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5.29 Diversions

In roff systems it is possible to format text as if for output, but instead of writing it immediately, one can divert the formatted text into a named storage area. It is retrieved later by specifying its name after a control character. The same name space is used for such diversions as for strings and macros; see Identifiers. Such text is sometimes said to be “stored in a macro”, but this coinage obscures the important distinction between macros and strings on one hand and diversions on the other; the former store unformatted input text, and the latter capture formatted output. Diversions also do not interpret arguments. Applications of diversions include “keeps” (preventing a page break from occurring at an inconvenient place by forcing a set of output lines to be set as a group), footnotes, tables of contents, and indices. For orthogonality it is said that GNU troff is in the top-level diversion if no diversion is active (that is, formatted output is being “diverted” immediately to the output device).

Dereferencing an undefined diversion will create an empty one of that name and cause a warning in category ‘mac’ to be emitted. See Warnings, for information about the enablement and suppression of warnings. A diversion does not exist for the purpose of testing with the d conditional operator until its initial definition ends (see Operators in Conditionals). The following requests are used to create and alter diversions.

Request: .di [name]
Request: .da [name]

Start collecting formatted output in a diversion called name. The da request appends to a diversion called name, creating it if necessary. If name already exists as an alias, the target of the alias is replaced or appended to; recall Strings. The pending output line is diverted as well. Switching to another environment (with the ev request) before invoking di or da avoids including any pending output line in the diversion; see Environments.

Invoking di or da without an argument stops diverting output to the diversion named by the most recent corresponding request. If di or da is called without an argument when there is no current diversion, a warning in category ‘di’ is produced. See Warnings, for information about the enablement and suppression of warnings.

Before the diversion.
.di yyy
In the diversion.
After the diversion.
    ⇒ After the diversion.
    ⇒ Before the diversion.  In the diversion.

GNU troff supports box requests to exclude a partially collected line from a diversion, as this is often desirable.

Request: .box [name]
Request: .boxa [name]

Divert (or append) output to name, similarly to the di and da requests, respectively. Any pending output line is not included in the diversion. Without an argument, stop diverting output; any pending output line inside the diversion is discarded.

Before the box.
.box xxx
In the box.
Hidden treasure.
After the box.
    ⇒ Before the box.  After the box.
    ⇒ In the box.

Apart from pending output line inclusion and the request names that populate them, boxes are handled exactly as diversions are. All of the following groff language elements can be used with them interchangeably.

Register: \n[.z]
Register: \n[.d]

Diversions may be nested. The read-only string-valued register .z contains the name of the current diversion. The read-only register .d contains the current vertical place in the diversion. If the input text is not being diverted, .d reports the same location as the register nl.

Register: \n[.h]

The read-only register .h stores the high-water mark on the current page or in the current diversion. It corresponds to the text baseline of the lowest line on the page.113

.tm .h==\n[.h], nl==\n[nl]
    ⇒ .h==0, nl==-1
This is a test.
.sp 2
.tm .h==\n[.h], nl==\n[nl]
    ⇒ .h==40, nl==120

As implied by the example, vertical motion does not produce text baselines and thus does not increase the value interpolated by ‘\n[.h]’.

Register: \n[dn]
Register: \n[dl]

After completing a diversion, the writable registers dn and dl contain its vertical and horizontal sizes. Only the lines just processed are counted: for the computation of dn and dl, the requests da and boxa are handled as if di and box had been used, respectively—lines that have been already stored in the diversion (box) are not taken into account.

.\" Center text both horizontally and vertically.
.\" Macro .(c starts centering mode; .)c terminates it.
.\" Disable the escape character with .eo so that we
.\" don't have to double backslashes on the "\n"s.
.de (c
.  br
.  ev (c
.  evc 0
.  in 0
.  nf
.  di @c
.de )c
.  br
.  ev
.  di
.  nr @s (((\n[.t]u - \n[dn]u) / 2u) - 1v)
.  sp \n[@s]u
.  ce 1000
.  @c
.  ce 0
.  sp \n[@s]u
.  br
.  fi
.  rr @s
.  rm @c
Escape sequence: \!anything
Escape sequence: \?anything\?

Transparently embed anything into the current diversion, preventing requests, macro calls, and escape sequences from being interpreted when read into a diversion. This is useful for preventing them from taking effect until the diverted text is actually output. The \! escape sequence transparently embeds input up to and including the end of the line. The \? escape sequence transparently embeds input until its own next occurrence.

anything may not contain newlines; use \! by itself to embed newlines in a diversion. The escape sequence \? is also recognized in copy mode and turned into a single internal code; it is this code that terminates anything. Thus the following example prints 4.

.nr x 1
.di d
.nr x 2
.di e
.nr x 3
.di f
.nr x 4

Both escape sequences read the data in copy mode.

If \! is used in the top-level diversion, its argument is directly embedded into GNU troff’s intermediate output. This can be used, for example, to control a postprocessor that processes the data before it is sent to an output driver.

The \? escape used in the top-level diversion produces no output at all; its argument is simply ignored.

Request: .output contents

Emit contents directly to GNU troff’s intermediate output (subject to copy mode interpretation); this is similar to \! used at the top level. An initial neutral double quote in contents is stripped to allow embedding of leading spaces.

This request can’t be used before the first page has started—if you get an error, simply insert .br before the output request.

Use with caution! It is normally only needed for mark-up used by a postprocessor that does something with the output before sending it to the output device, filtering out contents again.

Request: .asciify div

Unformat the diversion div in a way such that Unicode basic Latin (ASCII) characters, characters translated with the trin request, space characters, and some escape sequences, that were formatted and diverted into div are treated like ordinary input characters when div is reread. Doing so can be useful in conjunction with the writem request. asciify can be also used for gross hacks; for example, the following sets register n to 1.

.tr @.
.di x
@nr n 1
.tr @@
.asciify x

asciify cannot return all items in a diversion to their source equivalent: nodes such as those produced by the \N escape sequence will remain nodes, so the result cannot be guaranteed to be a pure string. See Copy Mode. Glyph parameters such as the type face and size are not preserved; use unformat to achieve that.

Request: .unformat div

Like asciify, unformat the diversion div. However, unformat handles only tabs and spaces between words, the latter usually arising from spaces or newlines in the input. Tabs are treated as input tokens, and spaces become adjustable again. The vertical sizes of lines are not preserved, but glyph information (font, type size, space width, and so on) is retained.

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