6 Text Preprocessing

6.1 Writing Preprocessor Files

The combination of the two features makes text in files in the scribble/text language be read as strings, which get printed out when the module is required, for example, when a file is given as an argument to racket. (In these example the left part shows the source input, and the right part the printed result.)

#lang scribble/text Programming languages should be designed not by piling feature on top of feature, but blah blah blah.

→

Programming languages should be designed not by piling feature on top of feature, but blah blah blah.

Using @-forms, we can define and use Racket functions.

#lang scribble/text @(require racket/list) @(define Foo "Preprocessing") @(define (3x . x)    ;; racket syntax here    (add-between (list x x x) " ")) @Foo languages should be designed not by piling feature on top of feature, but @3x{blah}.

→

Preprocessing languages should be designed not by piling feature on top of feature, but blah blah blah.

As demonstrated in this case, the output function simply scans nested list structures recursively, which makes them convenient for function results. In addition, output prints most values similarly to display — notable exceptions are void and false values which cause no output to appear. This can be used for convenient conditional output.

#lang scribble/text @(define (errors n)    (list n          " error"          (and (not (= n 1)) "s"))) You have @errors[3] in your code, I fixed @errors[1].

→

You have 3 errors in your code, I fixed 1 error.

Using the scribble @-forms syntax, you can write functions more conveniently too.

#lang scribble/text @(define (errors n)    ;; note the use of `unless'    @list{@n error@unless[(= n 1)]{s}}) You have @errors[3] in your code, I fixed @errors[1].

→

You have 3 errors in your code, I fixed 1 error.

Following the details of the scribble reader, you may notice that in these examples there are newline strings after each definition, yet they do not show in the output. To make it easier to write definitions, newlines after definitions and indentation spaces before them are ignored.

#lang scribble/text @(define (plural n)    (unless (= n 1) "s")) @(define (errors n)    @list{@n error@plural[n]}) You have @errors[3] in your code,   @(define fixed 1) I fixed @errors[fixed].

→

You have 3 errors in your code, I fixed 1 error.

These end-of-line newline strings are not ignored when they follow other kinds of expressions, which may lead to redundant empty lines in the output.

#lang scribble/text @(define (count n str)    (for/list ([i (in-range 1 (add1 n))])      @list{@i @str,@"\n"})) Start... @count[3]{Mississippi} ... and I'm done.

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi, ... and I'm done.

There are several ways to avoid having such empty lines in your output. The simplest way is to arrange for the function call’s form to end right before the next line begins, but this is often not too convenient. An alternative is to use a @; comment, which makes the scribble reader ignore everything that follows it up to and including the newline. (These methods can be applied to the line that precedes the function call too, but the results are likely to have what looks like erroneous indentation. More about this below.)

#lang scribble/text @(define (count n str)    (for/list ([i (in-range 1 (+ n 1))])      @list{@i @str,@"\n"})) Start... @count[3]{Mississippi }... done once. Start again... @count[3]{Massachusetts}@; ... and I'm done again.

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi, ... done once. Start again... 1 Massachusetts, 2 Massachusetts, 3 Massachusetts, ... and I'm done again.

A better approach is to generate newlines only when needed.

#lang scribble/text @(require racket/list) @(define (counts n str)    (add-between     (for/list ([i (in-range 1 (+ n 1))])       @list{@i @str,})     "\n")) Start... @counts[3]{Mississippi} ... and I'm done.

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi, ... and I'm done.

In fact, this is common enough that the scribble/text language provides a convenient facility: add-newlines is a function that is similar to add-between using a newline string as the default separator, except that false and void values are filtered out before doing so.

#lang scribble/text @(define (count n str)    (add-newlines     (for/list ([i (in-range 1 (+ n 1))])       @list{@i @str,}))) Start... @count[3]{Mississippi} ... and I'm done.

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi, ... and I'm done.

#lang scribble/text @(define (count n str)    (add-newlines     (for/list ([i (in-range 1 (+ n 1))])       @(and (even? i) @list{@i @str,})))) Start... @count[6]{Mississippi} ... and I'm done.

→

Start... 2 Mississippi, 4 Mississippi, 6 Mississippi, ... and I'm done.

The separator can be set to any value.

#lang scribble/text @(define (count n str)    (add-newlines #:sep ",\n"     (for/list ([i (in-range 1 (+ n 1))])       @list{@i @str}))) Start... @count[3]{Mississippi}. ... and I'm done.

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi. ... and I'm done.

6.2 Defining Functions and More

(Note: most of the tips in this section are applicable to any code that uses the Scribble @-form syntax.)

Because the Scribble reader is uniform, you can use it in place of any expression where it is more convenient. (By convention, we use a plain S-expression syntax when we want a Racket expression escape, and an @-form for expressions that render as text, which, in the scribble/text language, is any value-producing expression.) For example, you can use an @-form for a function that you define.

#lang scribble/text @(define @bold[text] @list{*@|text|*}) An @bold{important} note.

→

An *important* note.

This is not commonly done, since most functions that operate with text will need to accept a variable number of arguments. In fact, this leads to a common problem: what if we want to write a function that consumes a number of “text arguments” rathen than a single “rest-like” body? The common solution for this is to provide the separate text arguments in the S-expression part of an @-form.

#lang scribble/text @(define (choose 1st 2nd)    @list{Either @1st, or @|2nd|@"."}) @(define who "us") @choose[@list{you're with @who}         @list{against @who}]

→

Either you're with us, or against us.

You can even use @-forms with a Racket quote or quasiquote as the “head” part to make it shorter, or use a macro to get grouping of sub-parts without dealing with quotes.

#lang scribble/text @(define (choose 1st 2nd)    @list{Either @1st, or @|2nd|@"."}) @(define who "us") @choose[@list{you're with @who}         @list{against @who}] @(define-syntax-rule (compare (x ...) ...)    (add-newlines     (list (list "* " x ...) ...))) Shopping list: @compare[@{apples}          @{oranges}          @{@(* 2 3) bananas}]

→

Either you're with us, or against us. Shopping list: * apples * oranges * 6 bananas

Yet another solution is to look at the text values and split the input arguments based on a specific token. Using match can make it convenient — you can even specify the patterns with @-forms.

#lang scribble/text @(require racket/match) @(define (features . text)    (match text      [@list{@|1st|@...             ---             @|2nd|@...}       @list{>> Pros <<             @1st;             >> Cons <<             @|2nd|.}])) @features{fast,           reliable           ---           expensive,           ugly}

→

>> Pros << fast, reliable; >> Cons << expensive, ugly.

In particular, it is often convenient to split the input by lines, identified by delimiting "\n" strings. Since this can be useful, a split-lines function is provided.

#lang scribble/text @(require racket/list) @(define (features . text)    (add-between (split-lines text)                 ", ")) @features{red           fast           reliable}.

→

red, fast, reliable.

Finally, the Scribble reader accepts any expression as the head part of an @-form — even an @ form. This makes it possible to get a number of text bodies by defining a curried function, where each step accepts any number of arguments. This, however, means that the number of body expressions must be fixed.

#lang scribble/text @(define ((choose . 1st) . 2nd)    @list{Either you're @1st, or @|2nd|.}) @(define who "me") @@choose{with @who}{against @who}

→

Either you're with me, or against me.

6.3 Using Printouts

Because the preprocessor language simply displays each toplevel value as the file is run, it is possible to print text directly as part of the output.

#lang scribble/text First @display{Second} Third

→

First Second Third

Taking this further, it is possible to write functions that output some text instead of returning values that represent the text.

#lang scribble/text @(define (count n)    (for ([i (in-range 1 (+ n 1))])      (printf "~a Mississippi,\n" i))) Start... @count[3]@; avoid an empty line ... and I'm done.

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi, ... and I'm done.

This can be used to produce a lot of output text, even infinite.

#lang scribble/text @(define (count n)    (printf "~a Mississippi,\n" n)    (count (add1 n))) Start... @count[1] this line is never printed!

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi, 4 Mississippi, 5 Mississippi, ...

However, you should be careful not to mix returning values with printouts, as the results are rarely desirable.

#lang scribble/text @list{1 @display{two} 3}

→

two1  3

Note that you don’t need side-effects if you want infinite output. The output function iterates thunks and (composable) promises, so you can create a loop that is delayed in either form.

#lang scribble/text @(define (count n)    (cons @list{@n Mississippi,@"\n"}          (lambda ()            (count (add1 n))))) Start... @count[1] this line is never printed!

→

Start... 1 Mississippi, 2 Mississippi, 3 Mississippi, 4 Mississippi, 5 Mississippi, ...

6.4 Indentation in Preprocessed output

An issue that can be very important in many preprocessor applications is the indentation of the output. This can be crucial in some cases, if you’re generating code for an indentation-sensitive language (e.g., Haskell, Python, or C preprocessor directives). To get a better understanding of how the pieces interact, you may want to review how the Scribble reader section, but also remember that you can use quoted forms to see how some form is read.

#lang scribble/text @(format "~s" '@list{                  a                    b                  c})

→

(list "a" "\n" "  " "b" "\n" "c")

The Scribble reader ignores indentation spaces in its body. This is an intentional feature, since you usually do not want an expression to depend on its position in the source. But the question is how can we render some output text with proper indentation. The output function achieves that by assigning a special meaning to lists: when a newline is part of a list’s contents, it causes the following text to appear with indentation that corresponds to the column position at the beginning of the list. In most cases, this makes the output appear “as intended” when lists are used for nested pieces of text — either from a literal list expression, or an expression that evaluates to a list, or when a list is passed on as a value; either as a toplevel expression, or as a nested value; either appearing after spaces, or after other output.

#lang scribble/text foo @list{1           2           3}

→

foo 1     2     3

#lang scribble/text @(define (block . text)    @list{begin            @text          end}) @block{first        second        @block{          third          fourth}        last}

→

begin   first   second   begin     third     fourth   end   last end

#lang scribble/text @(define (enumerate . items)    (add-newlines #:sep ";\n"     (for/list ([i (in-naturals 1)]                [item (in-list items)])       @list{@|i|. @item}))) Todo: @enumerate[@list{Install Racket}                  @list{Hack, hack, hack}                  @list{Profit}].

→

Todo: 1. Install Racket;       2. Hack, hack, hack;       3. Profit.

There are, however, cases when you need more refined control over the output. The scribble/text provides a few functions for such cases. The splice function is used to group together a number of values but avoid introducing a new indentation context.

#lang scribble/text @(define (block . text)    @splice{{      blah(@text);    }}) start   @splice{foo();           loop:}   @list{if (something) @block{one,                               two}} end

→

start   foo(); loop:   if (something) {     blah(one,          two);   } end

The disable-prefix function disables all indentation printouts in its contents, including the indentation before the body of the disable-prefix value itself. It is useful, for example, to print out CPP directives.

#lang scribble/text @(define (((IFFOO . var) . expr1) . expr2)    (define (array e1 e2)      @list{[@e1,             @e2]})    @list{var @var;          @disable-prefix{#ifdef FOO}          @var = @array[expr1 expr2];          @disable-prefix{#else}          @var = @array[expr2 expr1];          @disable-prefix{#endif}}) function blah(something, something_else) {   @disable-prefix{#include "stuff.inc"}   @@@IFFOO{i}{something}{something_else} }

→

function blah(something, something_else) { #include "stuff.inc"   var i; #ifdef FOO   i = [something,        something_else]; #else   i = [something_else,        something]; #endif }

If there are values after a disable-prefix value on the same line, they will get indented to the goal column (unless the output is already beyond it).

#lang scribble/text @(define (thunk name . body)    @list{function @name() {            @body          }}) @(define (ifdef cond then else)    @list{@disable-prefix{#}ifdef @cond            @then          @disable-prefix{#}else            @else          @disable-prefix{#}endif}) @thunk['do_stuff]{   init();   @ifdef["HAS_BLAH"     @list{var x = blah();}     @thunk['blah]{       @ifdef["BLEHOS"         @list{@disable-prefix{#}@;                 include <bleh.h>               bleh();}         @list{error("no bleh");}]     }]   more_stuff(); }

→

function do_stuff() {   init(); # ifdef HAS_BLAH     var x = blah(); # else     function blah() { #     ifdef BLEHOS #       include <bleh.h>         bleh(); #     else         error("no bleh"); #     endif     } # endif   more_stuff(); }

There are cases where each line should be prefixed with some string other than a plain indentation. The add-prefix function causes its contents to be printed using some given string prefix for every line. The prefix gets accumulated to an existing indentation, and indentation in the contents gets added to the prefix.

#lang scribble/text @(define (comment . body)    @add-prefix["// "]{@body}) @comment{add : int int -> string} char *foo(int x, int y) {   @comment{     skeleton:       allocate a string       print the expression into it       @comment{...more work...}   }   char *buf = malloc(@comment{FIXME!                               This is bad}                      100); }

→

// add : int int -> string char *foo(int x, int y) {   // skeleton:   //   allocate a string   //   print the expression into it   //   // ...more work...   char *buf = malloc(// FIXME!                      // This is bad                      100); }

When combining add-prefix and disable-prefix there is an additional value that can be useful: flush. This is a value that causes output to print the current indentation and prefix. This makes it possible to get the “ignored as a prefix” property of disable-prefix but only for a nested prefix.

#lang scribble/text @(define (comment . text)    (list flush          @add-prefix[" *"]{            @disable-prefix{/*} @text */})) function foo(x) {   @comment{blah            more blah            yet more blah}   if (x < 0) {     @comment{even more              blah here              @comment{even                       nested}}     do_stuff();   } }

→

function foo(x) {   /* blah    * more blah    * yet more blah */   if (x < 0) {     /* even more      * blah here      * /* even      *  * nested */ */     do_stuff();   } }

6.5 Using External Files

Using additional files that contain code for your preprocessing is trivial: the preprocessor source is still source code in a module, so you can require additional files with utility functions.

#lang scribble/text @(require "itemize.rkt") Todo: @itemize[@list{Hack some}          @list{Sleep some}          @list{Hack some                more}] itemize.rkt:  #lang racket (provide itemize) (define (itemize . items)   (add-between (map (lambda (item)                       (list "* " item))                     items)                "\n"))

→

Todo: * Hack some * Sleep some * Hack some   more

Note that the at-exp language can often be useful here, since such files need to deal with texts. Using it, it is easy to include a lot of textual content.

#lang scribble/text @(require "stuff.rkt") Todo: @itemize[@list{Hack some}          @list{Sleep some}          @list{Hack some                more}] @summary stuff.rkt:  #lang at-exp racket/base (require racket/list) (provide (all-defined-out)) (define (itemize . items)   (add-between (map (lambda (item)                       @list{* @item})                     items)                "\n")) (define summary   @list{If that's not enough,         I don't know what is.})

→

Todo: * Hack some * Sleep some * Hack some   more If that's not enough, I don't know what is.

Of course, the extreme side of this will be to put all of your content in a plain Racket module, using @-forms for convenience. However, there is no need to use the preprocessor language in this case; instead, you can (require scribble/text), which will get all of the bindings that are available in the scribble/text language. Using output, switching from a preprocessed files to a Racket file is very easy —- choosing one or the other depends on whether it is more convenient to write a text file with occasional Racket expressions or the other way.

#lang at-exp racket/base (require scribble/text racket/list) (define (itemize . items)   (add-between (map (lambda (item)                       @list{* @item})                     items)                "\n")) (define summary   @list{If that's not enough,         I don't know what is.}) (output  @list{    Todo:    @itemize[@list{Hack some}             @list{Sleep some}             @list{Hack some                   more}]    @summary  })

→

Todo: * Hack some * Sleep some * Hack some   more If that's not enough, I don't know what is.

However, you might run into a case where it is desirable to include a mostly-text file from a preprocessor file. It might be because you prefer to split the source text to several files, or because you need to preprocess a file without even a #lang header (for example, an HTML template file that is the result of an external editor). For these cases, the scribble/text language provides an include form that includes a file in the preprocessor syntax (where the default parsing mode is text).

#lang scribble/text @(require racket/list) @(define (itemize . items)    (list     "<ul>"     (add-between      (map (lambda (item)             @list{<li>@|item|</li>})           items)      "\n")     "</ul>")) @(define title "Todo") @(define summary    @list{If that's not enough,          I don't know what is.}) @include["template.html"] template.html:  <html> <head><title>@|title|</title></head> <body>   <h1>@|title|</h1>   @itemize[@list{Hack some}            @list{Sleep some}            @list{Hack some                  more}]   <p><i>@|summary|</i></p> </body> </html>

→

<html> <head><title>Todo</title></head> <body>   <h1>Todo</h1>   <ul><li>Hack some</li>       <li>Sleep some</li>       <li>Hack some           more</li></ul>   <p><i>If that's not enough,         I don't know what is.</i></p> </body> </html>

(Using require with a text file in the scribble/text language will not work as intended: using the preprocessor language means that the text is displayed when the module is invoked, so the required file’s contents will be printed before any of the requiring module’s text does. If you find yourself in such a situation, it is better to switch to a Racket-with-@-expressions file as shown above.)