A pagetree is a hierarchical list of Pollen output files. A pagetree source file has the extension .ptree. A pagetree provides a convenient way of separating the structure of the pages from the page sources, and navigating around this structure.
Pagetrees are made of pagenodes. Usually these pagenodes will be names of output files in your project. (If you think it would’ve been more logical to just call them “pages,” perhaps. When I think of a web page, I think of a file on a disk. Whereas pagenodes may — and often do — refer to files that don’t yet exist.)
Books and other long documents are usually organized in a structured way — at minimum they have a sequence of pages, but more often they have sections with subsequences within. Individual Pollen source files don’t know anything about how they’re connected to other files. In theory, you could maintain this information within each source file. This would be a poor use of human energy. Let the pagetree figure it out.
Test whether possible-pagetree is a valid pagetree. It must be a txexpr? where all elements are pagenode?, and each is unique within possible-pagetree (not counting the root node).
Test whether possible-pagenode is a valid pagenode. A pagenode can be any symbol? that is not whitespace/nbsp? Every leaf of a pagetree is a pagenode. In practice, your pagenodes will likely be names of output files.
Pagenodes are symbols (rather than strings) so that pagetrees will be valid tagged X-expressions, which is a more convenient format for validation & processing.
A parameter that defines the default pagetree used by pagetree navigation functions (e.g., parent-pagenode, chidren, et al.) if another is not explicitly specified. Initialized to #f.
Return the pagenode immediately before p. For previous*, return all the pagenodes before p, in sequence. In both cases, return #f if there aren’t any pagenodes. The root pagenode is ignored.
Return the pagenode immediately after p. For next*, return all the pagenodes after p, in sequence. In both cases, return #f if there aren’t any pagenodes. The root pagenode is ignored.
Convert path p to a pagenode — meaning, make it relative to current-project-root, run it through ->output-path, and convert it to a symbol. Does not tell you whether the resultant pagenode actually exists in the current pagetree (for that, use in-pagetree?).
A pagetree is a hierarchical list of Pollen output files. A pagetree source file has the extension .ptree. A pagetree provides a convenient way of separating the structure of the pages from the page sources, and navigating around this structure.
Pagetrees are made of pagenodes. Usually these pagenodes will be names of output files in your project. (If you think it would’ve been more logical to just call them “pages,” perhaps. When I think of a web page, I think of a file on a disk. Whereas pagenodes may — and often do — refer to files that don’t yet exist.)
Books and other long documents are usually organized in a structured way — at minimum they have a sequence of pages, but more often they have sections with subsequences within. Individual Pollen source files don’t know anything about how they’re connected to other files. In theory, you could maintain this information within each source file. This would be a poor use of human energy. Let the pagetree figure it out.
Test whether possible-pagetree is a valid pagetree. It must be a txexpr? where all elements are pagenode?, and each is unique within possible-pagetree (not counting the root node).
Test whether possible-pagenode is a valid pagenode. A pagenode can be any symbol? that is not whitespace/nbsp? Every leaf of a pagetree is a pagenode. In practice, your pagenodes will likely be names of output files.
Pagenodes are symbols (rather than strings) so that pagetrees will be valid tagged X-expressions, which is a more convenient format for validation & processing.
A parameter that defines the default pagetree used by pagetree navigation functions (e.g., parent-pagenode, chidren, et al.) if another is not explicitly specified. Initialized to #f.
Return the pagenode immediately before p. For previous*, return all the pagenodes before p, in sequence. In both cases, return #f if there aren’t any pagenodes. The root pagenode is ignored.
Return the pagenode immediately after p. For next*, return all the pagenodes after p, in sequence. In both cases, return #f if there aren’t any pagenodes. The root pagenode is ignored.
Convert path p to a pagenode — meaning, make it relative to current-project-root, run it through ->output-path, and convert it to a symbol. Does not tell you whether the resultant pagenode actually exists in the current pagetree (for that, use in-pagetree?).
\ No newline at end of file
diff --git a/doc/second-tutorial.html b/doc/second-tutorial.html
index 9d06bed..6456639 100644
--- a/doc/second-tutorial.html
+++ b/doc/second-tutorial.html
@@ -3,8 +3,8 @@
(strong () "so") " happy to be writing this."))
You should now be able to recognize this as an X-expression. In authoring mode, Pollen parses your Markdown into the corresponding HTML entities, but then provides the data as an X-expression rather than finished HTML.
The empty parentheses () after p and strong signal that the tag’s attributes are empty. When you write an X-expression without attributes, these parentheses are optional — (tag () "text") and (tag "text") are equivalent — but Pollen will always print X-expressions this way.
From what you learned in the last section, it should be evident that this X-expression corresponds to HTML that looks like this:
<root><h1 id="my-article">Deep Thought</h1><p>I am <strong>so</strong> happy to be writing this.</p></root>
“But what’s this root tag? That’s not HTML.” An X-expression must have a root tag, so in the spirit of obviousness, every X-expression produced by a source file in authoring mode will start with root. If you don’t need it, you can discard it. But it also creates a useful hook for further processing, as we’ll see later.
By the way, as review, let’s remind ourselves how this is different from preprocessor mode. Let’s take the same Markdown content, but this time put it into a preprocessor source file called article.md.pp.
"article.md.pp"
#lang pollen
Deep Thought
============
I am **so** happy to be writing this.
When you run this file in DrRacket, you’ll see:
Deep Thought ============
I am **so** happy to be writing this.
Hopefully, this result makes sense to you: when you run Markdown source in preprocessor mode, you get Markdown. When you run Markdown source in authoring mode, you get an X-expression.
6.4Templates
So how do you convert an X-expression into a finished file? You use a Pollen template, which takes data from an X-expression and converts it to the target format.
If you’ve used other web-publishing systems, templates are probably a familiar idea. Templates in Pollen are in some ways similar to the ones you’ve seen before, but in other ways different.
First, the similarities. At its simplest, a template holds boilerplate material that you want to reuse across multiple pages. For instance, in a set of HTML pages, you might have layout and navigation elements that stay the same, while the content changes. In that case, you could put the layout and navigation in the template, and keep the content in your Pollen source files. When you want to add a new page, you can make a new source file and just use it with the existing template. Moreover, if you want to change the layout and navigation globally, you can just change the template, rather than changing the source files.
Pollen templates, like others, can also have conditional features — meaning, you can embed simple code in your templates that allows them to change based on the content in the page. For instance, a template could show or hide “previous page” and “next page” links depending on whether there’s actually a previous or next page.
The major difference with Pollen templates is that there’s no special “template language” you need to learn, with magic delimiters and whatnot. Instead, you can use all the same Pollen commands in a template that you can in authoring mode or preprocessor mode.
To see a template in action, let’s return to the source file we started in the last section:
"article.html.pmd"
#lang pollen
Deep Thought
============
I am **so** happy to be writing this.
Last time, I had you run this file in DrRacket to see the X-expression it produced. This time, load it in the project server. You’ll see something like this:
Deep Thought
I am so happy to be writing this.
Here, you’re seeing the X-expression from your source combined with an HTML template, which adds the necessary boilerplate for the finished HTML:
<html><head><meta charset="UTF-8" /></head><body>
<root><h1 id="my-article">Deep Thought</h1><p>I am
<strong>so</strong> happy to be writing this.</p></root>
- </body></html>
But wait — where did the template come from? When you view an authoring-mode source file in the project server without specifying a template, Pollen helps you out and uses its fallback template. The fallback template is just a minimal template that’s used as a last resort. Under ordinary circumstances, seeing the fallback template usually signals a problem (e.g., Pollen couldn’t find the template you asked for).
But we can learn a few things from the fallback template about how to make an HTML template.
But within a template, we need to explicitly convert from X-expression to HTML. So we wrap this X-expression with our second key ingredient, the Pollen command ->html:
By convention, every Pollen source file makes its output available through the variable doc. A source file in preprocessor mode puts its text result in doc. And a source file in authoring mode puts its X-expression result in doc.
Under the hood, a template is just a partial program that relies on a set of variables defined by another source file (fancy name: lexical context). So if you ran this template on its own, nothing would happen, because doc isn’t defined. But when you run it in the context of another source file, it picks up the doc that’s defined by that file.
Caution — despite the name, a Pollen template is not necessarily a file of the type suggested by its extension. For instance, fallback.html is a file that ultimately produces HTML, but it’s not actually written in HTML.
It could be, however. Here’s an equivalent way of writing fallback.html that inserts doc into actual HTML, rather than making the whole thing an X-expression.
<html><head><meta charset="UTF-8"></head>
<body>◊(->html doc)</body></html>
Notice that we still need to use the ->html function, but this time, instead of surrounding a larger X-expression, it just goes around doc.
Truly, there is no difference between these two methods. Use whichever works best for you. I often prefer the second method because I like to build & test HTML layouts by hand using placeholder content to make sure all the fiddly bits work. Then it’s easy to replace the placeholder content with (->htmldoc), and it becomes a template.
6.4.2Making a custom template
We’ll use these three ingredients to make our own template for article.html.pmd.
In general, template files can have any name you want. But by default, Pollen will first look for a file in your project directory called template.ext, where ext matches the output-file extension of the source file. So if your source file is database.xml.pmd, Pollen will look for template.xml. And for article.html.pmd, Pollen will look for template.html.
Therefore, to set up a custom template, all we need to do is create a file called template.html in our project directory, and make sure it has the three key ingredients we saw in the fallback template. Pollen will automatically apply it to article.html.pmd when we view it in the project server.
But don’t take my word for it. In your project directory, create a new file called template.html:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>Custom template</title></head>
<body>◊(->html doc)</body>
</html>
Recall from the last section that this is the same as the fallback template, but written out in HTML, and with a title element added. In fact, you can now refresh article.html in the project server. Does it look different? No — it won’t, because the resulting template is the same. You should notice, however, that the title of the browser window is now “Custom template,” because Pollen is relying on your new template file, rather than the fallback template.
Let’s change our custom template by adding a style block:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>Custom template</title>
<style type="text/css">
body {padding: 3em; font-size: 20px;}
h1 {background: gray; color: white;}
strong {color: red;}
</style></head>
<body>◊(->html doc)</body>
</html>
When you refresh article.html in the project server, you’ll see that the heading now has a gray background, and one word in the text is red.
Feel free to add other settings to template.html, or update the text in article.html, and see how the page changes. As you’d expect, the project server keeps an eye on both your source files and your template files, and if one changes, it will refresh the output file automatically.
6.4.3Inserting specific source data into templates
In the last example, we used doc to insert the entire content of the source file — as an X-expression — into the template.
But what if you want to only insert part of your source file into the template? For instance, you’ll look like a dork if the title on each page is “Custom template.” So let’s fix that.
When you’re working in a template, Pollen provides a select function that lets you extract the content of a specific tag, like so: ◊(select ’tag-name doc), which means “get the content of tag-name out of doc and put it here.”
Let’s suppose that we’d rather use the name of the article — Deep Thought — as the page title. We’re going to put a ◊(select ...) command inside the <title> tag.
Beyond that, we just need to know the tag name that contains the title. If we have a little Markdown expertise, we might already know that this part of our Markdown source:
Deep Thought
============
is going to produce a tag named h1.
What if we don’t have all the Markdown conversions memorized? No problem. We can still figure out the tag name by running the article.html.pmd source file in DrRacket and looking at the X-expression that results:
’(root (h1 ((id "my-article")) "Deep Thought") (p () "I am "
- (strong () "so") " happy to be writing this."))
Either way, now we know that the text Deep Thought lives in the h1 tag. So we update our template accordingly (for brevity, I’m going to omit the style tag in these examples, but it’s fine to leave it in):
"template.html"
<html>
<head><meta charset="UTF-8">
<title>◊(select 'h1 doc)</title></head>
<body>◊(->html doc)</body>
</html>
When you refresh the page in the project server, the page title will now appear as “Deep Thought.” Of course, you can also combine static and dynamic elements in your template, like so:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>◊(select 'h1 doc), by MB</title></head>
<body>◊(->html doc)</body>
</html>
The page title will now be “Deep Thought, by MB”.
A couple notes on command syntax. We inserted the select and ->html commands using Racket-mode syntax. We could also use text-mode syntax and write the commands this way:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>◊select['h1 doc], by MB</title></head>
<body>◊->html[doc]</body>
</html>
This is exactly equivalent to the previous example. Skeptics are welcome to confirm this by checking the result in the project server.
Finally, notice that in the select command, the tag name 'h1 is written with a quote mark, whereas doc is not. This is an easy place to get tripped up, but the rule is simple: you don’t use a quote mark when you’re referring to the name of an existing function or variable (like select or doc). But you do need a quote mark when you’re using the text as a literal value.
Racket (and hence Pollen) makes a distinction between Symbols (e.g. 'h1) and Strings (e.g. "h1"). Without getting into the weeds, just note for now that the tag of an X-expression is always a symbol, not a string. But if you write ◊(select "h1" doc), the command will still work, because Pollen will treat it as ◊(select 'h1 doc), consistent with a general policy of not being persnickety about input types when the intention is clear.
6.4.4Linking to an external CSS file
If you’re a super web hotshot, you probably don’t put your CSS selectors in the <head> tag. Instead, you link to an external CSS file. So it will not surprise you that in Pollen, you can do this by adding the usual <link> tag to your HTML template, in this case a file called styles.css:
Fans of hand-coded CSS, I trust you to take it from here: make your styles.css file, and enjoy the results.
But folks who paid attention during the First tutorial might be wondering “Can we link to a dynamically generated styles.css.pp file?”
Yes, of course. Here’s the rule of thumb: when you’re making links between files — whether CSS, or HTML, or anything else — Pollen doesn’t care whether the file is static or dynamic. You just refer to it by its ultimate name, in this case styles.css. If a static styles.css file exists, Pollen will use that. If it doesn’t, Pollen will look for a source file it can use to make styles.css, and generate it on the spot. (You can also start with a static file, and change it to be dynamic later, and Pollen will do the right thing.)
So to use a dynamic CSS file, we don’t need to make any changes to template.html. We just need to add styles.css.pp to the project directory:
"styles.css.pp"
#lang pollen
◊(define h1-color "blue")
◊(define strong-color "green")
body {padding: 3em; font-size: 20px;}
h1 {background: ◊|h1-color|; color: white;}
strong {color: ◊|strong-color|;}
Now, when you refresh article.html in the project server, Pollen will generate the styles.css file it needs, and you’ll see the new colors in the page. As before, if you update styles.css.pp, Pollen will notice and regenerate the CSS file when you refresh the page.
Can you add multiple dynamic style sheets? Yes.
+ </body></html>
But wait — where did the template come from? When you view an authoring-mode source file in the project server without specifying a template, Pollen helps you out and uses its fallback template. The fallback template is just a minimal template that’s used as a last resort. Under ordinary circumstances, seeing the fallback template usually signals a problem (e.g., Pollen couldn’t find the template you asked for).
But we can learn a few things from the fallback template about how to make an HTML template.
But within a template, we need to explicitly convert from X-expression to HTML. So we wrap this X-expression with our second key ingredient, the Pollen command ->html:
By convention, every Pollen source file makes its output available through the variable doc. A source file in preprocessor mode puts its text result in doc. And a source file in authoring mode puts its X-expression result in doc.
Under the hood, a template is just a partial program that relies on a set of variables defined by another source file (fancy name: lexical context). So if you ran this template on its own, nothing would happen, because doc isn’t defined. But when you run it in the context of another source file, it picks up the doc that’s defined by that file.
Caution — despite the name, a Pollen template is not necessarily a file of the type suggested by its extension. For instance, fallback.html is a file that ultimately produces HTML, but it’s not actually written in HTML.
It could be, however. Here’s an equivalent way of writing fallback.html that inserts doc into actual HTML, rather than making the whole thing an X-expression.
<html><head><meta charset="UTF-8"></head>
<body>◊(->html doc)</body></html>
Notice that we still need to use the ->html function, but this time, instead of surrounding a larger X-expression, it just goes around doc.
Truly, there is no difference between these two methods. Use whichever works best for you. I often prefer the second method because I like to build & test HTML layouts by hand using placeholder content to make sure all the fiddly bits work. Then it’s easy to replace the placeholder content with (->htmldoc), and it becomes a template.
6.4.2Making a custom template
We’ll use these three ingredients to make our own template for article.html.pmd.
In general, template files can have any name you want. But by default, Pollen will first look for a file in your project directory called template.ext, where ext matches the output-file extension of the source file. So if your source file is database.xml.pmd, Pollen will look for template.xml. And for article.html.pmd, Pollen will look for template.html.
Therefore, to set up a custom template, all we need to do is create a file called template.html in our project directory, and make sure it has the three key ingredients we saw in the fallback template. Pollen will automatically apply it to article.html.pmd when we view it in the project server.
But don’t take my word for it. In your project directory, create a new file called template.html:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>Custom template</title></head>
<body>◊(->html doc)</body>
</html>
Recall from the last section that this is the same as the fallback template, but written out in HTML, and with a title element added. In fact, you can now refresh article.html in the project server. Does it look different? No — it won’t, because the resulting template is the same. You should notice, however, that the title of the browser window is now “Custom template,” because Pollen is relying on your new template file, rather than the fallback template.
Let’s change our custom template by adding a style block:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>Custom template</title>
<style type="text/css">
body {padding: 3em; font-size: 20px;}
h1 {background: gray; color: white;}
strong {color: red;}
</style></head>
<body>◊(->html doc)</body>
</html>
When you refresh article.html in the project server, you’ll see that the heading now has a gray background, and one word in the text is red.
Feel free to add other settings to template.html, or update the text in article.html, and see how the page changes. As you’d expect, the project server keeps an eye on both your source files and your template files, and if one changes, it will refresh the output file automatically.
6.4.3Inserting specific source data into templates
In the last example, we used doc to insert the entire content of the source file — as an X-expression — into the template.
But what if you want to only insert part of your source file into the template? For instance, you’ll look like a dork if the title on each page is “Custom template.” So let’s fix that.
When you’re working in a template, Pollen provides a select function that lets you extract the content of a specific tag, like so: ◊(select ’tag-name doc), which means “get the content of tag-name out of doc and put it here.”
Let’s suppose that we’d rather use the name of the article — Deep Thought — as the page title. We’re going to put a ◊(select ...) command inside the <title> tag.
Beyond that, we just need to know the tag name that contains the title. If we have a little Markdown expertise, we might already know that this part of our Markdown source:
Deep Thought
============
is going to produce a tag named h1.
What if we don’t have all the Markdown conversions memorized? No problem. We can still figure out the tag name by running the article.html.pmd source file in DrRacket and looking at the X-expression that results:
’(root (h1 ((id "my-article")) "Deep Thought") (p () "I am "
+ (strong () "so") " happy to be writing this."))
Either way, now we know that the text Deep Thought lives in the h1 tag. So we update our template accordingly (for brevity, I’m going to omit the style tag in these examples, but it’s fine to leave it in):
"template.html"
<html>
<head><meta charset="UTF-8">
<title>◊(select 'h1 doc)</title></head>
<body>◊(->html doc)</body>
</html>
When you refresh the page in the project server, the page title will now appear as “Deep Thought.” Of course, you can also combine static and dynamic elements in your template, like so:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>◊(select 'h1 doc), by MB</title></head>
<body>◊(->html doc)</body>
</html>
The page title will now be “Deep Thought, by MB”.
A couple notes on command syntax. We inserted the select and ->html commands using Racket-mode syntax. We could also use text-mode syntax and write the commands this way:
"template.html"
<html>
<head><meta charset="UTF-8">
<title>◊select['h1 doc], by MB</title></head>
<body>◊->html[doc]</body>
</html>
This is exactly equivalent to the previous example. Skeptics are welcome to confirm this by checking the result in the project server.
Finally, notice that in the select command, the tag name 'h1 is written with a quote mark, whereas doc is not. This is an easy place to get tripped up, but the rule is simple: you don’t use a quote mark when you’re referring to the name of an existing function or variable (like select or doc). But you do need a quote mark when you’re using the text as a literal value.
Racket (and hence Pollen) makes a distinction between Symbols (e.g. 'h1) and Strings (e.g. "h1"). Without getting into the weeds, just note for now that the tag of an X-expression is always a symbol, not a string. But if you write ◊(select "h1" doc), the command will still work, because Pollen will treat it as ◊(select 'h1 doc), consistent with a general policy of not being persnickety about input types when the intention is clear.
6.4.4Linking to an external CSS file
If you’re a super web hotshot, you probably don’t put your CSS selectors in the <head> tag. Instead, you link to an external CSS file. So it will not surprise you that in Pollen, you can do this by adding the usual <link> tag to your HTML template, in this case a file called styles.css:
Fans of hand-coded CSS, I trust you to take it from here: make your styles.css file, and enjoy the results.
But folks who paid attention during the First tutorial might be wondering “Can we link to a dynamically generated styles.css.pp file?”
Yes, of course. Here’s the rule of thumb: when you’re making links between files — whether CSS, or HTML, or anything else — Pollen doesn’t care whether the file is static or dynamic. You just refer to it by its ultimate name, in this case styles.css. If a static styles.css file exists, Pollen will use that. If it doesn’t, Pollen will look for a source file it can use to make styles.css, and generate it on the spot. (You can also start with a static file, and change it to be dynamic later, and Pollen will do the right thing.)
So to use a dynamic CSS file, we don’t need to make any changes to template.html. We just need to add styles.css.pp to the project directory:
"styles.css.pp"
#lang pollen
◊(define h1-color "blue")
◊(define strong-color "green")
body {padding: 3em; font-size: 20px;}
h1 {background: ◊|h1-color|; color: white;}
strong {color: ◊|strong-color|;}
Now, when you refresh article.html in the project server, Pollen will generate the styles.css file it needs, and you’ll see the new colors in the page. As before, if you update styles.css.pp, Pollen will notice and regenerate the CSS file when you refresh the page.
Can you add multiple dynamic style sheets? Yes.
Can you mix dynamic and static style sheets? Yes.
Can you add a dynamic JavaScript file? Yes.
- You’re getting the general idea, right? So let’s move on.
6.5Intermission
If you only need one page for your article, you can stop here. You now know everything necessary to publish a single-page article using authoring mode. You know how to create the mandatory ingredients — a source file and a template — and you also know how to link to an optional CSS file, which can be dynamically generated.
If you want to create a multi-page article, however, you need to get through one more big idea. This might be a good time to take a break.
6.6Pagetrees
A pagetree is a hierarchical list of Pollen pages. When you have multiple pages in your project, the pagetree establishes relationships among those pages. At its most basic, a pagetree establishes a linear sequence for the pages. But pagetrees can also establish hierarchical relationships — for instance, a book-length project can be organized into chapters, the chapters into sections, and so on. The pagetree doesn’t impose any semantics on the organization of your project. It’s just a tree, and it’s up to you how many layers to establish, what those layers mean, and so on.
Pagemap might’ve been an equally good name, and perhaps more consistent with similar concepts in other web-publishing systems. But I avoided it out of deference to Racket’s map function, which means something completely different.
6.6.1Pagetree navigation
Pagetrees are used in various ways throughout Pollen. But the most obvious use for a pagetree is to add navigational links to your pages. Obviously, in a multi-page article, readers need a way of getting from one page to the next. In this part of the tutorial, we’ll expand our sample article from one page to three, and see how to create “previous page” and “next page” links in our template that are dynamically generated relative to the current page.
6.6.2Using the automatic pagetree
You’ve actually already been exposed to pagetrees (though I didn’t tell you about it at the time). Recall that the dashboard of the project server is located at http://localhost:8080/index.ptree. The list of files you see in the dashboard is a pagetree that Pollen creates by reading the files in the current directory and arranging them in alphabetical order.
If the multiple pages in your project are already ordered by filename, then you can rely on this automatic pagetree.
From earlier in the tutorial, you have a Markdown source file called article.html.pmd that looks like this:
"article.html.pmd"
#lang pollen
Deep Thought
============
I am **so** happy to be writing this.
Let’s supplement this source file by creating two others for the project:
"barticle.html.pmd"
#lang pollen
Barticle Title
==============
The wonderful second part of the article.
"carticle.html.pmd"
#lang pollen
Carticle Title
==============
The terrific third part.
As before, you can fill these source files with any sample Markdown content you like. Moreover, you don’t have to use the filenames barticle.html.pmd and carticle.html.pmd — the point is that the intended sequence needs to match the alphabetic sorting of the filenames.
We’ll reuse the template.html and styles.css files from earlier in the tutorial. Move or delete the other tutorial files so that your dashboard in the project server shows only these five files:
article.html.pmd
barticle.html.pmd
carticle.html.pmd
styles.css (or styles.css.pp)
template.html
If you click on any of the three Markdown sources, you will see it converted into HTML using template.html, with styles appiled from styles.css.
The automatic pagetree for this project is exactly what you see in the dashboard: a list of the three article files, followed by styles.css and template.html.
6.6.3Adding navigation links to the template with here
Recall from earlier in the tutorial that the content of your source file is made available in the template through the special variable doc. Likewise, the name of the current source file is made available through the special variable here.
To make any navigation link — up, down, sideways — the general idea is that we use here as input to a pagetree-navigation function, which then looks up the answer in the current pagetree.
First, let’s just see here on its own. Update your template as follows:
If you refresh article.html, you will now see the line “The current page is called article.html.” Switch to barticle.html, and you’ll see “The current page is called barticle.html.” Makes sense, right?
Now let’s use pagetree functions to show the names of the previous and next pages. Consistent with the usual policy of obviousness, these functions are called previous and next:
Refresh barticle.html. You’ll now see that “The current page is called barticle.html. The previous is article.html. The next is carticle.html.” So far, so good: we’re correctly deriving the previous and next pages from the automatic pagetree.
All that’s left is to add hyperlinks, which is easy:
The previous is <a href="◊|(previous here)|">◊|(previous here)|</a>.
The next is <a href="◊|(next here)|">◊|(next here)|</a>.
</body>
</html>
Refresh barticle.html, and you’ll see that the names of the previous and next pages are now hyperlinks to those pages. Click through and convince yourself that it works.
The documentation for pagetree Navigation will tell you about the other functions available for generating navigation links.
6.6.4Handling navigation boundaries with conditionals
If you clicked through to article.html or carticle.html, you might’ve noticed a couple problems. Because article.html is the first page in the automatic pagetree, it doesn’t have any previous page it can link to. And the next-page link for carticle.html is styles.css, which is strictly correct — it is, in fact, the next file in the automatic pagetree — but it’s not part of our article, so we’d rather stop the navigation there.
One way to fix the problem would be to have three separate template files — the standard one with both previous- and next-page links, one with only a next-page link, and one with only a previous-page link.
But since we have a whole programming language available in Pollen, that’s a dull-witted way to solve the problem. The better way is to add conditionals to the template to selectively change the navigation. That keeps things simple, because we’ll still have only one template.html to deal with.
To handle article.html, we want to hide the previous-page navigation link when there’s no previous page. As it turns out, if the previous function can’t find a previous page, it will return false. So we just need to wrap our previous-page navigation in the when/block command like so:
◊when/block[(previous here)]{The previous is <a href="◊|(previous here)|">◊|(previous here)|</a>.}
The next is <a href="◊|(next here)|">◊|(next here)|</a>.
</body>
</html>
The basic structure of when/block is ◊when/block[condition]{insert-this-text}. Note the square braces around the condition, and the curly braces around the text. Using (previous here) as the condition is shorthand for “when (previous here) does not return false...”
Programmers in the audience might be getting anxious about the repeated use of (previous here) — you’re welcome to store that value in a variable, and everything will work the same way:
◊when/block[prev-page]{The previous is <a href="◊|prev-page|">◊|prev-page|</a>.}
The next is <a href="◊|(next here)|">◊|(next here)|</a>.
</body>
</html>
We need a different technique for handling the end of the next-page navigation, because we’re not reaching the actual end of the pagetree. We’re just reaching the end of the pages we care about navigating through.
What condition will help us detect this? Here, we can notice that the names of our article pages all contain the string article. While you’d probably want a more robust condition for a real project, in this tutorial, what we’ll do is hide the next-page navigation if the name of the next page doesn’t contain “article”. As we did before, we wrap our navigation line in the when/block function:
The next is <a href="◊|(next here)|">◊|(next here)|</a>.}
</body>
</html>
This time, the condition is (regexp-match"article"(->string(nexthere))). How were you supposed to know this? You weren’t. That’s why this is a tutorial. Without going on a lengthy detour, the regexp-match function returns true if the first string (in this case, "article") is found inside the second string (in this case, we convert (nexthere) to a string by wrapping it in ->string).
In any case, even if some of the programmy bits went over your head just now, relax and paste the code into your template. What you’ll see when you refresh carticle.html is that the next-page link is gone. So now our template lets us navigate among the pages of our article, and the conditionals handle the end pages correctly.
6.6.5Making a pagetree file
I didn’t want to dwell on programming complications in the last conditional. Why? The extra programming was necessary only because we made life somewhat difficult for ourselves by relying on the automatic pagetree. A better way to solve the problem is to avoid it altogether by making a pagetree file.
Pagetree source files have a different syntax and status than other Pollen source files, so they are parsed using their own Pollen dialect. To invoke this dialect, you just start the file with #lang pollen and name the file with the ptree extension, for instance my-project.ptree. While you can have as many pagetrees in your project as you want, Pollen will accord primary status to the one named index.ptree.
So let’s make an index.ptree file. At its simplest, a pagetree file can just be a list of files in the intended order. In DrRacket, create a new file in your project directory as follows:
Pretty boring, I know. But behind the scenes, Pollen’s pagetree parser is making sure your tree is valid (e.g., no duplicate or malformed names). Today it’s boring, but on the day you have a long and complicated pagetree, you will be grateful.
Notice that the names in this pagetree are the names of output files, not source files. This is deliberate, so that neither you nor Pollen has to care which files are static vs. dynamic. This next pagetree wouldn’t be wrong in the sense of bad syntax — the pagetree parser won’t complain — but it would be wrong in the sense of not-what-you-want, because it refers to source names rather than output names:
"bad-index.ptree"
#lang pollen
carticle.html.pmd
article.html.pmd
barticle.html.pmd
You also probably noticed that the files are in a different order than they were in the automatic pagetree: carticle.html is first, followed by article.html and then barticle.html. This too is deliberate, so we can see what happens with a differently ordered pagetree.
Pagetrees don’t change nearly as often as other source files, so as a performance optimization, the project server does not dynamically reflect changes to pagetrees. To see the effect of this new pagetree on our project, you’ll need to go to your terminal window and stop the project server with ctrl+C, and then restart it. Which will take all of three seconds.
Now refresh carticle.html. You’ll notice that the navigation links are different. You won’t see a previous-page link — because carticle.html is now the first page in the pagetree — and the next page will show up as article.html. Click through to article.html, and you’ll see the navigation likewise updated. Click through to barticle.html, and you’ll see ...
BAM! An error page with a yellow box that says Can’t convert #f to string. What happened? We switched to using our own pagetree file but we didn’t update our template conditionals. Once you reach barticle.html, the value of (nexthere) is false, which means the (->string(nexthere)) command in the template conditional is trying to convert false into a string. Hence the error.
So let’s go back and fix that. Because we don’t have extraneous files in our pagetree anymore, we can change the second conditional in the template to work the same way as the first:
◊when/block[prev-page]{The previous is <a href="◊|prev-page|">◊|prev-page|</a>.}
◊(define next-page (next here))
◊when/block[next-page]{
The next is <a href="◊|next-page|">◊|next-page|</a>.}
</body>
</html>
Refresh barticle.html — because you’re updating the template, you don’t need to restart the project server — and you’ll see the right result. The previous-page link goes to article.html, and the next-page link is hidden.
6.6.6index.ptree & the project server
One more thing to show you before we wrap up this tutorial. Remember that the dashboard of the project server is at http://localhost:8080/index.ptree? By default, the project server will synthesize a pagetree from an alphbetical directory listing.
But if you put your own index.ptree file in the directory, the project server will use that for the dashboard instead. In fact, visit http://localhost:8080/index.ptree now and you’ll see what I mean. Consistent with the index.ptree you made, you’ll now see carticle.html, article.html, and barticle.html, but not template.html nor styles.css (even though they’re still in the project directory).
6.7Second tutorial complete
That was a big tutorial. I commend you for your tenacity and patience. But in this tutorial, you made a giant leap forward. Despite the silly examples, you now know everything you need to make multi-page articles — books, even — using Markdown authoring mode in Pollen. If this is all you ever use Pollen for, it’ll be a big improvement over ordinary Markdown.
But there’s more. We haven’t even gotten into the more elaborate automation that’s possible with Pollen, nor Pollen’s own markup language. We’ll cover that in the third tutorial.