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#lang scribble/manual

@(require racket/runtime-path scribble/example quadwriter pitfall/page pitfall/color racket/format (for-label txexpr (except-in pollen #%module-begin) xml racket/base racket/draw quadwriter)
pollen/scribblings/mb-tools quad/pict)

@(define my-eval (make-base-eval))
@(my-eval `(require quad quad/pict))


@title[#:style 'toc]{Quad: document processor}

@author[(author+email "Matthew Butterick" "mb@mbtype.com")]

@(define-runtime-path quads.png "quads.png")
@(image quads.png #:scale 0.4)

@margin-note{Quad is in progress. It works, but it is unstable — I am still changing things, small and large — and thus I make no commitment to maintain the API in its current state.}


@section{Installing Quad & Quadwriter} 

At the command line:
@verbatim{raco pkg install quad}

After that, you can update the package like so:
@verbatim{raco pkg update quad}

Or, without the command line: Launch Dr­Racket. Use the @onscreen{File} → @onscreen{Install Package ...} command to install @racketmodname[quad].

Either way, @racketmodname[quadwriter] is installed as part of the @racketmodname[quad] package.

@margin-note{If you're new to Racket and want to configure your system to use the terminal commands, follow the @link["https://beautifulracket.com/setup.html#full-setup"]{instructions here}.}

@section{What is Quad?}

A document processor, which means that it:

@itemlist[#:style 'ordered
  @item{Computes the layout of your document from a series of formatting codes (not unlike a web browser)}

  @item{Renders to PDF (not unlike a word processor).}
]

For instance, LaTeX is a document processor. So are web browsers. Quad borrows from both traditions — it's an attempt to modernize the good ideas in LaTeX, and generalize the good ideas in web browsers, while bypassing some of the limitations of LaTeX (e.g., no Unicode) and of web browsers (e.g., performance and error recovery are valued above all).

Document processors sit opposite WYSIWYG tools like Microsoft Word and Adobe InDesign. There, the user controls the layout by manipulating a representation of the page on the screen. This is fine as far as it goes. But changes to the layout — for instance, a new page size — often require a new round of manual adjustments. 

A document processor, by contrast, relies on markup codes within the text to determine the layout programmatically. Compared to WYSIWYG, this approach offers less granular control. But it also creates a more flexible relationship between the source and its possible layouts. 

Another benefit of document processors is that it permits every document to have a high-level, text-based source file that's independent of any particular output format.

Much of the font-parsing and PDF-rendering code in Quad is adapted from @link["http://github.com/foliojs/"]{FolioJS} by Devon Govett. I thank Mr. Govett for figuring out a lot of details that would've made me squeal in agony. 

@subsection{How does Quad work?}

Quad produces PDFs using three ingredients: 

@itemlist[#:style 'ordered
  @item{A @bold{font engine} that handles glyph shaping and positioning using standard TTF or OTF font files.}

  @item{A @bold{layout engine} that converts typesetting instructions into an output-independent layout — e.g., putting characters into lines, and lines into pages.}

  @item{A @bold{PDF engine} that takes this layout and renders it as a finished PDF file.}
]


For the most part, neither Quad nor Quadwriter rely much on @racketmodname[racket/draw]. In particular, Quad completely ignores Racket's PDF-drawing functions, which are provided by @link["https://www.pango.org/"]{Pango}, because of major shortcomings in the kind of PDFs it produces (for instance, it doesn't support hyperlinks).


@section{What is Quadwriter?}

A demo app built with Quad. It takes a text-based source file as input, calculates the typesetting and layout, and then outputs a PDF.

You can fiddle with it & then submit issues and feature requests at the @link["http://github.com/mbutterick/quad"]{Quad repo}. 


@section{Quadwriter quick tour}


Open DrRacket (or whatever editor you prefer) and start a new document with @code{#lang quadwriter/markdown} as the first line:


@fileblock["test.rkt"
@codeblock|{
#lang quadwriter/markdown
Brennan and Dale like fancy sauce.
}|
]


Save the document. Any place, any name is fine. 

@onscreen{Run} the document. You'll get REPL output like this:

@repl-output{
quadwriter: atomize: 2ms
quadwriter: hyphenate: 1ms
quadwriter: line-wrap: 21ms
quadwriter: col-wrap: 0ms
quadwriter: page-wrap: 0ms
quadwriter: position: 1ms
quadwriter: draw: 75ms
quadwriter: wrote PDF to /Users/Desktop/test.pdf
}

Congratulations — you just made your first PDF. If you want to have a look, either open the file manually, or enter this command on the REPL, which will open the PDF in your default viewer:

@terminal{
> (view-output)
}

Next, on the REPL enter this:

@terminal{
> doc
}

You will see the actual input to Quadwriter, which is called a @tech{Q-expression}:

@repl-output{
'(q () (q ((page-margin-left "120") (page-margin-top "80") (page-margin-bottom "120") (font-family "text") (line-height "17")) (q ((keep-first-lines "2") (keep-last-lines "3") (font-size "1em") (character-tracking "0") (hyphenate "true") (display "g49598")) "Brennan and Dale like fancy sauce.")))
}

In the demos that follow, the input language will change slightly. But the PDF will be rendered the same way (by running the source file) and you can always look at @racket[doc] or use @racket[view-output].


@subsection{Quadwriter & Markdown}

I @link["https://docs.racket-lang.org/pollen/second-tutorial.html#%28part._the-case-against-markdown%29"]{don't recommend} that writers adopt Markdown for serious projects. But for goofing around, why not.

Our first version of @racket["test.rkt"] had one line of plain text:

@fileblock["test.rkt"
@codeblock|{
#lang quadwriter/markdown
Brennan and Dale like fancy sauce.
}|
]

Behind the scenes, @racketmodname[quadwriter/markdown] is doing more heavy lifting than this sample suggests. We can type our source in Markdown notation, and it will automatically be converted to the appropriate Quad formatting commands to make things look right. 

For instance, try this sample, which combines a Markdown heading, bullet list, code block, and bold and italic formatting:

@fileblock["test.rkt"
@codeblock|{
#lang quadwriter/markdown
# Did you know?

__Brennan__ and **Dale** like:

* *Fancy* sauce
* _Chicken_ fingers

```
And they love to code
```
}|
]

You're welcome to paste in bigger Markdown files that you have laying around and see what happens. As a demo language, I'm sure there are tortured agglomerations of Markdown notation that will confuse @racketmodname[quadwriter/markdown]. But vanilla files should be fine.

Back to the demo. Curious characters can do this:

@terminal{
> doc
}

To see this:

@repl-output{
'(q
  ()
  (q
   ((page-margin-left "120") (page-margin-top "80") (page-margin-bottom "120") (font-family "text") (line-height "17"))
   (para-break)
   (q ((font-family "heading") (first-line-indent "0") (display "block") (font-size "20") (line-height "24.0") (border-width-top "0.5") (border-inset-top "9") (inset-bottom "-3") (inset-top "6") (keep-with-next "true") (id "did-you-know")) "Did you know?")
   ···
}

This is the first part of the @tech{Q-expression} that the source file produces when it runs and exports via @racket[doc]. This @tech{Q-expression} is passed to Quadwriter for layout and rendering.

@margin-note{Yes, you can generate your own @tech{Q-expressions} by other means and pass them to @racketmodname[quadwriter] for layout & rendering. See @racket[render-pdf].}

@margin-note{Mac OS note: I have no connection to the @link["https://skim-app.sourceforge.io/"]{Skim PDF reader}, but it has an auto-refresh feature that monitors a PDF for changes. This cooperates nicely with Quadwriter during editing sessions: you can have a window on the PDF that updates automatically when you recompile the source file (say, in DrRacket).}

@subsection{Quadwriter & HTML}

Suppose Markdown is just not your thing. You prefer to enter your HTML-style markup the old-fashioned way — by hand. I hear you. So let's switch to the @racket[quadwriter/html] dialect. First we try our simple test:

@fileblock["test.rkt"
@codeblock|{
#lang quadwriter/html
Brennan and Dale like fancy sauce.
}|
]

We get the same PDF result as before, again because a short line of plain text is the same in this dialect as the last.

But if we want to reproduce the result of the Markdown notation, this time we use the equivalent HTML-ish markup tags:

@fileblock["test.rkt"
@codeblock|{
#lang quadwriter/html
◊h1{Did you know?}

◊strong{Brennan} and ◊strong{Dale} like:

◊ul{
◊li{◊em{Fancy} sauce}
◊li{◊em{Chicken} fingers}
}

◊pre{
◊code{
And they love to code
}
}
}|
]

The special @litchar{◊} character is called a @deftech{lozenge}. It introduces markup tags. @link["https://docs.racket-lang.org/pollen/pollen-command-syntax.html#%28part._the-lozenge%29"]{Instructions for typing it}, but for now it suffices to copy & paste, or use the @onscreen{Insert Command Char} button in the DrRacket toolbar.

Under the hood, the @racketmodname[quadwriter/markdown] dialect is converting the Markdown surface notation into markup tags that look like this. So the @racket[quadwriter/html] dialect just lets us start with those tags. 

Curious characters can prove that this is so by again typing at the REPL:

@terminal{
> doc
}

This @tech{Q-expression} is exactly the same as the one that resulted with the @racketmodname[quadwriter/markdown] source file.

@subsection{Quadwriter & Q-expressions}

@racketmodname[quadwriter/markdown] showed high-level notation (= a generous way of describing Markdown) that generated a @tech{Q-expression}. Then @racketmodname[quadwriter/html] showed a mid-level notation that generated another (identical) @tech{Q-expression}.

If we wish, we can also skip the notational foofaraw and just write @tech{Q-expressions} directly in our source file. We do this with the basic @racketmodname[quadwriter] language. 

Recall our very first example:

@fileblock["test.rkt"
@codeblock|{
#lang quadwriter/html
Brennan and Dale like fancy sauce.
}|
]

In the REPL, the @racket[doc] was this @tech{Q-expression}:

@repl-output{
'(q () (q ((page-margin-left "120") (page-margin-top "80") (page-margin-bottom "120") (font-family "text") (line-height "17")) "Brennan and Dale like fancy sauce."))
}

Let's copy this @tech{Q-expression} and use it as our new source code. This time, however, we'll switch to plain @code{#lang quadwriter} (instead of the @racket[markup] or @racket[markdown] dialects):

@fileblock["test.rkt"
@codeblock|{
#lang quadwriter
'(q () (q ((page-margin-left "120") (page-margin-top "80") 
(page-margin-bottom "120") (font-family "text") 
(line-height "17")) "Brennan and Dale like fancy sauce."))
}|
]

This produces the same one-line PDF as before.

Likewise, we can pick up the @racket[doc] from our more complex example:


@codeblock|{
#lang quadwriter/markdown
# Did you know?

__Brennan__ and **Dale** like:

* *Fancy* sauce
* _Chicken_ fingers

```
And they love to code
```
}|


And again, use the resulting @tech{Q-expression} in @racket[doc] as the source for a new @racket[quadwriter] program, which will result in the same PDF.

It is also possible to mix @code{#lang quadwriter/html} and @tech{Q-expressions}, allowing us to escape to lower-level @tech{Q-expressions} when needed.

@codeblock|{
#lang quadwriter/html

◊h1{A nice image}

Check out this ◊b{nice} image:

◊q[#:line-height "false" #:image-width "400" #:line-align "center"
   #:image-file "nice-image.png" #:image-alt "A nice image"
   #:display "block"]{}
}|

@subsection{Setting section-level attributes}

Even if you're using a @racketmodname[quadwriter] dialect, you can still set section-level formatting attributes for the document. For instance, suppose we wanted to make our original @racketmodname[quadwriter/markdown] example 24 points and red, and put the PDF on wide tabloid (17in × 11in) paper. We can add these section-level attributes to the beginning of our source file as keyword arguments:

@fileblock["test.rkt"
@codeblock|{
#lang quadwriter/markdown

#:page-size "tabloid"
#:page-orientation "wide"
#:font-size 18
#:font-color "red"

Brennan and Dale like fancy sauce.
}|
]

Any of the @secref{Markup} attributes documented below can be used as keyword arguments. The syntax follows the pattern above: one attribute + value pair per line, with the attribute prefixed with @litchar{#:} to make it a keyword, followed by the value.

This keyword syntax works in the @racketmodname[quadwriter], @racketmodname[quadwriter/markdown], and @racketmodname[quadwriter/html] languages. The idea is to make it easy to adjust the default layout behavior without going outside the source file.


@subsection{Invoking Quadwriter as a library}

Part of the idea of @racketmodname[quad] and @racketmodname[quadwriter] is to make typographic layout & PDF generation a service that can be built into other Racket apps and languages. 

Let's see how this works by doing document layout and rendering from within good old @racketmodname[racket/base]:

@fileblock["test.rkt"
@codeblock|{
#lang racket/base
(require quadwriter)
(define qx `(q "Brennan likes fancy sauce."
                ,para-break
                "Dale hates fancy sauce."))
(define pdf-path "~/Desktop/new.pdf")
(render-pdf qx pdf-path)
}|
]

Here, we create a little @tech{Q-expression}, which we pass to @racket[render-pdf] with a @racket[pdf-path] argument.

@subsection{Combining Quadwriter with Pollen}

Fans of @racketmodname[pollen] might be glad to hear that @racketmodname[quadwriter] can be used to handle layout and PDF rendering for Pollen source files. As usual we start with a Pollen source file, this time with the @racket[pdf.pm] extension to indicate that it's a Pollen markup file that will produce a PDF:

@fileblock["test.pdf.pm"
@codeblock|{
#lang pollen

Brennan likes fancy sauce.

Dale hates fancy sauce.

}|
]

Then we add a simple @racket["pollen.rkt"] that converts the output of our source file into a @tech{Q-expression}:

@fileblock["pollen.rkt"
@codeblock|{
#lang racket
(require pollen/decode quadwriter)
(provide root render-pdf)
 
(define (root . xs)
  `(q ,@(add-between (decode-paragraphs xs 'q) para-break)))
}|
]

All we're doing here is wrapping our paragraphs in @racket[q] tags (rather than the default @racket[p] tags) and then adding explicit Quadwriter paragraph breaks between them (see @racket[para-break]).

Finally, we add a @racket["template.pdf.p"] that passes the @racket[doc] from the Pollen source to @racket[render-pdf]:

@fileblock["template.pdf.p"
@codeblock|{
◊(render-pdf doc #false)
}|
]

In this case, we pass @racket[#false] as the path argument to @racket[render-pdf] so that it returns the actual bytes, which the Pollen renderer will put in the right place.

You can fire up the Pollen project server and see how this works. As usual with Pollen sources, when you make changes to the source file, the rendered PDF will be dynamically updated.


@margin-note{Though a @racketmodname[quadwriter] source file and a @racketmodname[pollen] source file both export something called @racket[doc], these exports don't share any deeper connection. (The name was chosen to be consistent with Scribble, which also exports a @racket[doc].)}

@subsection{Quick tour complete}

In the usual Racket tradition, @racket[quadwriter] and its dialects are just compiling a document from a higher-level representation to a lower-level representation. 

If you're a writer, you might prefer to use the high-level representation (like @racketmodname[quadwriter/markdown]) so that your experience is optimized for ease of use.

If you're a developer, you might prefer to use the lower-level representation for precision. For instance, a @racketmodname[pollen] author who wanted to generate a PDF could design tag functions that emit Q-expressions, and then pass the result to @racket[render-pdf].

Or, you can aim somewhere in between. Like everything else in Racket, you can design functions & macros to emit the pieces of a @tech{Q-expression} using whatever interface you prefer. 


@section{Quadwriter: developer guide}

@defmodule[quadwriter #:link-target? #f #:no-declare]

@defmodulelang*/no-declare[(quadwriter
quadwriter/markdown
quadwriter/html)]

@declare-exporting[quadwriter]

@defthing[doc qexpr?]{
Every source file written in a @racketmodname[quadwriter] dialect exports an identifier called @racket[doc] that contains the @tech{Q-expression} that results from running the source.
}

@subsection{Q-expressions}

A @deftech{Q-expression} is an @seclink["X-expressions" #:doc '(lib "pollen/scribblings/pollen.scrbl")]{X-expression}, but more restricted:

@racketgrammar[
#:literals (list q)
qexpr string
      (list q (list (list attr-name attr-val) ...) qexpr ...)
      (list q (list qexpr ...))
]

This grammar means that a @tech{Q-expression} is either a) a string, b) an X-expression whose tag is @racket[q] and whose elements are themselves Q-expressions.

@examples[#:eval my-eval
(qexpr? "Hello world")
(qexpr? '(q "Hello world"))
(qexpr? '(q () "Hello world"))
(qexpr? '(q ((font-color "pink")) "Hello world"))
(qexpr? '(q ((font-color "pink")) (q "Hello world")))
(code:comment @#,t{malformed Q-expressions})
(qexpr? 42)
(qexpr? '(div "Hello world"))
(qexpr? '(q (("pink" font-color)) "Hello world"))
] 

Because Q-expressions are a subset of X-expressions, you can apply any tools that work with X-expressions (for instance, the @racketmodname[txexpr] library).

@margin-note{Unlike X-expressions, Q-expressions do not support character entities or CDATA, because those are inherent to XML-ish markup.}



@subsection{Markup}

@subsection{Hard breaks}


@deftogether[(@defthing[line-break qexpr?]
@defthing[column-break qexpr?]
@defthing[page-break qexpr?])]{
The Q-expressions @racketresult['#,line-break], @racketresult['#,column-break], and @racketresult['#,page-break], respectively. Quadwriter will automatically insert  these breaks as needed. But you can also add them explicitly (aka ``hard'' breaks) by inserting the @tech{Q-expression} denoting the break.
}


@defthing[para-break qexpr?]{
The @tech{Q-expression} @racketresult['#,para-break]. Used to denote the start of a new paragraph. The default @racket[space-before] a paragraph is 0; the default @racket[space-after] is 60% of the paragraph's @racket[line-height].
}


@defthing[section-break qexpr?]{
The @tech{Q-expression} @racketresult['#,section-break]. Used to denote the start of a new section.

A section is a contiguous series of pages. Each section has its own @secref{Section-level_attributes}. A document without any explicit section breaks still has one section (that includes all the pages).
}

@subsection{Drawing quads}

@defmodule[quadwriter/draw]

@deftech{Drawing quads} can be used to put arbitrary text or shapes in the document, either in the midst of the text flow, or at arbitrary locations. Drawing quads can be used to implement headers and footers, line numbers, title pages, and so on.

@defthing[line qexpr?]{
Draws a line. Four attributes are required: @racket[x1] and @racket[y1] (which determine the starting point) and @racket[x2] and @racket[y2] (which determine the ending point). Each of these values is a @tech{dimension string}. These distances are interpreted as relative to the upper left corner of the quad.

Optional attributes are @racket[stroke] (a @tech{dimension string} that controls the thickness of the line) and @racket[color] (a a @tech{hex color} string or @tech{named color} string that controls the color of the line).
}

@defthing[text qexpr?]{
Draws a single text string without line wrapping. One attribute is required: @racket[string], which is the string to be drawn. 

A text-drawing quad will also inherit the current @secref["Font_attributes"], which can also be set separately.
}


@subsection{Attributes}

These are the attributes that can be used inside a @tech{Q-expression} passed to @racketmodname[quadwriter]. Inside a Q-expression, every attribute is a @tech[#:doc '(lib "scribblings/guide/guide.scrbl")]{symbol}, and every attribute value is a @tech[#:doc '(lib "scribblings/guide/guide.scrbl")]{string}.

A @deftech{dimension string} represents a distance in the plane. If unitless, it is treated as points (where 1 point = 1/72 of an inch). If the number has @racket[in], @racket[cm], or @racket[mm] as a suffix, it is treated as inches, centimeters, or millimeters respectively. If the number has @racket[p] or @racket[pica] as a suffix or infix, it is treated as a pica / point measurement. A pica is 12 points, so @racket[12p] is 72 points, and @racket[3p9] is 45 points. If the number has @racket[em] as a suffix, it is treated as an em measurement, which is a multiple of the current font size.

@subsubsection{Document-level attributes}

Attributes that can only be set once for the whole document.

@defthing[#:kind "attribute" output-path symbol?]{
Output path for the rendered PDF. Default is the name of the source file with its extension changed to @racket[.pdf]. For instance, @racket["my-source.rkt"] would become @racket["my-source.pdf"]. Unsaved source files are rendered as @racket["untitled.pdf"].
}

@deftogether[(@defthing[#:kind "attribute" pdf-title symbol?]
              @defthing[#:kind "attribute" pdf-author symbol?]
              @defthing[#:kind "attribute" pdf-subject symbol?]
              @defthing[#:kind "attribute" pdf-keywords symbol?])]{
Strings that are used to fill in the corresponding @link["https://helpx.adobe.com/acrobat/using/pdf-properties-metadata.html"]{PDF metadata} fields. Default for each is the empty string.
}


@subsubsection{Section-level attributes}

Attributes that can be set for each section.


@deftogether[(@defthing[#:kind "attribute" page-size symbol?]
              @defthing[#:kind "attribute" page-orientation symbol?])]{
The usual way of setting the overall page dimensions of the rendered PDF. The value of @racket[page-size] is a @tech{named page size}. The value of @racket[page-orientation] can be either @racket["tall"] or @racket["portrait"] (which both put the longer edge vertically) or @racket["wide"] or @racket["landscape"] (which put the longer edge horizontally).

The @deftech{named page sizes} are listed below. Names are case-insensitive. Dimensions below are in points.


@(tabular 
#:sep @hspace[2] 
(cons (list @bold{name} @bold{short edge} @bold{long edge})
(map (λ (args) (map (λ (x) (tt (~a x))) args)) 
(sort (hash->list page-sizes) string<? #:key car))))
}


@deftogether[(@defthing[#:kind "attribute" page-width symbol?]
              @defthing[#:kind "attribute" page-height symbol?])]{
The unusual way of setting the overall page dimensions of the rendered PDF. Both values are given as a @tech{dimension string}.
}

@deftogether[(@defthing[#:kind "attribute" page-margin-top symbol?]
              @defthing[#:kind "attribute" page-margin-bottom symbol?]
              @defthing[#:kind "attribute" page-margin-left symbol?]
              @defthing[#:kind "attribute" page-margin-right symbol?])]{
Inset values from the page edges. Value is a @tech{dimension string}. Default values depend on size of the page: they are chosen to be not completely bananas.
}

@defthing[#:kind "attribute" page-margin-gutter symbol?]{
Extra space added to the inner margin of page. Value is a @tech{dimension string}. On right-hand pages, the gutter will be added to the left margin. On left-hand pages, it will be added to the right margin.  Default is @racket[0].
}

@defthing[#:kind "attribute" page-number-start symbol?]{
First page number used. Value is an integer. Default is @racket[1].
}

@defthing[#:kind "attribute" page-side-start symbol?]{
Side that first page appears on. Value is @racket["left"], @racket["right"], or @racket["next"]. Default is @racket["right"]. If the value is @racket["left"] or @racket["right"], a blank page will be inserted if necessary.
}

@deftogether[(@defthing[#:kind "attribute" column-count symbol?]
              @defthing[#:kind "attribute" column-gap symbol?])]{
Columns per page. @racket[column-count] is a positive integer; @racket[column-gap] (the space between columns) is a @tech{dimension string}.
}

@defthing[#:kind "attribute" footer-display symbol?]{
Whether footer is displayed. Default is @racket["false"]. Footer is suppressed if this value is @racket["none"] or @racket["false"].
}

@defthing[#:kind "attribute" footer-text symbol?]{
Text displayed in footer. Default is @racket["false"], which will lead to default text being used for the footer (= a combination of page number, document name, and time / date).
}


@subsubsection{Block-level attributes}

A block is a paragraph or other rectangular item (say, a blockquote or code block) with paragraph breaks around it.

@margin-note{Block-level attributes are ignored unless the quad is a block-level element. To explicitly promote a quad to a block-level element, use the @racket[display] attribute with value @racket["block"].}

@deftogether[(@defthing[#:kind "attribute" inset-top symbol?]
              @defthing[#:kind "attribute" inset-bottom symbol?]
              @defthing[#:kind "attribute" inset-left symbol?]
              @defthing[#:kind "attribute" inset-right symbol?])]{
Inset values increase the layout boundary of the quad. Value is a @tech{dimension string}. @racket["0"] by default.
}

@deftogether[(@defthing[#:kind "attribute" border-inset-top symbol?]
              @defthing[#:kind "attribute" border-inset-bottom symbol?]
              @defthing[#:kind "attribute" border-inset-left symbol?]
              @defthing[#:kind "attribute" border-inset-right symbol?])]{
Border-inset values do not change the layout boundary of the quad. Rather, they change the position of the border (if any) relative to the layout boundary. Value is a @tech{dimension string}. @racket["0"] by default (meaning, the border sits on the layout boundary).
}

@deftogether[(@defthing[#:kind "attribute" border-width-top symbol?]
              @defthing[#:kind "attribute" border-width-bottom symbol?]
              @defthing[#:kind "attribute" border-width-left symbol?]
              @defthing[#:kind "attribute" border-width-right symbol?])]{
Width of the border on each edge of the quad. Value is a @tech{dimension string}. @racket["0"] by default (meaning no border).
}

@deftogether[(@defthing[#:kind "attribute" border-color-top symbol?]
              @defthing[#:kind "attribute" border-color-bottom symbol?]
              @defthing[#:kind "attribute" border-color-left symbol?]
              @defthing[#:kind "attribute" border-color-right symbol?])]{
Color of the border on each edge of the quad. Value is a @tech{hex color} string or @tech{named color} string.
}

@defthing[#:kind "attribute" background-color symbol?]{
Color of the background of the quad. Value is a @tech{hex color} string or @tech{named color} string.
}

@deftogether[(@defthing[#:kind "attribute" space-before symbol?]
              @defthing[#:kind "attribute" space-after symbol?])]{
Vertical space added around a block. Value is a @tech{dimension string}.
}

@deftogether[(@defthing[#:kind "attribute" keep-first-lines symbol?]
              @defthing[#:kind "attribute" keep-last-lines symbol?])]{
How many lines of the quad are kept together near a page break. @racket[keep-first-lines] sets the minimum number of lines that appear before a page break; @racket[keep-last-lines] sets the minimum number that appear after. In both cases, they take a non-negative integer string as a value, or @racket["all"]. 

If the value (of one or both attributes) is @racket["all"], then all the lines of the quad are kept on the same page. Be careful with this option — it's possible to make a single quad that's longer than one page, in which case @racketmodname[quadwriter] will ignore the setting to prevent the annihilation of the universe.
}

@defthing[#:kind "attribute" keep-with-next symbol?]{
Whether a quad appears on the same page with the following quad. Activated only when value is @racket["true"]. Essentially this is the ``nonbreaking paragraph space''.
}

@defthing[#:kind "attribute" line-align symbol?]{
How the lines are aligned horizontally in the quad. Possibilities are @racket["left"], @racket["center"], @racket["left"], @racket["justify"], @racket["inner"], and @racket["outer"].

@racket["inner"] and @racket["outer"] align the line toward (or away from) the gutter. So on right-hand pages, @racket["inner"] alignment is the same as @racket["left"], and @racket["outer"] is the same as @racket["right"]. On left-hand pages, vice versa. 

The last line of a paragraph with @racket["justify"] alignment will only be justified if the space left over is reasonably small. Otherwise it will be left-aligned. This is because the last line of the paragraph may only have a few words on it.
}

@defthing[#:kind "attribute" first-line-indent symbol?]{
The indent of the first line in the quad. Value is a @tech{dimension string}.
}


@defthing[#:kind "attribute" line-wrap symbol?]{
Selects the linebreak algorithm. A value of @racket["best"] or @racket["kp"] invokes the @link["http://defoe.sourceforge.net/folio/knuth-plass.html"]{Knuth–Plass linebreaking algorithm}, which finds the optimal set of linebreaks (defined as the set that gives the most even spacing throughout the paragraph). Otherwise, you get the ordinary linebreak algorithm, which just puts as many words as it can on each line. The Knuth–Plass algorithm is slower, of course.
}

@defthing[#:kind "attribute" hyphenate symbol?]{
Whether the block is hyphenated. Activated only when value is @racket["true"]. 
}

@defthing[#:kind "attribute" clip symbol?]{
Whether the contents of the block are clipped to its boundary. Activated only when value is @racket["true"]. 
}

@deftogether[(@defthing[#:kind "attribute" image-file symbol?]
              @defthing[#:kind "attribute" image-alt symbol?]
              @defthing[#:kind "attribute" image-height symbol?]
              @defthing[#:kind "attribute" image-width symbol?])]{
Specify a quad with an image (either @racket{.png} or @racket{.jpeg}). @racket[image-file] is a string containg the path to the image file. @racket[image-alt] is optional text. 

@racket[image-height] and @racket[image-width] are optional sizing values, each of which is a @tech{dimension string}. If neither @racket[image-height] nor @racket[image-width] are provided, the image is displayed at ``full size'' (meaning one pixel = one point, or 72 dpi). If both @racket[image-height] and @racket[image-width] are provided, the image is displayed at exactly that size. If only @racket[image-height] or @racket[image-width] is provided, the image is scaled by the proportion implied by the value. That is, if @racket[image-height] is @racket{50} and the image is 200 pixels high by 100 pixels wide, then the image will be displayed 50 pixels high by 25 pixels wide.
}

@subsubsection{Positioning attributes}


@defthing[#:kind "attribute" repeat symbol?]{
Moves the quad from its existing position and repeats it within the document according to the attribute value, which is a @tech{query string} like @racket["section[this]:page[*]"]. This attribute is resolved before @racket[parent], so a parent value like @racket["page[this]:line[first]"] will refer to the first line on the page that the quad has landed on as a result of the repeat operation.
}

@defthing[#:kind "attribute" parent symbol?]{
Moves the quad from its existing position in the layout into a new parent quad. Value is a @tech{query string} like @racket["page[this]:line[2]"].
}

@defthing[#:kind "attribute" anchor-from-parent symbol?]{
Forces the quad to attach to its parent using the designated anchor on the parent. Value is an @tech{anchor string}. See @secref["Layout_model"] for an explanation of the anchoring system.
}

@defthing[#:kind "attribute" anchor-to symbol?]{
Forces the current quad to attach to its antecedent using the designated anchor on the current quad. Value is an @tech{anchor string}. See @secref["Layout_model"] for an explanation of the anchoring system.
}

@subsubsection{Font attributes}


@defthing[#:kind "attribute" font-size symbol?]{
Sets the point size for text. Value is a @tech{dimension string}.
}

@defthing[#:kind "attribute" font-family symbol?]{
Name of the font family. Value is a string with the font-family name. See @secref["Fonts"] for where these names come from.
}

@defthing[#:kind "attribute" font-color symbol?]{
The color of the rendered font. Value is a @tech{hex color} string or @tech{named color} string.
}

@defthing[#:kind "attribute" font-bold symbol?]{
Whether the quad has bold styling applied. Activated only when value is @racket["true"]. 
}

@defthing[#:kind "attribute" font-italic symbol?]{
Whether the quad has italic styling applied. Activated only when value is @racket["true"]. 
}

@defthing[#:kind "attribute" font-underline symbol?]{
Whether the quad has an underline applied. Activated only when value is @racket["true"]. 
}

@defthing[#:kind "attribute" font-features symbol?]{
Sets OpenType layout features. @racket[font-features] takes a @deftech{feature string}, which is an alternating list of OT feature tags and values, separated by white space. For instance, @racket{liga 0 smcp 1} would deactivate the ligature feature and activate the small-cap feature. If the feature string is prefixed with @racket["+"], rather than replacing the current feature settings, it amends the features that would otherwise apply.

@margin-note{Fonts with OpenType layout features may be configured so that certain features, like ligatures, are activated by default. Your font will display these layout features even though there is no @racket[font-features] attribute in your Q-expression. You can, however, still turn them off with @racket[font-features].}
}
 

@defthing[#:kind "attribute" font-tracking symbol?]{
Space between characters. Value is a @tech{dimension string}.
}

@defthing[#:kind "attribute" font-baseline-shift symbol?]{
Vertical offset of font baseline (positive values move the baseline up, negative down). Value is a @tech{dimension string}.
}

@defthing[#:kind "attribute" font-case symbol?]{
Case transformation of string. Possibilities are @racket["uppercase"], @racket["lowercase"], or @racket["capitalize"] (= first letter of each word is uppercase, the rest is lowercase).
}

@subsubsection{Other attributes}

@defthing[#:kind "attribute" display symbol?]{
Sets the display type. Value is a string. Supply @racket["block"] as a value of this attribute to make the quad behave as a block-level element.
}


@defthing[#:kind "attribute" line-height symbol?]{
Sets the distance between baselines. Value is a @tech{dimension string}.
}

@defthing[#:kind "attribute" draw-debug symbol?]{
Controls whether debug boxes are drawn. Value can be @racket["false"] (default) or @racket["true"].
}

TK: OT feature attributes, bullet attributes

@subsection{Query strings}

Certain quad attributes (like @racket[repeat] and @racket[parent]) accept a @deftech{query string} as a value. A query string lets us refer to quads that will eventually exist in the layout (like lines, pages, and sections) while we're still in the markup.

A query string consists of query pieces chained together with colons @litchar{:} in between. Each query piece has a target and a subscript.

A @deftech{query target} can be @racket[doc], @racket[section], @racket[page], @racket[column], @racket[block], or @racket[line], referring to the corresponding layout entities.

A @deftech{query subscript} follows the query target in square brackets. Some subscripts refer to single quads; some refer to multiples. If any query piece contains a subscript that refers to multiple quads, then the result of the whole query will be a list of quads, or @racket[#false] if no matches were found. Similarly, if no subscript refers to multiple quads, then the result of the whole query will be a single quad, or @racket[#false] if no match is found.

Possible query subscripts are:

@itemlist[

@item{@racket[this]: refers to the current target, relative to the location of the quad in the layout. So @racket["page[this]"] means the current page, and @racket["section[this]"] means the current section.}

@item{@racket[prev] and @racket[next]: respectively refer to the previous or next target, relative to the location of the quad in the layout. So @racket["page[prev]"] means the page before the current page, and @racket["section[next]"] means the section after the current one.}

@item{a positive integer, which is interpreted as the nth item (counting from 1) within the previous quad in the query. So the second page of the document would be @racket["doc[this]:page[2]"].}

@item{a negative integer, which is interpreted as the nth item (counting from the end) within the previous quad in the query. So the next-to-last page of the current section would be @racket["section[this]:page[-2]"].}

@item{@racket[i..j], where @racket[i] and @racket[j] are positive or negative integers, with the meanings given above. The @racket[..] in between means the range of items from the ith to the jth, inclusive. So the second through fourth pages of the previous section would be @racket["section[prev]:page[2..4]"].}

@item{@racket[first] and @racket[last], which are the same as @racket[1] and @racket[-1], respectively.}

@item{@racket[all] or @racket[*], which refers to all matching items. So all the pages in the previous section would be @racket["section[prev]:pages[all]"] or @racket["section[prev]:pages[*]"]. All the lines on the third page of every section would be @racket["section[*]:page[3]:line[*]"].}

@item{@racket[rest], which refers to all items after the first. So all the pages in the document that do not fall on the first page of a section would be @racket["section[*]:page[rest]"].}

@item{@racket[odd] or @racket[even], which respectively refer to all the items with an odd or even numerical subscript. So if a page has five lines, @racket["page[this]:line[odd]"] would refer to the first, third, and fifth, and @racket["page[this]:line[even]"] to the second and fourth.}

]

TK: querying quads by name, and @racket[left] and @racket[right]


@subsection{Rendering}

@defproc[
(render-pdf
[qx qexpr?]
[pdf-path (or/c path? path-string? #false) #false]
[base-dir (or/c path? path-string?) (current-directory)]
[#:replace replace? any/c #true]
[#:compress compress? any/c #true])
(or/c void? bytes?)]{
Compute the layout for @racket[qx] and render it as a PDF to @racket[pdf-path]. If @racket[pdf-path] is @racket[#false], then the rendered PDF is returned as a @tech[#:doc '(lib "scribblings/guide/guide.scrbl")]{byte string}. Otherwise it is written to @racket[pdf-path]. The default is @racket[#false].

The optional @racket[base-dir] argument sets a base directory for resolution of any relative path names passed as attribute values. The default is @racket[(current-directory)]. 

The optional @racket[replace?] argument controls whether an existing file is automatically overwritten. The default is @racket[#true].

The optional @racket[compress?] argument controls whether data inside the resulting PDF is compressed. The default is @racket[#true].
}


@subsection{Fonts}

A design goal of Quadwriter is to treat document layout as the result of a program. Along those lines, fonts are handled differently than usual. When you use a word processor, you choose from whatever fonts might be installed on your system. 

Quadwriter, by contrast, prefers to rely on fonts that are @emph{in the same directory} as your other project source files. This is a feature: it means that everything necessary to render the document can travel together in the same directory. You can re-render it anywhere with identical results. You never have the problem — still with us after 35 years of desktop word processing — that ``oh, you need to install such-and-such font in your system before it will work.'' Bah!

Quadwriter supports the usual TrueType (@racket[.ttf]) and OpenType (@racket[.otf]) font files. It also supports WOFF files (@racket[.woff]). To add fonts to your Quadwriter experience:

@itemlist[#:style 'ordered

@item{Within your project directory, create a subdirectory called @racket["fonts"].}

@item{Within @racket["fonts"], create a subdirectory for each font family you want to use in your Quadwriter document. The names of these subdirectories will become the acceptable values for the @racket[font-family] attribute in your documents.}

@item{If there is only one font file in the family subdirectory, then it is used every time the font family is requested.}

@item{Alternatively, you can specify styled variants by creating within the family directory style subdirectories called @racket["regular"], @racket["bold"], @racket["italic"], and @racket["bold-italic"].}

@item{Oh, you dislike making subdirectories? Then you can also specify styled variants by putting a set of four font files at the top level of the family subdirectory whose filenames include @racket["bold"] and @racket["italic"] (denoting the bold italic style), @racket["bold"] (denoting bold), @racket["italic"] (denoting italic), and neither @racket["bold"] nor @racket["italic"] (denoting regular).}
]

Though this system may seem like a lot of housekeeping, it's nice for two reasons. First, we use the filesystem to map font names to font files, and avoid having another configuration file floating around our project. Second, we create a layer of abstraction between font names and files. This makes it easy to change the fonts in the document: you just put new fonts in the appropriate font-family directory, and you don't need to faff about with the source file itself.

TK: example of font setup

@subsubsection{Default font families}

Quadwriter typesets documents by looking up families with the names below. You can override the default selections by providing a family in the @racket[fonts] folder of your project that has the same name.

@defthing[#:kind "font directory" text path-string?]{
Used for all body text. Default is a serif font.
}

@defthing[#:kind "font directory" heading path-string?]{
Used for headings. Default is a sans serif font.
}

@defthing[#:kind "font directory" code path-string?]{
Used for code. Default is a monospaced font.
}

@defthing[#:kind "font directory" blockquote path-string?]{
Used for blockquote boxes. Default is a sans serif font.
}


@defthing[#:kind "font directory" default path-string?]{
Used for any miscellaneous elements. Default is same as @racket[text].
}


@defthing[#:kind "font directory" fallback-emoji path-string?]{
Fallback only. Used for emoji not present in the currently selected font.
}

@defthing[#:kind "font directory" fallback-math path-string?]{
Fallback only. Used for math symbols not present in the currently selected font.
}

@defthing[#:kind "font directory" fallback path-string?]{
Fallback only. Used for other glyphs not present in the currently selected font.
}

@subsubsection{System fonts}

Yes, if you're feeling lazy, you can use the name of a built-in system font family in any field that takes a family name, and Quad will comply. 

If you do this, bear in mind that your source file will no longer necessarily be portable between systems, because it depends on a certain font already being available on that system. (@italic{Portable} = one can run the source file on a different machine and get an equivalent result.) I include this option because I can imagine plenty of uses for Quad where ease outweighs portability. In which case, have at it. 

The concern about portability pertains only to the source file. Once you generate an output PDF from the source file, the PDF itself will always be portable. Regardless of whether you invoke a system font or project font in your source file, the font will be embedded within the PDF. The PDF will then display correctly on any platform, with the correct fonts.


@subsection{Colors}

A @deftech{hex color} is a case-insensitive string of six hex digits prefixed with @litchar{#}, such as @racket["#fe456a"] or @racket["#cc6633"]. The pairs of digits represent the red, green, and blue components of the color respectively, each pair taking on hex values between 0 (@racket["00"]) and 255 (@racket["ff"]), inclusive. As optional shorthand, a three-digit hex color such as @racket["#c63"] is equivalent to @racket["#cc6633"].

A @deftech{named color} is a hex color with a pre-existing name.

@(define (tuple->hexstring triple)
(apply string-append "#" (map (λ(x) (~r x #:base 16 #:min-width 2 #:pad-string "0")) triple)))

@(tabular 
#:sep @hspace[2] 
(cons (list @bold{name} @bold{hex color equivalent})
(map (λ (args) (list (tt (car args)) (tt (tuple->hexstring (cdr args))))) 
(sort (hash->list named-colors) string<? #:key car))))



@subsection{Utility}

@defproc[
(view-output)
void?]{
On the REPL, after running a @racketmodname[quadwriter] dialect and generating a PDF, this function will open the PDF.
}



@section{Quad: the details}

@defmodule[quad]

As mentioned above, The @racket[quad] library itself knows as little as it can about typography and fonts and pictures. Nor does it even assert a document model like Scribble. Rather, it offers a generic geometric represntation of layout elements. In turn, these elements can be combined into more useful pieces (e.g., @racketmodname[quadwriter]).

@subsection{Data model: the quad}

The eponymous @racket[quad] is a structure type that represents a rectangular layout area. This rectangle is used for layout purposes only. It is not enforced during the rendering phase. Meaning, once positioned, a quad's drawing function can access this rectangle, but does not need to stay within it.

Each quad has nested @deftech{elements}, which is a (possibly empty) list of subquads. Given a certain element, the quad containing it is called its @deftech{parent} quad. There are no restrictions on what kind of quad can be nested in another.


@subsection{Wrapping}

Wrapping is a optional phase where lists of quads are broken into sublists of a certain size. In @racketmodname[quadwriter], the list of words is wrapped to produce a list of lines of a certain horizontal width. In turn, the list of lines is wrapped to produce a list of pages of a certain vertical height.

@subsection{Layout model}

The heart of Quad's layout model is its system of @deftech{anchor points}. A quad is positioned in a layout by aligning its anchor point to an anchor point on the previous quad.

Each quad has a set of 11 anchor points on its perimeter. 

Eight points are named for the compass directions: @racket['n] (= top center) @racket['e] (= right center) @racket['s] (= bottom center) @racket['w] (= left center) @racket['ne] (= upper right) @racket['se] (= lower right) @racket['sw] (= lower left) @racket['nw] (= upper left). The center of the quad is @racket['c]. 
 
The other two anchor points are @racket['baseline-in] and @racket['baseline-out] (or just @racket['bi] and @racket['bo]). These points are also on the quad perimeter. They allow quads containing typeset text to be aligned according to adjacent baselines. The exact location of these points depends on the direction of the script and the @link["https://docs.microsoft.com/en-us/typography/opentype/spec/hhea"]{internal @racket[ascender] value} of the font. For instance, in left-to-right languages, @racket['baseline-in] is on the left edge, and @racket['baseline-out] is on the right. The vertical position of these points depends on the font associated with the quad. If no font is specified, the @racket['baseline-in] and @racket['baseline-out] anchors are vertically aligned with the southern edge. In that case, again supposing a left-to-right language, they would occupy the same positions as @racket['sw] and @racket['se].

(As an attribute value, these anchor points become @deftech{anchor strings}: @racket["n"] or @racket["e"] or @racket["bi"] and so on.)


By default, each subquad will ultimately be positioned relative to the immediately preceding subquad (or, if it's the first subquad, the parent). Optionally, a subquad can attach to the parent. 

How does a quad know which anchor points to use? Each quad specifies a @deftech{to anchor} on its own perimeter, and a @deftech{from anchor} on the previous quad's perimeter. The quad is positioned by moving it until its to anchor matches the position of the (already positioned) from anchor. Think of it like two tiny magnets clicking together.

A key benefit of the anchor-point system is that it gets rid of notions of ``horizontal'', ``vertical'', ``up'', ``down'', etc. Quads flow in whatever direction is implied by their anchor points.

@examples[#:label #f #:eval my-eval
(define parent (make-quad #:size '(25 25)))
(define child (make-quad #:size '(15 15)))

(quad->pict (position (attach-to parent 'e child 'w)))
(quad->pict (position (attach-to parent 'nw child 'se)))
(quad->pict (position (attach-to parent 'w child 'e)))
(quad->pict (position (attach-to parent 's child 'n)))
(quad->pict (position (attach-to parent 'e child 'n)))
]


``Wait a minute — why is the child quad specifying @emph{both} anchor points? Shouldn't the @racket[from] anchor be specified by the parent quad?'' It could, but it would make the layout system less flexible, because all the child quads hanging onto a certain parent quad would have to emanate from a single point. This way, every child quad can attach to its parent (or its neighbor) in whatever way it prefers.


@subsection[#:tag "quad-rendering"]{Rendering}

Once the quads have been positioned, they are passed to the renderer, which recursively visits each quad and calls its drawing function.

Though every quad has a size field, this is just the size used during layout and positioning. Quad doesn't know (or care) about whether the drawing stays within those bounds.


@section{What are your plans for Quad?}

Some things I personally plan to use Quad for:

@itemlist[#:style 'ordered

@item{@bold{A simple word processor}. Quadwriter is the demo of this.}

@item{@bold{Font sample documents}. In my work as a @link["https://mbtype.com"]{type designer}, I have to put together PDFs of fonts. To date, I have done them by hand, but I would like to just write programs to generate them.}

@item{@bold{Book publishing}. My wife is a lawyer and wants to publish a book about a certain area of the law that involves a zillion fiddly charts. If I had to do it by hand, it would take months. But with a Quad program, it could be easy.}

]

@subsection{Getting more help}

Questions about Quad and Quadwriter can be posted in the @link["https://github.com/mbutterick/pollen-users/"]{@tt{pollen-users} discussion area.} 


@subsection{Why is it called Quad?}

In letterpress printing, a @italic{quad} was a piece of metal used as spacing material within a line.



@(linebreak)
@(linebreak)
@(linebreak)


@italic{``A way of doing something original is by trying something
so painstaking that nobody else has ever bothered with it.'' — Brian Eno}