2 Quick tour

2.1 Creating a source file

Assuming you’ve installed Racket & Pollen, launch DrRacket.

Open a new document. Change the top line to:

#lang pollen

The first line of every Pollen source file will start with #lang pollen.

2.2 Running a source file

Add a second line to your source file so it reads:

#lang pollen

Hello world

Click the Run button. In the interactions window, you’ll see the result:

Hello world

Not bad. I think Pollen just won the Hello World Tournament.

You can work with Pollen source files in any text editor. The key advantage of DrRacket is that you can preview the results by running the file.

Try editing your source file:

#lang pollen

Goodbye Stranger

Breakfast in America

Take the Long Way Home

You don’t have to use Supertramp song titles. Any text will do. When you click Run again, you’ll see whatever you typed:

Goodbye Stranger
Breakfast in America
Take the Long Way Home

We won’t do it a third time. You get the point — any plain text is valid within a Pollen source file, and gets printed as is. You never have to perform the incantations of typical programming languages:

In Pollen, what you write is what you get.

2.3 Naming, saving, and rendering a source file

Save this file with the name hello.txt.pp in any convenient directory. The desktop is fine.

Open a terminal window and issue two commands:

After a moment, a new file will appear called hello.txt. Let’s see what’s in it:

You’ve just learned three things:

• Pollen commands in the terminal begin with raco pollen, followed by a specific command (in this case render) and sometimes an argument (in this case hello.txt.pp).

• The render command takes the ouput from your source file — meaning, the result you previewed in DrRacket in the previous step — and saves it to another file.

• The name of the output file is the same as the source file, minus the Pollen source extension. So hello.txt.pp becomes hello.txt.

Try editing the text in the hello.txt.pp source file and running raco pollen render hello.txt.pp again. The old hello.txt will be replaced with a new one showing your changes. And so you’ve learned a fourth thing:

• Pollen works by rendering output files from source files. Output files can be overwritten. Therefore, you should only make edits to your source files.

2.4 The project server

You’ve just learned two ways to see the output of a Pollen source file — first, you ran it in DrRacket. Then, you rendered it to an output file.

Now here’s a third: the Pollen project server. Here’s how you start it. Return to your terminal window and issue two commands:

After a moment, you’ll see the startup message:

Open a web browser and point it at http://localhost:8080/index.ptree. The top of the window will say Project root. Below that will be a listing of the files in the directory.

Among them will be hello.txt, with a greyed-out .pp extension. Click on it, and you’ll be taken to http://localhost:8080/hello.txt, where you’ll see:

That’s the boring part. Here’s the good part. Leave the project server running. Open your source file again in DrRacket and edit it as follows:

Go back to your web browser and reload http://localhost:8080/hello.txt. Now you’ll see this:

Notice what happened — the Pollen project server dynamically regenerated the output file (hello.txt) from the source file (hello.txt.pp) after you edited the source. If you like, try making some more changes to hello.txt.pp, and reloading the browser to see the updates in hello.txt.

2.5 Intermission

That covers input & output. Now let’s circle back and look at what else you can do with Pollen (beyond the epic achievement of displaying plain text in a web browser).

For the rest of this tutorial, I recommend keeping two windows on screen: a web-browser window pointed at your project server (the main URL is http://localhost:8080/index.ptree) and the DrRacket editing window.

2.6 Pollen as a preprocessor

A preprocessor is a tool for making systematic, automated changes to a source file before the main processing happens. A preprocessor can also be used to add programming logic to files that otherwise don’t support it.

For instance, HTML. In DrRacket, create a new file called margin.html.pp in your project directory:

The “.pp” file extension — which you saw before, with hello.txt.pp — stands for “Pollen preprocessor.” You can use the Pollen preprocessor with any text-based file by inserting #lang pollen as the first line, and adding the .pp file extension.

But for now, go to your project dashboard and click on margin.html. You should see a black box containing the text “5em is the inset.”

Let’s suppose you want to change the inset to 30%. Without a preprocessor, you’d have to search & replace each value. But with a preprocessor, you can move the inset value into a variable, and update it from that one location. So first, introduce a variable called my-inset by using the define command:

The ◊ character is called a lozenge. In Pollen, the lozenge is a special character that marks anything Pollen should interpret as a command (rather than plain text).

How to type a lozenge:
Mac: option + shift + V
Windows: holding down alt, type 9674 on the num pad
Ubuntu: ctrl + shift + U, then 25CA

The whole command ◊define[my-inset]{30%} means “create a variable called my-inset and give it the value 30%.”

Then put the variable into the HTML like so, this time using the ◊ character with the variable name in the two places the value appears:

Now reload margin.html. You’ll see that the size of the margin has changed (because of the change to the style attribute) and so has the text of the HTML. If you like, try editing my-inset with different values and reloading the page. You can also try using define to create another variable (for instance, to change the color of the box border).

Still, this is the tiniest tip of the iceberg. The Pollen preprocessor gives you access to everything in the Racket programming language — including math functions, text manipulation, and so on.

2.7 Markdown mode

When used as a preprocessor, Pollen’s rule is that what you write is what you get. But if you’re targeting HTML, who wants to type out all those <tedious>tags</tedious>? You can make Pollen do the heavy lifting by using it as a source decoder.

For instance, Markdown mode. Markdown is a simplified notation system for HTML. You can use Pollen’s Markdown decoder by inserting #lang pollen as the first line, and adding the .pmd file extension.

Try it. In DrRacket, create a file with the following lines and save it as downtown.html.pmd:

As before, go to the dashboard for the project server. This time, click the link for downtown.html. You’ll see something like this:

As usual, you’re welcome to edit downtown.html.pmd and then refresh the web browser to see the changes.

In Markdown mode, you can still embed Pollen commands within the source as you did in preprocessor mode. Just keep in mind that your commands need to produce valid Markdown (as opposed to raw HTML). For instance, use define to create a variable called metal, and insert it into the Markdown:

Refresh downtown.html in the browser:

Pollen is handling two tasks here: interpreting the commands in the source, and then converting the Markdown to HTML. But what if you wanted to use Pollen as a preprocessor that outputs a Markdown file? No problem — just change the source name from downtown.html.pmd to downtown.md.pp. Changing the extension from .pmd to .pp switches Pollen from Markdown mode back to preprocessor mode. And changing the base name from downtown.html to downtown.md updates the name of the output file.

2.8 Markup mode

If all you need to do is produce basic HTML, Markdown is great. But if you need to do semantic markup or other kinds of custom markup, it’s not flexible enough.

In that case, you can use Pollen markup mode. To use Pollen markup, insert #lang pollen as the first line of your source file, and add a .pm file extension.

Compared to Markdown mode, Pollen markup mode is wide open. Markdown mode gives you a limited set of formatting tools (i.e., the ones supported by Markdown). But in markup mode, you can use any tags you want. Markdown mode decodes the source in a fixed way (i.e., with the Markdown decoder). But markup mode lets you build any decoder you want.

Let’s convert our Markdown example into Pollen markup. Marking up content is simple: insert the lozenge character (◊) followed by the name of the tag (◊tag), followed by the content of the tag in curly braces (◊tag{content}). In DrRacket, create a new file called uptown.html.pm as follows:

Go to the project dashboard and click on uptown.html. You’ll see something like this:

That’s not right. What happened?

We marked up the source using a combination of standard HTML tags (strong, em) and nonstandard ones (headline, items, item, link). This is valid Pollen markup. (In fact, if you look at the generated source, you’ll see that they didn’t disappear.) But since we’re targeting HTML, we need to convert our custom tags into valid HTML tags.

For that, we’ll make a special file called directory-require.rkt. This is a file in the standard Racket language that provides helper functions to decode the source. The definitions won’t make sense yet. But this is the quick tour, so all you need to do is copy, paste, and save:

Return to the project dashboard and click on uptown.html. Now you’ll get the right result:

Markup mode takes a little more effort to set up. But it also allows you more flexibility. If you want to do semantic markup, or convert your source into multiple output formats, or handle complex page layouts — it’s the way to go.

2.9 Templates

The HTML pages we just made looked pretty dull. For the last stop on the quick tour, let’s fix that.

Pollen source files that are written in Markdown or markup mode (i.e., .pmd or .pm files) are rendered with a template. A template is not a standalone Pollen source file. It’s a file of the output type — e.g., CSS, HTML, XML — where you put the stuff that needs to be consistent between output files. The template also contains template variables that mark where values from the Pollen source file should be inserted.

When it needs a template, Pollen first looks for a file in the project directory named template.[output extension of source]. For uptown.html.pm, the output extension will be .html, thus Pollen will look for template.html.

So let’s create template.html. Make a new file that with the following lines and save it to the same directory as uptown.html.pm:

This is a simple HTML file that should look familiar, except for the two template variables. The first, here, contains the name of the current source file. As before, the lozenge character marks it as a Pollen command rather than text, so you write it as ◊here. The other command, ◊->html{◊doc}, takes the content from the source file, which is contained in a variable called doc, and converts it to HTML with a Pollen function called ->html.

Go back to your web browser and reload uptown.html. (Or downtown.html — both will work.) The page will be rendered with the new template.html. As before, you can edit the template or the source and the project server will dynamically update the output file.

2.10 PS for Scribble users

Pollen can also be used as a dynamic preview server for Scribble files. From your terminal, do the following:

On the project dashboard, you’ll see your filename.scrbl files listed as filename.html. This may not represent the ultimate structure of your Scribble project — you may end up combining multiple Scribble source files into one HTML file, or making multiple HTML files from one Scribble source — but it’s handy for checking your work as you go.

2.11 The end of the beginning

Now you’ve seen the key features of Pollen. What do you think?

• “So it’s like WordPress, but harder to use?” I was a happy WordPress user for several years. If you need a blog, it’s great. But the farther you get from blogs, the more it becomes like teaching an elephant to pirouette. And for those who like to solve problems with programming, PHP is, um, limited.

• “What about pairing a Python template system and Python web server?” Good idea. I even tried it. But Python template systems don’t offer you Python — they offer you limited dialects that aren’t very Pythonic. Also, Python’s handing of XML-ish data is cumbersome.

• “Haven’t you heard of Jekyll?” Yes. If everything you need to write is expressible in Markdown, it’s great. If you need more than that, you’re stuck.

• “Sounds a lot like LaTeX. Why not use that?” Also a good idea. LaTeX gets a lot of things right. But it wasn’t designed for web publishing.

• “Eh, there are plenty of adequate options. Why should I learn a system written in Racket, which I’ve never used?” A salient objection. It’s also the question I asked myself before I committed to Racket. But publishing systems that are author- or designer-friendly tend to be programmer-hostile, and vice versa. Racket is the only language I found that could meet my requirements.

But don’t take my word for it. The rest of this documentation will show you the useful and sophisticated things you can do with Pollen. If there’s another tool that suits you better, great. Keep in mind that I didn’t make Pollen because I’m a programmer. I’m a writer who wants to make electronic books that are better than the ones we have now. And for that, I needed a better tool.