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#lang scribble/doc
@(require scribble/manual scribble/bnf scribble/eval "utils.rkt"
          (for-syntax racket/base)
          (for-label (only-in scribble/reader
                              use-at-readtable)))

@(define read-eval (make-base-eval))
@(interaction-eval #:eval read-eval (require (for-syntax racket/base)))

@(define (at-exp-racket)
   @racket[#, @hash-lang[] #, @racketmodname[at-exp] #, @racketidfont{racket}])

@title[#:tag "reader"]{@"@" Syntax}

The Scribble @"@" notation is designed to be a convenient facility for
free-form text in Racket code, where ``@"@"'' was chosen as one of the
least-used characters in existing Racket code. An @"@"-expression is
simply an S-expression in disguise.

Typically, @"@" notation is enabled through
@racketmodname[scribble/base] or similar languages, but you can also
add @"@" notation to an S-expression-based language using the
@racketmodname[at-exp] meta-language. For example,

@verbatim[#:indent 2]|{
  #lang at-exp racket
  (define v '@op{str})
}|

is equivalent to 

@racketmod[
racket
(define v '(op "str"))
]

Using @at-exp-racket[] is probably the easiest way to try the examples
in this chapter.

@;--------------------------------------------------------------------
@section{The Scribble Syntax at a Glance}

To review @secref["how-to:reader"], the concrete syntax of @deftech{@"@"-forms}
is roughly

@racketblock[
 @#,BNF-seq[@litchar["@"]
            @nonterm{cmd}
            @litchar{[} @kleenestar{@nonterm{datum}} @litchar{]}
            @litchar["{"] @kleenestar{@nonterm{text-body}} @litchar["}"]]
]

where all three parts after @litchar["@"] are optional, but at least
one should be present.  (Spaces are not allowed between the
three parts.)  Roughly, a form matching the above grammar is read as

@racketblock[
  (@#,nonterm{cmd} @#,kleenestar{@nonterm{datum}} @#,kleenestar{@nonterm{parsed-body}})
]

where @nonterm{parsed-body} is the translation of each
@nonterm{text-body} in the input.  Thus, the initial @nonterm{cmd}
determines the Racket code that the input is translated into.  The
common case is when @nonterm{cmd} is a Racket identifier, which reads
as a plain Racket form, with datum arguments and/or string arguments.

Here is one example:

@scribble-examples|==={
  @foo{blah blah blah}
}===|

The example shows how an input syntax is read as Racket syntax, not
what it evaluates to. If you want to see the translation of an example
into S-expression form, add a quote in front of it in a
@at-exp-racket[] module. For example, running

@verbatim[#:indent 2]|{
  #lang at-exp racket
  '@foo{blah blah blah}
}|

in DrRacket prints the output

@nested[#:style 'inset]{@racketresult[(foo "blah blah blah")]}

while omitting the quote

@verbatim[#:indent 2]|{
  #lang at-exp racket
  @foo{blah blah blah}
}|

triggers a syntax error because @racket[foo] is not bound, and

@verbatim[#:indent 2]|{
  #lang at-exp racket
  (define (foo str) (printf "He wrote ~s.\n" str))
  @foo{blah blah blah}
}|

prints the output

@nested[#:style 'inset]{@racketoutput{He wrote "blah blah blah".}}

Here are more examples of @tech{@"@"-forms}:

@scribble-examples|==={
  @foo{blah "blah" (`blah'?)}
  @foo[1 2]{3 4}
  @foo[1 2 3 4]
  @foo[#:width 2]{blah blah}
  @foo{blah blah
       yada yada}
  @foo{
    blah blah
    yada yada
  }
}===|

As seen in the last example, multiple lines and the newlines that
separate them are parsed to multiple Racket strings.  More generally,
a @nonterm{text-body} is made of text, newlines, and nested
@tech{@"@"-forms}, where the syntax for @tech{@"@"-forms} is the same whether it's
in a @nonterm{text-body} context as in a Racket context.  A
@nonterm{text-body} that isn't an @tech{@"@"-form} is converted to a string
expression for its @nonterm{parsed-body}; newlines and following
indentations are converted to @racket["\n"] and all-space string
expressions.

@scribble-examples|==={
  @foo{bar @baz{3}
       blah}
  @foo{@b{@u[3] @u{4}}
       blah}
  @C{while (*(p++))
       *p = '\n';}
}===|

The command part of an @tech{@"@"-form} is optional as well. In that case,
the @tech{@"@"-form} is read as a list, which usually counts as a function
application, but it also useful when quoted with the usual Racket
@racket[quote]:

@scribble-examples|==={
  @{blah blah}
  @{blah @[3]}
  '@{foo
     bar
     baz}
}===|

Finally, we can also drop the datum and text parts, which leaves us with
only the command---which is read as is, not within a parenthesized
form.  This is not useful when reading Racket code, but it can be used
inside a text block to escape a Racket identifier.  A vertical bar
(@litchar{|}) can be used to delimit the escaped identifier when
needed.

@scribble-examples|==={
  @foo
  @{blah @foo blah}
  @{blah @foo: blah}
  @{blah @|foo|: blah}
}===|

Actually, the command part can be any Racket expression (that does not
start with @litchar["["], @litchar["{"], or @litchar["|"]), which is
particularly useful with such escapes since they can be used with any
expression.

@scribble-examples|==={
  @foo{(+ 1 2) -> @(+ 1 2)!}
  @foo{A @"string" escape}
  @"@"
}===|

Note that an escaped Racket string is merged with the surrounding text
as a special case.  This is useful if you want to use the special
characters in your string, but escaping braces are not necessary if
they are balanced.

@scribble-examples|==={
  @foo{eli@"@"barzilay.org}
  @foo{A @"{" begins a block}
  @C{while (*(p++)) {
       *p = '\n';
     }}
}===|

In some cases, a text contains many literal @"@"s, which can be
cumbersome to quote individually.  For such case, braces have an
alternative syntax: A block of text can begin with a
``@litchar["|{"]'' and terminated accordingly with a
``@litchar["}|"]''.  Furthermore, any nested @tech{@"@"-forms} must begin
with a ``@litchar["|@"]''.

@scribble-examples|==={
  @foo|{bar}@{baz}|
  @foo|{bar |@x{X} baz}|
  @foo|{bar |@x|{@}| baz}|
}===|

In cases when even this is not convenient enough, punctuation
characters can be added between the @litchar{|} and the braces and the
@"@" in nested forms.  (The punctuation is mirrored for parentheses
and @litchar{<>}s.)  With this extension, @tech{@"@"-form} syntax can be used as a
``here string'' replacement.

@scribble-examples|==={
  @foo|--{bar}@|{baz}--|
  @foo|<<{bar}@|{baz}>>|
}===|

On the flip side of this is, how can an @"@" sign be used in Racket
code?  This is almost never an issue, because Racket strings and
characters are still read the same, and @litchar["@"] is set as a
non-terminating reader macro so it can be used in Racket identifiers
anywhere except in the first character of an identifier.  When
@litchar["@"] must appear as the first character of an identifier, you
must quote the identifier just like other non-standard characters in
normal S-expression syntax: with a backslash or with vertical bars.

@scribble-examples|==={
  (define \@email "foo@bar.com")
  (define |@atchar| #\@)
}===|

Note that spaces are not allowed before a @litchar{[} or a
@litchar["{"], or they will be part of the following text (or Racket
code).  (More on using braces in body texts below.)

@scribble-examples|==={
  @foo{bar @baz[2 3] {4 5}}
}===|

Finally, remember that @tech{@"@"-forms} are just an alternate form of
S-expressions. Identifiers still get their meaning, as in any
Racket code, through the lexical context in which they appear.
Specifically, when the above @tech{@"@"-form} appears in a Racket expression
context, the lexical environment must provide bindings for
@racket[foo] as a procedure or a macro; it can be defined, required,
or bound locally (with @racket[let], for example).

@; FIXME: unfortunate code duplication
@interaction[
(eval:alts
  (let* ([formatter (lambda (fmt)
          (lambda args (format fmt (apply string-append args))))]
         [bf (formatter "*~a*")]
         [it (formatter "/~a/")]
         [ul (formatter "_~a_")]
         [text string-append])
    #,(tt "@text{@it{Note}: @bf{This is @ul{not} a pipe}.}"))
  (let* ([formatter (lambda (fmt)
          (lambda args (format fmt (apply string-append args))))]
         [bf (formatter "*~a*")]
         [it (formatter "/~a/")]
         [ul (formatter "_~a_")]
         [text string-append])
    @text{@it{Note}: @bf{This is @ul{not} a pipe}.}))
]

@;--------------------------------------------------------------------
@section{The Command Part}

Besides being a Racket identifier, the @nonterm{cmd} part of an
@tech{@"@"-form} can have Racket punctuation prefixes, which will end up
wrapping the @italic{whole} expression.

@scribble-examples|==={
  @`',@foo{blah}
  @#`#'#,@foo{blah}
}===|

When writing Racket code, this means that @litchar|{@`',@foo{blah}}|
is exactly the same as @litchar|{`@',@foo{blah}}| and
@litchar|{`',@@foo{blah}}|, but unlike the latter two, the first
construct can appear in body texts with the same meaning, whereas the
other two would not work (see below).

After the optional punctuation prefix, the @nonterm{cmd} itself is not
limited to identifiers; it can be @italic{any} Racket expression.

@scribble-examples|==={
  @(lambda (x) x){blah}
  @`(unquote foo){blah}
}===|

In addition, the command can be omitted altogether, which will omit it
from the translation, resulting in an S-expression that usually
contains, say, just strings:

@scribble-examples|==={
  @{foo bar
    baz}
  @'{foo bar
     baz}
}===|

If the command part begins with a @litchar{;} (with no newline between
the @litchar["@"] and the @litchar{;}), then the construct is a
comment.  There are two comment forms, one for arbitrary-text and
possibly nested comments, and another one for line comments:

@racketblock[
@#,BNF-seq[@litchar["@;{"] @kleenestar{@nonterm{any}} @litchar["}"]]

@#,BNF-seq[@litchar["@;"] @kleenestar{@nonterm{anything-else-without-newline}}]
]

In the first form, the commented body must still parse correctly; see
the description of the body syntax below.  In the second form, all
text from the @litchar["@;"] to the end of the line @italic{and} all
following spaces (or tabs) are part of the comment (similar to
@litchar{%} comments in TeX).

@scribble-examples|==={
  @foo{bar @; comment
       baz@;
       blah}
}===|

Tip: if you use an editor in some Scheme mode without support for
@tech{@"@"-forms}, balanced comments can be confusing, since the open brace
looks commented out, and the closing one isn't.  In such cases it is
useful to ``comment'' out the closing brace too:

@verbatim[#:indent 2]|==={
  @;{
    ...
  ;}
}===|

so the editor does not treat the file as having unbalanced
parentheses.

If only the @nonterm{cmd} part of an @tech{@"@"-form} is specified, then the
result is the command part only, without an extra set of parenthesis.
This makes it suitable for Racket escapes in body texts.  (More on this
below, in the description of the body part.)

@scribble-examples|==={
  @foo{x @y z}
  @foo{x @(* y 2) z}
  @{@foo bar}
}===|

Finally, note that there are currently no special rules for using
@litchar["@"] in the command itself, which can lead to things like:

@scribble-examples|==={
  @@foo{bar}{baz}
}===|

@;--------------------------------------------------------------------
@section{The Datum Part}

The datum part can contains arbitrary Racket expressions, which
are simply stacked before the body text arguments:

@scribble-examples|==={
  @foo[1 (* 2 3)]{bar}
  @foo[@bar{...}]{blah}
}===|

The body part can still be omitted, which is essentially an
alternative syntax for plain (non-textual) S-expressions:

@scribble-examples|==={
  @foo[bar]
  @foo{bar @f[x] baz}
}===|

The datum part can be empty, which makes no difference, except when
the body is omitted.  It is more common, however, to use an empty body
for the same purpose.

@scribble-examples|==={
  @foo[]{bar}
  @foo[]
  @foo
  @foo{}
}===|

The most common use of the datum part is for Racket forms that expect
keyword-value arguments that precede the body of text arguments.

@scribble-examples|==={
  @foo[#:style 'big]{bar}
}===|

@;--------------------------------------------------------------------
@section{The Body Part}

The syntax of the body part is intended to be as convenient as
possible for free text.  It can contain almost any text---the only
characters with special meaning is @litchar["@"] for sub-@tech{@"@"-forms},
and @litchar["}"] for the end of the text.  In addition, a
@litchar["{"] is allowed as part of the text, and it makes the
matching @litchar["}"] be part of the text too---so balanced braces
are valid text.

@scribble-examples|==={
  @foo{f{o}o}
  @foo{{{}}{}}
}===|

As described above, the text turns to a sequence of string arguments
for the resulting form.  Spaces at the beginning and end of lines are
discarded, and newlines turn to individual @racket["\n"] strings
(i.e., they are not merged with other body parts); see also the
information about newlines and indentation below. Spaces are
@italic{not} discarded if they appear after the open @litchar["{"]
(before the closing @litchar["}"]) when there is also text that
follows (precedes) it; specifically, they are preserved in a
single-line body.

@scribble-examples|==={
  @foo{bar}
  @foo{ bar }
  @foo[1]{ bar }
}===|

If @litchar["@"] appears in a body, then it is interpreted as Racket
code, which means that the @"@"-reader is applied recursively, and the
resulting syntax appears as part of the S-expression, among other
string contents.

@scribble-examples|==={
  @foo{a @bar{b} c}
}===|

If the nested @"@" construct has only a command---no body or datum
parts---it will not appear in a subform.  Given that the command part
can be any Racket expression, this makes @"@" a general escape to
arbitrary Racket code.

@scribble-examples|==={
  @foo{a @bar c}
  @foo{a @(bar 2) c}
}===|

This is particularly useful with strings, which can be used to include
arbitrary text.

@scribble-examples|==={
  @foo{A @"}" marks the end}
}===|

Note that the escaped string is (intentionally) merged with the rest
of the text.  This works for @litchar["@"] too:

@scribble-examples|==={
  @foo{The prefix: @"@".}
  @foo{@"@x{y}" --> (x "y")}
}===|

@;--------------------------------------------------------------------
@subsection[#:tag "alt-body-syntax"]{Alternative Body Syntax}

In addition to the above, there is an alternative syntax for the body,
one that specifies a new marker for its end: use @litchar["|{"] for
the opening marker to have the text terminated by a @litchar["}|"].

@scribble-examples|==={
  @foo|{...}|
  @foo|{"}" follows "{"}|
  @foo|{Nesting |{is}| ok}|
}===|

This applies to sub-@tech{@"@"-forms} too---the @litchar["@"] must be
prefixed with a @litchar{|}:

@scribble-examples|==={
  @foo|{Maze
        |@bar{is}
        Life!}|
  @t|{In |@i|{sub|@"@"s}| too}|
}===|

Note that the subform uses its own delimiters, @litchar{{...}} or
@litchar{|{...}|}.  This means that you can copy and paste Scribble
text with @tech{@"@"-forms} freely, just prefix the @litchar["@"] if the
immediate surrounding text has a prefix.

For even better control, you can add characters in the opening
delimiter, between the @litchar{|} and the @litchar["{"].
Characters that are put there (non alphanumeric ASCII characters only,
excluding @litchar["{"] and @litchar["@"]) should also be used for
sub-@tech{@"@"-forms}, and the end-of-body marker should have these characters
in reverse order with paren-like characters (@litchar{(},
@litchar{[}, @litchar{<}) mirrored.

@scribble-examples|==={
  @foo|<<<{@x{foo} |@{bar}|.}>>>|
  @foo|!!{X |!!@b{Y}...}!!|
}===|

Finally, remember that you can use an expression escape with a Racket
string for confusing situations.  This works well when you only need
to quote short pieces, and the above works well when you have larger
multi-line body texts.

@;--------------------------------------------------------------------
@subsection{Racket Expression Escapes}

In some cases, you may want to use a Racket identifier (or a number or
a boolean etc.) in a position that touches the following text; in
these situations you should surround the escaped Racket expression by
a pair of @litchar{|} characters.  The text inside the bars is
parsed as a Racket expression.

@scribble-examples|==={
  @foo{foo@bar.}
  @foo{foo@|bar|.}
  @foo{foo@3.}
  @foo{foo@|3|.}
}===|

This form is a generic Racket expression escape, there is no body text
or datum part when you use this form.

@scribble-examples|==={
  @foo{foo@|(f 1)|{bar}}
  @foo{foo@|bar|[1]{baz}}
}===|

This works for string expressions too, but note that unlike the above,
the string is (intentionally) not merged with the rest of the text:

@scribble-examples|==={
  @foo{x@"y"z}
  @foo{x@|"y"|z}
}===|

Expression escapes also work with @italic{any} number of expressions,

@scribble-examples|==={
  @foo{x@|1 (+ 2 3) 4|y}
  @foo{x@|*
          *|y}
}===|

It seems that @litchar["@||"] has no purpose---but remember that these escapes
are never merged with the surrounding text, which can be useful when
you want to control the sub expressions in the form.

@scribble-examples|==={
  @foo{Alice@||Bob@|
       |Carol}
}===|

Note that @litchar["@|{...}|"] can be parsed as either an escape expression or
as the Racket command part of an @tech{@"@"-form}.  The latter is used in this case
(since there is little point in Racket code that uses braces.

@scribble-examples|==={
  @|{blah}|
}===|

@;--------------------------------------------------------------------
@subsection{Comments}

As noted above, there are two kinds of @tech{@"@"-form} comments: @litchar|{@;{...}}| is
a (nestable) comment for a whole body of text (following the same
rules for @tech{@"@"-forms}), and @litchar|{@;...}| is a line-comment.

@scribble-examples|==={
  @foo{First line@;{there is still a
                    newline here;}
       Second line}
}===|

One useful property of line-comments is that they continue to the end
of the line @italic{and} all following spaces (or tabs).  Using this,
you can get further control of the subforms.

@scribble-examples|==={
  @foo{A long @;
       single-@;
       string arg.}
}===|

Note how this is different from using @litchar["@||"]s in that strings
around it are not merged.

@;--------------------------------------------------------------------
@subsection{Spaces, Newlines, and Indentation}

The @tech{@"@"-form} syntax treats spaces and newlines in a special way is
meant to be sensible for dealing with text.  As mentioned above,
spaces at the beginning and end of body lines are discarded, except
for spaces between a @litchar["{"] and text, or between text and a
@litchar["}"].

@scribble-examples|==={
  @foo{bar}
  @foo{ bar }
  @foo{ bar
       baz }
}===|

A single newline that follows an open brace or precedes a closing
brace is discarded, unless there are only newlines in the body; other
newlines are read as a @racket["\n"] string

@scribble-examples|==={
  @foo{bar
  }
  @foo{
    bar
  }
  @foo{

    bar

  }
  @foo{
    bar

    baz
  }
  @foo{
  }
  @foo{

  }
  @foo{ bar
       baz }
}===|

Spaces at the beginning of body lines do not appear in the resulting
S-expressions, but the column of each line is noticed, and all-space
indentation strings are added so the result has the same indentation.
A indentation string is added to each line according to its distance
from the leftmost syntax object (except for empty lines).  (Note: if
you try these examples on a Racket REPL, you should be aware that
the reader does not know about the ``@litchar{> }'' prompt.)

@scribble-examples|==={
  @foo{
    bar
    baz
    blah
  }
  @foo{
    begin
      x++;
    end}
  @foo{
      a
     b
    c}
}===|

If the first string came from the opening @litchar["{"] line, it is
not prepended with an indentation (but it can affect the leftmost
syntax object used for indentation).  This makes sense when formatting
structured code as well as text (see the last example in the following
block).

@scribble-examples|==={
  @foo{bar
         baz
       bbb}
  @foo{ bar
          baz
        bbb}
  @foo{bar
     baz
     bbb}
  @foo{ bar
     baz
     bbb}
  @foo{ bar
     baz
       bbb}
  @text{Some @b{bold
    text}, and
    more text.}
}===|

Note that each @"@"-form is parsed to an S-expression that has its own
indentation.  This means that Scribble source can be indented like
code, but if indentation matters then you may need to apply
indentation of the outer item to all lines of the inner one.  For
example, in

@litchar/lines|==={
  @code{
    begin
      i = 1, r = 1
      @bold{while i < n do
              r *= i++
            done}
    end
  }
}===|

a formatter will need to apply the 2-space indentation to the
rendering of the @racket[bold] body.

Note that to get a first-line text to be counted as a leftmost line,
line and column accounting should be on for the input port
(@racket[use-at-readtable] turns them on for the current input port).
Without this,

@litchar/lines|==={
  @foo{x1
         x2
         x3}
}===|

will not have 2-space indentations in the parsed S-expression if
source accounting is not on, but

@litchar/lines|==={
  @foo{x1
         x2
       x3}
}===|

will (due to the last line).  Pay attention to this, as it can be a
problem with Racket code, for example:

@litchar/lines|==={
  @code{(define (foo x)
          (+ x 1))}
}===|

For rare situations where spaces at the beginning (or end) of lines
matter, you can begin (or end) a line with a @litchar["@||"].

@scribble-examples|==={
  @foo{
    @|| bar @||
    @|| baz}
}===|

@; --------------------------------------------------
@(close-eval read-eval)

