexible,
extensible and programmable than either of them. It’s useful both for
typesetting documents (such as this very documentation) written in the SILE
language, and as a processing system for styling and outputting structured
data.
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