[unix-history] / usr / doc / trofftut / tt13

.NH
Environments
.PP
As we mentioned, there is a potential problem
when going across a page boundary:
parameters like size and font
for a page title may well be different from those
in effect in the text when the page boundary occurs.
.UL troff
provides a very general way to deal with this and
similar situations.
There are three `environments',
each of which has independently settable versions of
many of the parameters associated with processing,
including size, font, line and title lengths,
fill/nofill mode, tab stops, and even partially collected lines.
Thus the titling problem may be readily solved by processing the main text
in one environment and titles in a separate one
with its own suitable parameters.
.PP
The command
.BD .ev\ n
shifts to environment
.BD n ;
.BD n
must be 0, 1 or 2.
The command
.BD .ev
with no argument returns to the
previous environment.
Environment names are maintained in a stack, so calls
for different environments may be nested and unwound consistently.
.PP
Suppose we say that the main text is processed in environment 0,
which is where 
.UL troff
begins by default.
Then we can modify the new page macro
.BD .NP
to process titles in environment 1 like this:
.P1 2
^de NP
^ev 1	\e" shift to new environment
^lt 6i	\e" set parameters here
^ft R
^ps 10
\&... any other processing ...
^ev	\e" return to previous environment
^^
.P2
It is also possible to initialize the parameters for an environment
outside the
.BD .NP
macro,
but the version shown keeps all the processing in one place
and is thus easier to understand and change.
Commit	Line	Data
8340f87c BJ	1	.NH
	2	Environments
	3	.PP
	4	As we mentioned, there is a potential problem
	5	when going across a page boundary:
	6	parameters like size and font
	7	for a page title may well be different from those
	8	in effect in the text when the page boundary occurs.
	9	.UL troff
	10	provides a very general way to deal with this and
	11	similar situations.
	12	There are three `environments',
	13	each of which has independently settable versions of
	14	many of the parameters associated with processing,
	15	including size, font, line and title lengths,
	16	fill/nofill mode, tab stops, and even partially collected lines.
	17	Thus the titling problem may be readily solved by processing the main text
	18	in one environment and titles in a separate one
	19	with its own suitable parameters.
	20	.PP
	21	The command
	22	.BD .ev\ n
	23	shifts to environment
	24	.BD n ;
	25	.BD n
	26	must be 0, 1 or 2.
	27	The command
	28	.BD .ev
	29	with no argument returns to the
	30	previous environment.
	31	Environment names are maintained in a stack, so calls
	32	for different environments may be nested and unwound consistently.
	33	.PP
	34	Suppose we say that the main text is processed in environment 0,
	35	which is where
	36	.UL troff
	37	begins by default.
	38	Then we can modify the new page macro
	39	.BD .NP
	40	to process titles in environment 1 like this:
	41	.P1 2
	42	^de NP
	43	^ev 1 \e" shift to new environment
	44	^lt 6i \e" set parameters here
	45	^ft R
	46	^ps 10
	47	\&... any other processing ...
	48	^ev \e" return to previous environment
	49	^^
	50	.P2
	51	It is also possible to initialize the parameters for an environment
	52	outside the
	53	.BD .NP
	54	macro,
	55	but the version shown keeps all the processing in one place
	56	and is thus easier to understand and change.