3 mztext

mztext is another Racket-based preprocessing language. It can be used as a preprocessor in a similar way to mzpp since it also uses preprocessor/pp-run functionality. However, mztext uses a completely different processing principle, it is similar to TeX rather than the simple interleaving of text and Racket code done by mzpp.

Text is being input from file(s), and by default copied to the standard output. However, there are some magic sequences that trigger handlers that can take over this process—these handlers gain complete control over what is being read and what is printed, and at some point they hand control back to the main loop. On a high-level point of view, this is similar to “programming” in TeX, where macros accept as input the current input stream. The basic mechanism that makes this programming is a composite input port which is a prependable input port—so handlers are not limited to processing input and printing output, they can append their output back on the current input which will be reprocessed.

The bottom line of all this is that mztext is can perform more powerful preprocessing than the mzpp, since you can define your own language as the file is processed.

3.1 Invoking mztext

Use the -h flag to get the available flags. SEE above for an explanation of the --run flag.

3.2 mztext processing: the standard command dispatcher

mztext can use arbitrary magic sequences, but for convenience, there is a default built-in dispatcher that connects Racket code with the preprocessed text—by default, it is triggered by @. When file processing encounters this marker, control is transferred to the command dispatcher. In its turn, the command dispatcher reads a Racket expression (using read), evaluates it, and decides what to do next. In case of a simple Racket value, it is converted to a string and pushed back on the preprocessed input. For example, the following text:

generates this output:

An explanation of a few lines:

• @"bar", @(+ 1 2)—the Racket objects that is read is evaluated and displayed back on the input port which is then printed.

• @"@(* 3 4)" — demonstrates that the results are “printed” back on the input: the string that in this case contains another use of @ which will then get read back in, evaluated, and displayed.

• @(/ (read) 3)12 — demonstrates that the Racket code can do anything with the current input.

The complete behavior of the command dispatcher follows:

• If the marker sequence is followed by itself, then it is simply displayed, using the default, @@ outputs a @.

• Otherwise a Racket expression is read and evaluated, and the result is processed as follows:

  • If the result consists of multiple values, each one is processed,

  • If it is #<void> or #f, nothing is done,

  • If it is a structure of pairs, this structure is processed recursively,

  • If it is a promise, it is forced and its value is used instead,

  • Strings, bytes, and paths are pushed back on the input stream,

  • Symbols, numbers, and characters are converted to strings and pushed back on the input,

  • An input port will be perpended to the input, both processed as a single input,

  • Procedures of one or zero arity are treated in a special way—see below, other procedures cause an error

  • All other values are ignored.

• When this processing is done, and printable results have been re-added to the input port, control is returned to the main processing loop.

A built-in convenient behavior is that if the evaluation of the Racket expression returned a #<void> or #f value (or multiple values that are all #<void> or #f), then the next newline is swallowed using swallow-newline (see below) if there is just white spaces before it.

During evaluation, printed output is displayed as is, without re-processing. It is not hard to do that, but it is a little expensive, so the choice is to ignore it. (A nice thing to do is to redesign this so each evaluation is taken as a real filter, which is done in its own thread, so when a Racket expression is about to evaluated, it is done in a new thread, and the current input is wired to that thread’s output. However, this is much too heavy for a "simple" preprocesser...)

So far, we get a language that is roughly the same as we get from mzpp (with the added benefit of reprocessing generated text, which could be done in a better way using macros). The special treatment of procedure values is what allows more powerful constructs. There are handled by their arity (preferring a the nullary treatment over the unary one):

• A procedure of arity 0 is simply invoked, and its resulting value is used. The procedure can freely use the input stream to retrieve arguments. For example, here is how to define a standard C function header for use in a Racket extension file:

  @(define (cfunc)

  (format

  "Scheme_Object *~a(int argc, Scheme_Object *argv[])\n"

  (read-line)))

  @cfunc foo

  @cfunc bar

  ==>

  Scheme_Object * foo(int argc, Scheme_Object *argv[])

  Scheme_Object * bar(int argc, Scheme_Object *argv[])

  Note how read-line is used to retrieve an argument, and how this results in an extra space in the actual argument value. Replacing this with read will work slightly better, except that input will have to be a Racket token (in addition, this will not consume the final newline so the extra one in the format string should be removed). The get-arg function can be used to retrieve arguments more easily—by default, it will return any text enclosed by parenthesis, brackets, braces, or angle brackets (see below). For example:

  @(define (tt)

  (format "<tt>~a</tt>" (get-arg)))

  @(define (ref)

  (format "<a href=~s>~a</a>" (get-arg) (get-arg)))

  @(define (ttref)

  (format "<a href=~s>@tt{~a}</a>" (get-arg) (get-arg)))

  @(define (reftt)

  (format "<a href=~s>~a</a>" (get-arg) (tt)))

  @ttref{racket-lang.org}{Racket}

  @reftt{racket-lang.org}{Racket}

  ==>

  <a href="racket-lang.org"><tt>Racket</tt></a>

  <a href="racket-lang.org"><tt>Racket</tt></a>

  Note that in reftt we use tt without arguments since it will retrieve its own arguments. This makes ttref’s approach more natural, except that "calling" tt through a Racket string doesn’t seem natural. For this there is a defcommand command (see below) that can be used to define such functions without using Racket code:

  @defcommand{tt}{X}{<tt>X</tt>}

  @defcommand{ref}{url text}{<a href="url">text</a>}

  @defcommand{ttref}{url text}{<a href="url">@tt{text}</a>}

  @ttref{racket-lang.org}{Racket}

  ==>

  <a href="racket-lang.org"><tt>Racket</tt></a>

• A procedure of arity 1 is invoked differently—it is applied on a thunk that holds the "processing continuation". This application is not expected to return, instead, the procedure can decide to hand over control back to the main loop by using this thunk. This is a powerful facility that is rarely needed, similarly to the fact that call/cc is rarely needed in Racket.

Remember that when procedures are used, generated output is not reprocessed, just like evaluating other expressions.

3.3 Provided bindings

Similarly to mzpp, preprocessor/mztext contains both the implementation as well as user-visible bindings.

Dispatching-related bindings:

procedure (command-marker) → string?

(command-marker str) → void?

str : string?

A string parameter-like procedure that can be used to set a different command marker string. Defaults to @. It can also be set to #f which will disable the command dispatcher altogether. Note that this is a procedure—it cannot be used with parameterize.

Note that the standard command dispatcher uses the same facility, and it is added by default to the dispatcher list unless command-marker is set to #f.

(regexp-try-match #rx"^[ \t]*\r?\n" (stdin))

The result is that a newline will be swallowed if there is only whitespace from the current location to the end of the line. Note that as a general principle regexp-try-match should be preferred over regexp-match for mztext’s preprocessing.

• The name for the new command (the contents of this argument is converted to a string),

• The list of arguments (the contents of this is turned to a list of identifiers),

• Arbitrary text, with textual instances of the variables that denote places they are used.

For example, the sample code above:

is translated to the following definition expression:

(define (ttref)

(let ([url (get-arg)] [text (get-arg)])

(list "<a href=\"" url "\">@tt{" text "}</a>")))

which is then evaluated. Note that the arguments play a role as both Racket identifiers and textual markers.

If it is called with no arguments, it will use get-arg to get an input filename, therefore making it possible to use this as a dispatcher command as well.