.wh
dist [name] ¶Call macro name when the vertical position dist on the page
is reached or passed in the downward direction. The default scaling
unit is ‘v’. Non-negative values for dist set the trap
relative to the top of the page; negative values set the trap relative
to the bottom of the page. An existing visible trap (see below)
at dist is removed; this is wh
’s sole function if
name is missing.
A trap is sprung only if it is visible, meaning that its location is reachable on the page84 and it is not hidden by another trap at the same location already planted there.
An example of how a macro package might set headers and footers follows.
.de hd \" page header ' sp .5i . tl '\\*[Title]''\\*[Date]' ' sp .3i .. . .de fo \" page footer ' sp 1v . tl ''%'' ' bp .. . .wh 0 hd \" trap at top of the page .wh -1i fo \" trap one inch from bottom
A trap above the top or at or below the bottom of the page can be made
visible by either moving it into the page area or increasing the page
length so that the trap is on the page. Negative trap values always use
the current page length; they are not converted to an absolute
vertical position.
We can use the ptr
request to dump our page location traps to the
standard error stream (see Debugging). Their positions are reported
in basic units appropriate to the device; an nroff
device example
follows.
.pl 5i .wh -1i xx .ptr error→ xx -240 .pl 100i .ptr error→ xx -240
It is possible to have more than one trap at the same location (although
only one at a time can be visible); to achieve this, the traps must be
defined at different locations, then moved to the same place with the
ch
request. In the following example, the many empty lines
caused by the bp
request are not shown in the output.
.de a . nop a .. .de b . nop b .. .de c . nop c .. . .wh 1i a .wh 2i b .wh 3i c .bp ⇒ a b c
.ch b 1i .ch c 1i .bp ⇒ a
.ch a 0.5i .bp ⇒ a b
\n[.t]
¶The read-only register .t
holds the distance to the next vertical
position trap. If there are no traps between the current position and
the bottom of the page, it contains the distance to the page bottom.
Within a diversion, in the absence of a diversion trap, this distance is
the largest representable integer in basic units—effectively infinite.
.ch
name [dist] ¶Change the location of a trap by moving macro name to new location
dist, or by unplanting it altogether if dist is absent. The
default scaling unit is ‘v’. Parameters to ch
are specified
in the opposite order from wh
. If name is the earliest
planted macro of multiple traps at the same location, (re)moving it from
that location exposes the macro next least recently planted at the same
place.85
Changing a trap’s location is useful for building up footnotes in a diversion to allow more space at the bottom of the page for them.
The same macro can be installed simultaneously at multiple locations;
however, only the earliest-planted instance—that has not yet been
deleted with wh
—will be moved by ch
. The following
example (using an nroff
device) illustrates this
behavior.86 Blank lines have been elided from the output.
.de T Trap sprung at \\n(nlu. .br .. .wh 1i T .wh 2i T foo .sp 11i .bp .ch T 4i bar .sp 11i .bp .ch T 5i baz .sp 11i .bp .wh 5i .ch T 6i qux .sp 11i
⇒ foo ⇒ Trap sprung at 240u. ⇒ Trap sprung at 480u. ⇒ bar ⇒ Trap sprung at 480u. ⇒ Trap sprung at 960u. ⇒ baz ⇒ Trap sprung at 480u. ⇒ Trap sprung at 1200u. ⇒ qux ⇒ Trap sprung at 1440u.
\n[.ne]
¶The read-only register .ne
contains the amount of space that was
needed in the last ne
request that caused a trap to be sprung;
it is useful in conjunction with the .trunc
register. See Page Control.
Since the .ne
register is set only by traps, it doesn’t make
sense to interpolate it outside of macros called by traps.
\n[.trunc]
¶A read-only register containing the amount of vertical space truncated
from an sp
request by the most recently sprung vertical
position trap, or, if the trap was sprung by an ne
request,
minus the amount of vertical motion produced by the ne
request. In other words, at the point a trap is sprung, it
represents the difference of what the vertical position would have
been but for the trap, and what the vertical position actually is.
Since the .trunc
register is set only by traps, it doesn’t make
sense to interpolate it outside of macros called by traps.
\n[.pe]
¶A read-only register containing 1 while a page is being ejected
with the bp
request (or by the end of input), and
0 otherwise.
In the following example, only the second call to x
is
caused by bp
.
.de x \&.pe=\\n[.pe] .br .. .wh 1v x .wh 4v x A line. .br Another line. .br ⇒ A line. .pe=0 Another line. .pe=1
An important fact to consider while designing macros is that diversions
and traps do not interact normally. For example, if a trap calls a
header macro (while outputting a diversion) that tries to change the
font on the current page, the effect is not visible before the diversion
has completely been printed (except for input protected with \!
or \?
) since the data in the diversion is already formatted. In
most cases, this is not the expected behaviour.