Pollen uses a special character — the @italic{lozenge}, which looks like this: ◊ — to mark commands within a Pollen source file. So when you put a ◊ in your source, whatever comes next will be treated as a command. If you don't, it will just be interpreted as plain text.
I chose the lozenge as the command marker because a) it appears in almost every font, b) it's barely used in ordinary typesetting, c) it's not used in any programming language that I know of, and d) its shape and color allow it to stand out easily in code without being distracting.
Still, if you don't want to use the lozenge as your command marker, you can use something else. Set Pollen's @racket[world:command-marker] value to whatever character you want.
@margin-note{Scribble uses the @"@" sign as a delimiter. It's not a bad choice if you only work with Racket files. But as you use Pollen to work on other kinds of text-based files that commonly contain @"@" signs — HTML pages especially — it gets cumbersome. So I changed it.}
@item{The @italic{Racket arguments} appear between square brackets. Pollen is partly an interface to the Racket programming language. These arguments are entered using Racket conventions —e.g., a @code{string of text} needs to be put in quotes as a @code{"string of text"}. If you like programming, you'll end up using these frequently. If you don't, you won't.}
@item{The @italic{text argument} appears between braces (aka curly brackets). You can put any ordinary text here. Unlike with the Racket arguments, you don't put quotes around the text.}
If you're familiar with Racket expressions, you can use the Racket-mode commands to embed them within Pollen source files. It's simple: any Racket expression can become a Pollen command by adding @litchar["◊"] to the front. So in Racket, this code:
Even if you don't plan to write a lot of Racket-mode commands, you should be aware that under the hood, Pollen is converting all commands in text mode to Racket mode. So a text-mode command that looks like this:
Thus a text-mode command is just an alternate way of writing a Racket-mode command. (More broadly, all of Pollen is just an alternate way of using Racket.)
The corollary is that you can always write Pollen commands using whichever mode is more convenient or readable. For instance, the earlier example, written in the Racket mode:
By default, Pollen treats every command name as a @italic{tag function}. The default tag function creates a tagged X-expression with the command name as the tag, and the text argument as the content.
To streamline markup, Pollen doesn't restrict you to a certain set of tags, nor does it make you define your tags ahead of time. Just type a tag, and you can start using it.
The one restriction is that you can't invent names for tags that are already being used for other commands. For instance, @code{map} is a name permanently reserved by the Racket function @racket[map]. It's also a rarely-used HTML tag. But gosh, you really want to use it. Problem is, if you invoke it directly, Pollen will think you mean the other @racket[map]:
Though every command name starts out as a default tag function, it doesn't necessarily end there. You have two options for invoking other functions: defining your own, or invoking others from Racket.
Use the @racket[define] command to create your own function for a command name. After that, when you use the command name, you'll get the new behavior. For instance, recall this example showing the default tag-function behavior:
The replacement function has to accept any arguments that might get passed along, but it doesn't have to do anything with them. For instance, this function definition won't work because @code{strong} is going to get a text argument that it's not defined to handle:
You can attach any behavior to a command name. As your project evolves, you can also update the behavior of a command name. In that way, Pollen commands become a set of hooks to which you can attach more elaborate processing.
You aren't limited to functions you define. Any function from Racket, or any Racket library, can be invoked directly by using it as a command name. Here's the function @racket[range], which creates a list of numbers:
Hold on —that's not what we want. Where's the list of numbers? The problem here is that we didn't explicitly import the @racketmodname[racket/list] library, which contains the definition for @racket[range]. (If you need to find out what library contains a certain function, the Racket documentation will tell you.) Without @racketmodname[racket/list], Pollen just thinks we're trying to use @code{range} as a tag function (and if we had been, then @repl-output{'(range 1 20)} would've been the right result).
Let's return to the problem that surfaced in the last section — the fact that some command names can't be used as tag functions because they're already being used for other things. You can work around this by defining your own tag function with a non-conflicting name.
For instance, suppose we want to use @code{map} as a tag even though Racket is using it for its own function called @racket[map]. First, we invent a command name that doesn't conflict. Let's call it @code{my-map}. As you learned above, Pollen will treat a new command name as a tag function by default:
But @code{my-map} is not the tag we want. We need to define @code{my-map} to be a tag function for @code{map}. We can do this with the Pollen helper @racket[make-default-tag-function]. That function lives in @racket[pollen/tag], so we @racket[require] that too:
A Pollen command name usually refers to a function, but it can also refer to a @italic{variable}, which is a data value. Once you define the variable, you can insert it into your source by using the ◊ notation without any other arguments:
Be careful —if you include arguments, even blank ones, Pollen will treat the command name as a function. This won't work, because a variable is not a function:
@margin-note{To understand what happens here, recall the relationship between Pollen's command modes. The text-mode command @code{◊foo[]} becomes the Racket-mode command @code{(foo)}, which after variable substitution becomes @code{("bar")}. If you try to evaluate @code{("bar")} — e.g., in DrRacket — you'll get the same error.}
The reason we can simply drop @code{◊foo} into the text argument of another Pollen command is that the variable @code{foo} holds a string (i.e., a text value).
In preprocessor source files, Pollen will convert a variable to a string in a sensible way. For instance, numbers are easily converted:
@margin-note{In an unsaved DrRacket file, or a file without a special Pollen source extension, the @tt{#lang pollen} designation invokes the Pollen preprocessor by default. You can explicitly invoke preprocessor mode by starting a file with @tt{#lang pollen/pre}. See also @secref["Preprocessor___pp_extension_"].}
If the variable holds a container datatype (like a @racket[list], @racket[hash], or @racket[vector]), Pollen will produce the Racket text representation of the item. Here, @code{zam} is a @racket[list] of integers:
This feature is included for your convenience. But in general, your readers won't want to see the Racket representation of a container. So in these cases, you should convert to a string manually in some sensible way. Here, the integers in the list are converted to strings, which are then combined using @racket[string-join] from the @racketmodname[racket/string] library:
Moreover, Pollen will not perform @italic{any} automatic text conversion in Pollen markup source files. Suppose we take the example above — which worked as a preprocessor source file —and change the language to @racket[pollen/markup]:
@codeblock|{
#lang pollen/markup
◊(define zam (list 1 2 3))
The value of zam is ◊zam
}|
This time, the file will produce an error:
@errorblock{
pollen markup error: in '(root "The value of zam is " (1 2 3)), '(1 2 3) is not a valid element (must be txexpr, string, symbol, XML char, or cdata)
One special case to know about. In the examples above, there's a word space between the variable and the other text. But suppose you need to insert a variable into text so that there's no space in between. The simple ◊notation won't work, because it won't be clear where the variable name ends and the text begins.
The example fails because Pollen reads the whole string after the @litchar{◊} as the single variable name @code{edgepx}. Since @code{edgepx} isn't defined, it's treated as a tag function, and since Pollen can't convert a function to a string, we get an error.
In these situations, surround the variable name with vertical bars @litchar{◊|}like so@litchar{|} to explicitly indicate where the variable name ends. The bars are not treated as part of the name, nor are they included in the result. Once we do that, we get what we intended:
@italic{Metas} are key–value pairs embedded in a source file that are not included in the main output when the source is run, and collected into a separate hash table.
Metas are not a foundational abstraction. They're just a convenience — a place to store arbitrary pieces of information that you might want to use later.
@margin-note{Pollen occasionally uses metas. For instance, the @racket[get-template-for] function will look in the metas of a source file to see if a template is explicitly specified. The @racket[pollen/template] module also contains functions for working with metas, such as @racket[select-from-metas].}
@margin-note{If your @code{meta} includes a text argument between curly braces —or any other arguments aside from the initial key–value pair — they will be ignored.}
Second, the meta is collected into a hash table that is exported with the name @code{metas}. To see this hash table, run the file above in DrRacket, then move to the interactions window and type @exec{metas} at the prompt:
The only key that's automatically defined in every meta table is @code{here-path}, which is the absolute path to the source file. (Here, because the file hasn't been saved, you'll see the @code{unsaved-editor...} name instead.)
It doesn't matter how many metas you put in a source file or where you put them. They'll all be extracted and put into the @code{metas} hash table. The order of the metas is not preserved (because order is not preserved in a hash table). But if you have two metas with the same key, the later one will supersede the earlier one:
The middle part of a text-mode Pollen command contains the @italic{Racket arguments} @litchar{[}between square brackets.@litchar{]} Most often, you'll see these used to pass extra information to commands that operate on text.
But that's a lot of parentheses to think about. So here's the easy way. Anytime you use a tag function, there's a shortcut for inserting attributes. You can enter them as a series of symbol–string pairs between the Racket-argument brackets. The only caveat is that the symbols have to begin with a quote mark @litchar{'} and end with a colon @litchar{:}. So taken together, they look like this:
Since Pollen commands are really just Racket arguments underneath, you can use those too. Here, we'll define a variable called @code{name} and use it in the Racket arguments of @code{title}:
You can also use this area for @italic{keyword arguments}. Keyword arguments can be used to provide options for a particular Pollen command, to avoid redundancy. Suppose that instead of using the @code{h1 ... h6} tags, you want to consolidate them into one command called @code{heading} and select the level separately. You can do this with a keyword, in this case @racket[#:level], which is passed as a Racket argument:
The third part of a text-mode Pollen command is the text argument. The text argument @litchar{@"{"}appears between curly braces@litchar{@"}"}. It can contain any text you want. The text argument can also contain other Pollen commands with their own text arguments. And they can contain other Pollen commands ... and so on, all the way down.
Three small details to know about the text argument.
First, the only character that needs special handling in a text argument is the lozenge @litchar{◊}. A lozenge ordinarily marks a new command. So if you want an actual lozenge to appear in the text, you have to escape it by typing @litchar{◊"◊"}.
Second, the whitespace-trimming policy. Here's the short version: if there's a carriage return at either end of the text argument, it is trimmed, and whitespace at the end of each line is selectively trimmed in an intelligent way. So this text argument, with carriage returns on the ends:
Third, within a multiline text argument, newline characters become individual strings that are not merged with adjacent text. So what you end up with is a list of strings, not a single string. That's why in the last example, we got this:
Under most circumstances, these two tagged X-expressions will behave the same way. The biggest exception is with functions. A function that operates on multiline text arguments needs to be able to handle an indefinite number of strings. For instance, this @code{jejune} function only accepts a single argument. It will work with a single-line text argument, because that produces a single string:
The answer is to use a @italic{rest argument} in the function, which takes the ``rest'' of the arguments —however many there may be —and combines them into a single @racket[list]. If we rewrite @code{jejune} with a rest argument, we can fix the problem:
The Pollen language is a variant of Racket's own text-processing language, called Scribble. Thus, most things that can be done with Scribble syntax can also be done with Pollen syntax. For the sake of clarity & brevity, I've only shown you the highlights here. But if you want the full story, see @secref["reader" #:doc '(lib "scribblings/scribble/scribble.scrbl")] in the Scribble documentation.
