@ -465,64 +465,34 @@ But more often, you're going to want to use functions defined elsewhere, and sto
Any function in Racket's extensive libraries can be called by loading the library with the @racket[require] command, which will make all its functions and constants available with the usual Pollen command syntax:
@fileblock["article.html.pm" @codeblock{
@fileblock["article.html.pp" @codeblock{
#lang pollen
◊(require racket/math)
Pi is close to ◊|pi|.
The hyperbolic sine of pi is close to ◊(sinh pi).
◊(require racket/math)
Pi is close to ◊(number->string pi).
The hyperbolic sine of pi is close to ◊(number->string (sinh pi)).
}]
The result:
@repl-output{'(root "Pi is close to " 3.141592653589793 "." "\n" "The hyperbolic sine of pi is close to " 11.548739357257748 ".")}
One caveat — you're still in a Pollen markup file, so the return value of whatever function you call has to produce a string or an X-expression, so it can be merged into the document. @margin-note*{This is similar to the restriction introduced in the @seclink["Setting_up_a_preprocessor_source_file"]{first tutorial} where functions used in preprocessor files had to produce text.}
Pollen won't stop you from calling a function that returns an incompatible value, like @racket[plot], which returns a bitmap image:
@fileblock["article.html.pm" @codeblock{
#lang pollen
◊(require math plot)
Here's a sine wave:
◊(plot (function sin (- pi) pi #:label "y = sin(x)"))
}]
But it won't work when you try to run it in DrRacket or load it in the project server.
@repl-output{
'(root "Pi is close to " "3.141592653589793" "." "\n" "The hyperbolic sine of pi is close to " "11.548739357257748" ".")}
It would be fine, however, to call a different kind of @racket[plot] function that returned an SVG result, because any XML-ish data structure can be converted to an X-expression.
One caveat — you're still in a Pollen markup file, so the return value of whatever function you call has to produce a string or an X-expression, so it can be merged into the document. That's why we have @racket[number->string] wrapping the numerical values. @margin-note*{This is similar to the restriction introduced in the @seclink["Setting_up_a_preprocessor_source_file"]{first tutorial} where functions used in preprocessor files had to produce text.}
@margin-note{Super web nerds also know that binary data can be converted into XML-ish form by encoding the file as a base64 data URL — but if you know what I'm talking about, then you don't need my help to try it.}
For functions that don't return a string or an X-expression, you can always make a conversion by hand. For instance, consider @racket[range], a Racket function that returns a list of integers:
If your functions produce incompatible results, you'll get an error. For instance, look what happens when we remove @racket[number->string] from the example above.
@fileblock["article.html.pm" @codeblock{
#lang pollen
◊(require racket/list)
A list of integers: ◊(range 5)
◊(require racket/math)
Pi is close to ◊|pi|.
The hyperbolic sine of pi is close to ◊(sinh pi).
}]
This will produce an error in DrRacket:
@errorblock{
pollen markup error: in '(root "A list of integers: " (0 1 2 3 4)), '(0 1 2 3 4) is not a valid element (must be txexpr, string, symbol, XML char, or cdata)
}
In a case like this, you can explicitly convert the return value to a string (in whatever way makes sense):
@fileblock["article.html.pm" @codeblock{
#lang pollen
◊(require racket/list racket/string)
A list of integers: ◊(string-join (map number->string (range 5)))
}]
And get this output:
pollen markup error: in '(root "Pi is close to " 3.141592653589793 "." "\n" "The hyperbolic sine of pi is close to " 11.548739357257748 "."), 3.141592653589793 is not a valid element (must be txexpr, string, symbol, XML char, or cdata)}
@repl-output{'(root "A list of integers: " "0 1 2 3 4")}
@subsection[#:tag-prefix "tutorial-3"]{Using the @filepath{pollen.rkt} file}
