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A poor guide to Pollen, that amazing document formatting system in Racket
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README.md

Caveat Please note that this tutorial will no longer work with modern Pollen 😭. I would love to update it but I don't have the time right now. If you have any tips on how to make this work with Pollen today, I would love to hear from you: please open an issue! End caveat

A poor guide to Pollen

The official Pollen documentation is a rich and hearty repast. That may be why it took me longer than expected to get where I wanted to go.

So, here’s a poor and meagre introduction that may help others get started as quickly as possible.

Goal

I’m trying to write Pollen markup that generates Dave Liepmann’s Tufte CSS-compliant HTML. Tufte CSS has some nice rules for sidenotes and headings that I’d like to write for, without writing HTML.

Preliminaries

Install Racket

On Mac OS X, verify the installation location by running $ /Applications/Racket\ v6.2.1/bin/racket -v, then add the following to your ~/.bash_profile:

export PATH="$PATH:/Applications/Racket v6.2.1/bin"

Then install Pollen by running $ raco pkg install pollen and answering “yes” a couple of times.

Now. After reading portions of three longform tutorials (fourth available), a mini-tutorial, and a quick tour, you will likely be very excited about many things. But for purposes of this poor guide, you can forget everything about DrRacket, .pp Pollen preprocessor files, .pmd & Pollen-Markdown mode, the Pollen project server (raco pollen start), etc.

We’ll be writing Pollen markup directly in .html.pm files, rendering them from the command line with raco pollen render, and viewing just the result in a browser. Clone this repository and follow along.

N.B. I’m assuming some passing familiarity with a Lisp, like Common Lisp or Emacs Lisp or Scheme or Clojure. The examples that follow have enough weird Pollen awesomeness that I can’t stop and explain parens or symbols or ,@ unquote-splicing. But I will try to point out Racketisms to save some Googling, and it’s always a good idea to Learn X=Racket in Y Minutes.

Take One. Basic and not-so-basic functionality.

In the take1/ directory there are

  • two input files: Pollen markup in take1.html.pm and an HTML template template.html,
  • one helper file: the tufte.css file from Liepmann, and
  • out output file: take1.html which is generated by running raco pollen render take1.html.

The reason you could forget about Markdown and the Pollen preprocessor is because these are small fry compared to real Pollen markup. Compare the Pollen markup input and its rendered output:

take1-screenshot

In the Pollen markup file take1/take1.html.pm, we have some Racket function definitions starting with ◊, then a bunch of content. Wait, maybe I overstated the difference between Racket code versus content, because what’s the difference between

  • ◊(define (emphatic . xs) `(em ,@xs)), and
  • ◊emphatic{italicize things}? Or, worse,
  • ◊h1{Take One. Or, Let’s Try To Get the Basics.}?

Pollen text mode versus Racket mode: Pollen’s two syntaxes for calling functions

Here’s the first thing you need to know. There’s a one-to-one equivalence between

  • ◊foo[arg1 arg2]{final-arg}, which is called Pollen text mode, and
  • ◊(foo arg1 arg2 final-arg) which is Racket mode.

Both call a Racket function foo. This isomorphism allows you to treat foo as a plain Racket function using the latter, and as convenient syntactic sugar to define arbitrary X-exprs with the former—X-exprs which may resolve to HTML tags. Because…

Pollen forgives undefined functions

The other thing you need to know is also awesome: if Pollen finds you using functions-tags that you haven’t defined, it’ll treat them as plain HTML tags. This is why take1.html.pm can call ◊h1{Take One …} without anybody defining h1 anywhere.

Put the two of these factoids together and you see why we can use ◊span as

  • a Pollen tag via ◊span['((class "hidden") (id "id1"))]{my span},
  • and as a Racket function via ◊(span '((class "hidden") (id "id2")) "my other span"), without explicitly defining span.

Of course you can see that, while they’re equivalent, in practice it’s easier to use one syntax in for some situations and the other for others, if only because you don’t need to quote text when you use ◊span{look ma, no quotes!}.

Explaining aforementioned usages

Now you can understand those three lines of code above.

◊(define (emphatic . xs) `(em ,@xs)) is written using Racket mode syntax, and defines a function called emphatic with all arguments rolled into a list called xs (just like def foo(*args) in Python or varargin in other languages), and which returns the input arguments list xs prepended with the symbol em using a bit of that quasiquote magic the Lisp family is famed for.

We invoke this function using ◊emphatic{italicized text}, and it renders as <em>italicized text</em>.

And as Pollen smartly passes through undefined functions as tags, ◊h1{Take One. Or, Let’s Try To Get the Basics.} produces the appropriate <h1> tag.

Test your understanding: given this linky function,

◊(define (linky url . xs) `(a ((href ,url)) ,@xs))

what would

◊linky["http://github.com"]{slinky dress}

render as? You may need to refer to the documentation on unquote again. Note how this sophomoric implementation prevents linky from being given additional attributes like id or class.

Templates: packaging it into a tidy HTML file

So in the take1/ directory, run

$ raco pollen render take1.html

Unlike other markup translators I’ve used, Pollen doesn’t write to stdout, and will overwrite take1.html. For now it can just be opened it in a browser (we’ll talk about webservers in a subsequent take).

You may have noticed, even in the screenshot above, that there is some boilerplate in the rendered HTML that’s not in the origial Pollen markup, like head and meta tags. This comes from a template file, in our case called take1/template.html.

This template is also a Pollen file, despite lacking any Pollen-esque file extension. Note a couple of ◊s in it: the first one is just a bit of showing off—the contents of the first <h1> tag is grabbed via a Pollen function select and used to set the page’s <title>: <title>◊select['h1 doc]</title>. (In Racket mode, this would be ◊(select 'h1 doc).) The X-expr doc containing the abstract syntax tree (AST) of take1.html.pm is available when the template is rendered, which select can scan, à la getElementsByTagName.

A bit later, inside a Tufte CSS-specific <article> tag, another Pollen function ->html converts the AST to HTML. ->html is clearly tightly intertwined with its target format. For contrast, here's an alternative AST-to-plaintext converter:

◊(apply string-append (filter string? (flatten doc))) ; instead of (->html doc). Generates plaintext.

After flattening the nested lists and throwing away tags and attributes, concatenate the strings that remain and you have a pretty good plaintext representation of take1.html.pm.

Summary of Take One

The functionality demonstrated so far is pretty elementary. The Pollen syntax can seem peculiar, and one couldn’t be blamed for being uneasy with the thought of constantly dropping into Racket. Maybe worst of all is we don’t have any easy way to italicize text like Markdown.

But I am already awestruck by the vistas of flexibility Pollen’s approach reveals. Having your document and your template be fully-programmable in the same language dramatically lowers the activation energy for complex document workflows.

The classic gripe of technical writers with Markdown—its lack of footnotes—can readily be fixed by a couple of quick Racket functions: you could write your paper in Markdown, use Pollen annotations just for the footnotes, and output another Markdown file with footnotes all fancy. But I’d argue that Markdown offers precious little besides single-character markup compared to Pollen markup, hence my advice to throw oneself whole-heartedly on Pollen markup.

In the next take, we’ll see how to make Tufte CSS sidenotes and get proper paragraph tags, two much more cognitively burdensome tasks.

Take Two. Things are harder than they appear.

Now consider the contents of the take2/ directory. Again we have a Pollen markup file, a template, the Tufte CSS .css file, and the rendered HTML. Re-render the last with

$ raco pollen render take2.html

Sidenote sausage

As mentioned above, Tufte CSS has nice sidenotes. But a sidenote isn’t a single tag. Here’s how the sidenote sausage is made:

Flowing text.
<label for="LABEL" class="margin-toggle sidenote-number"></label>
<input id="LABEL" class="margin-toggle" type="checkbox"></input>
<span class="sidenote">SIDENOTE CONTENT</span>
More flowing text.

We’d like to get this from just

Flowing text.◊sidenote["LABEL"]{SIDENOTE CONTENT} More flowing text.

The Pollen magic from the first take doesn’t help here. At best we could squeeze all three of these tags into a single <sidenote> tag, which browsers could probably handle just fine. But can we find something more elegant, to just place three adjacent tags in the flow of the parent tag?

Matthew Butterick’s source code to Making a dual typed / untyped Racket library shows us how to do this. It defines a splice Racket function and includes a test for it, and here’s my version of it:

(define (splice xs)
  (apply append (for/list ([x (in-list xs)])
                  (if (and (txexpr? x) (member (get-tag x) '(splice-me)))
                      (get-elements x)
                      (list x)))))

(splice '(p "foo" (splice-me "bar") "zam")) ; should be equal to '(p "foo" "bar" "zam")

All this is worth puzzling over for a bit. As I make it out, splice searches the contents of an X-expr (which is a list, and without recursing into any nested sublist child-tags), looking for a child tag called splice-me. When it finds one, it replaces the child tag with its contents. So an input representing <p>foo <splice-me>bar</splice-me> zam</p> becomes <p>foo bar zam</p>. That txexpr and get-tag stuff is for dealing with the specifics of X-exprs (which are lists, i.e., S-exprs, but with extra pixie dust on top).

So if instead of enclosing the label, input, and span tags inside a <sidenote> tag like I bemoaned doing a minute ago, enclose them in a <splice-me> tag, and ask Racket to run splice on every sub-X-expr in the document.

Pollen has a neat way of doing this. It’s related to something I forgot to point out about the rendered HTML in Take One, that Pollen markup is enclosed in a <root> tag by Pollen convention. So define a root Racket function:

(define (root . xs)
  (decode `(decoded-root ,@xs) #:txexpr-elements-proc splice))

decode looks like a pretty powerful function that can operate on X-exprs—any X-exprs, including subsets of the doc AST (and the #: syntax is Racket for keyword arguments). Here, decode is used to apply splice to every child node of <root>, that is, the entire document, and put the results in a new tag, <decoded-root>.

So now, instead of the contents of the Pollen markup going in a <root> tag, they’ll be in <decoded-root>. Any <splice-me> tag will have its contents spliced into its parent tag.

Sidenote sausage cooked.

You are appalled that one had to write so much Racket to do something that one would think was a pretty common task—replacing one tag with mutliple adjacent ones. Why does Pollen give us any simpler way to do this? But that’s the point of having a fully-programmable document. Unix doesn’t have a command line utility to do every task, but the Unix philosophy of small tools that do one thing well and can be chained means it doesn’t have to. C doesn’t give you hash tables or linked lists: it gives you the tools to write them yourself. XML doesn’t specify every conceivable semantic tag: it embraces extensibility. The discomfort felt at performing surgery on your document’s complete AST soon evaporates into relief—and perhaps joy—at being able to surgically modify your document’s AST.

There are no newlines under the sun

One very nagging thing about the Pollen renders so far is that newlines are passed through, and that you immediately noticed when you looked at take1.html in a browser—it looked weird because it lacked <p> paragraph tags, and it was basically a single run-on paragraph.

Dealing with <br> and <p> tags, line breaks and paragraph tags, is thankfully something that you don’t have to write yourself, as Pollen comes with some fancy functionality to handle this. detect-paragraphs is pretty feature-rich, but to get the job done for take2.html, its default functionality is entirely sufficient.

It is similar to splice, discussed a second ago, in that it scans an X-expr without recursing and converts newlines to <p> and <br> tags appropriately. So like splice, we can ask decode to run detect-paragraphs on each sub-X-expr while it’s running splice. Here’s the expanded call to root:

◊(define (root . xs)
  (decode `(decoded-root ,@xs)
          #:txexpr-elements-proc (compose1 detect-paragraphs splice)
          #:exclude-tags '(pre)
          ))

Note that the #:txexpr-elements-proc keyword argument to decode must be a single function, not a list, but we happen to be using a functional programming language here. (compose1 detect-paragraphs splice) returns a single function that will apply splice first, then detect-paragraphs.

And what about that #:exclude-tags keyword argument? We don’t want to convert newlines in <pre> tags, and Pollen has thoughtfully given this high-level mechanism to prevent those from being decoded.

It may be a good exercise to figure out how to run splice on pre tags but not detect-paragraphs: detect’s high-level customizations might not be sufficient for this and one may have to write a couple of lines of Racket.

Summary of Take Two

This take is titled "things that are harder than they appear", but having seen what it takes to splice a tag’s children into its parent, I completely appreciate how Racket’s power can make mincemeat out of complex tasks once one understands the underlying data structures—and how Pollen dramatically reduces the activation energy to whipping up some Racket code.

Then we saw that there are other things which Pollen has thought about and provides out-of-the-box solutions, like newline detection, smart punctuation (which I haven’t demonstrated here), etc. Yet even thees are steeped in the Racket ethos, and are hugely programmable.

In the next and last (planned) take, we’ll set up some infrastructure to make Pollen authoring even easier. We’ll set up a custom Nginx webserver that lets the HTML page auto-refresh whenever the Pollen markup is saved.

Take Three, where we get all steampunk

So far, my Pollen workflow has been edit–save–compile–refresh, switching between a text editor (gvim), command line, and browser. This is tiring and here’s how I chose to streamline this.

First, I install Node.js, a popular cross-platform JavaScript runtime with a very large ecosystem, and run a ~100-line JavaScript program which starts a webserver and watches a single Pollen markup file for changes. When I save that file, Node calls raco to re-render the HTML and sends a server-sent event to any browser viewing the rendered HTML, telling it to refresh the page.

The HTML page is aware of server-sent events because of some JavaScript we embed in it, and JavaScript is also how it refreshes itself when it gets the command to do so. This is very handy while authoring, but such infrastructure code should be removed before uploading to a public webserver for general viewing. So the last thing we’ll do is make our Pollen markup aware of our desire to make a testing versus production version of the output using an environment variable. Our template.html, which contains HTML boilerplate, will check for a POLLEN environment variable, and include refresh logic only when in testing mode, otherwise leaving it out. This is the only Pollen-specific part of this take and is a snap given how much we know about Pollen now.

Aside I personally use Nginx with this HTTP push stream module, because Nginx is far more performant than Node when it comes to serving big webpages loading images, JavaScript libraries, JSON datasets, MathJax, etc. There’s also a lot less custom code—none, really. But Nginx doesn’t work (well) on Windows, and one has to compile it from scratch to get the HTTP push stream module (which abstracts server-sent events, WebSockets, etc.). Rather than force readers of this poor guide to front this high NRE, I made this Node.js alternative. And I chose Node because of my own familiarity with it, and because it is cross-platform. It should possible to do all this in Racket.

Setup and use

Step 0 As a preliminary step, in the take3/ directory, render the Pollen markup file:

$ POLLEN=TESTING raco pollen render take3.html

You could even read the input or output files, if you like. I note in passing that I’ve moved all Racket code out of the Pollen markup file into a take3/pollen.rkt Racket file, which Pollen loads as a module.

Step 1 As a first step, download and install Node.js. Works on Mac, Linux, and Windows, ARM and x86, the lot.

Step 2 Second, in the take3/ directory in a terminal, run

$ npm install

npm is the package manager for Node. It looks at take3/package.json and installs a handful of dependencies in the take3/node_modules directory.

Step 2.5 If you are using Internet Explorer, Edge, Opera Mini, or any browser that doesn’t support EventServer (full list of unsupporting browsers at Can I Use), copy the node_modules/event-source-polyfill/eventsource.min.js shim to the take3/public folder:

cp node_modules/event-source-polyfill/eventsource.min.js public/

Step 3 Third, run

$ node server.js take3.html.pm "POLLEN=TESTING raco pollen render take3.html"

This starts the Node application, including a webserver and a file watch on the take3.html.pm Pollen markup file. That string "POLLEN=TESTING raco …" is what will be executed when changes to take3.html.pm are detected.

Step 4 Next, visit http://localhost:3000/take3.html to view the rendered HTML being served by the Express.js webserver in Node.

Step 5 For my final trick, I position my browser so that I can see it and my text editor at the same time. I edit the take3.html.pm Pollen markup file and save it. In a second or three, the browser refreshes in same place, showing my changes.

Hurrah!

N.B. Fastidious readers may notice two browser refreshes for a single save. The file watching API in Node (and anywhere else) is a little persnickety and may choose to detect two changes, one immediately after another, when the file changed only a single time.

Rendering for production environments

I don’t want to get into the details of how server-sent events and their client-side counterpart, the EventSource API, are used here. The details are all in take3/server.js and in take3/template.html. Hopefully you can change the arguments to node server.js to work for different projects, without coding.

But do note that this auto-refresh trick works because of custom JavaScript embedded in template.html. We don’t want this code to be present when we publish a Pollen document for a production environment. It likely won’t cause any harm but really should be omitted if it’s not needed.

For this reason, template.html checks to see if a POLLEN environment variable is available. One can set this by prefixing the variable to raco command: POLLEN=TESTING raco …. The template assumes that, if this environment varialbe is defined, Pollen is being run in a testing environment—it doesn’t care what the actual contents of the string are.

And if the template decides it is in a testing environment, it will include a couple of <script> tags in the HTML header which load the JavaScript that facilitates the auto-refresh feature. If no POLLEN environment variable is defined, these scripts are omitted.

This is classic Pollen:

◊(if (getenv "POLLEN") "
<script src='/public/eventsource.min.js'></script>
<!-- ... ... ... -->
" "")

We use Racket’s if, and choose to embed either a multi-line string containing HTML and JavaScript code, or an empty string. Happily, HTML and JavaScript can use single- and double-quotes just as easily, so I had to do no escaping of quotes in the if form! I can copy-and-paste HTML/JavaScript from non-Pollen sources without any problems. If I insisted on using double-quote, though, I would have had to escape them within the multiline quote.

When rendering for production, simply omit the POLLEN=TESTING environment variable and the resulting HTML file will not mention refresh events: simply run $ raco pollen render take3.html and upload the resulting file to your web host.

A technical note on server.js The Node server sends browsers a message not only when it detects a file change, but also if it receives a POST request on /events/ID for some string ID—in this case, the Node server forwards the POST message to the browser. Therefore, one could have the server send a request to refresh via the following command in a terminal:

$ curl -f -s -X POST http://localhost:3000/events/refreshme -d 'change'

Note how the ID of the event (or channel) is refreshme and the message is change, which is the same channel and message that the browsers are looking for. If instead of -d 'change' one had -d 'hi', one would see hi in the browser JavaScript console. This provides another useful way to communicate from your shell to the browser. A script that combines Pollen rendering and POSTing a refresh request to the server (to forward to browsers) is included in the take3/testing.sh script.

Summary of Take 3

In this last take of this poor guide to Pollen, we were able to use Racket’s multiline string support to effortlessly embed HTML and JavaScript in a document’s rendered output based on if/then logic.

But this take really focused on infrastructure to make the Pollen authoring process smoother. We cobbled together some JavaScript, script kiddie style, to watch a file for changes, rerun the Pollen renderer when changes were detected, and use server-sent events and the EventSource web technologies to communicate to the browser to refresh the page, getting the latest content.

Epilogue

The first take of this poor guide revisited the basic features of Pollen that one needs to know to start being productive in it. The second looked in-depth into one situation where one needed a bit of common Racket to achieve some pedestrian results. The third and final take set up some infrastructure to auto-refresh a document in a browser when it was saved.

By the end of this guide, I hope you are comfortable writing Pollen documents and confident in Pollen’s ability to transform its markup into any HTML you want. And by extension, any other format too.

Using Pollen markup has been much more liberating than using Markdown with custom Pandoc writers written in Lua because, although Pandoc does give custom writers an AST, it is not as flexible as having the full X-exprs. Although it looks a bit more “pointy” than Markdown, I think the benefits of Pollen markup far make up for it.

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