[TUHS] troff environments, traps, and diversions (was: TeX and groff)

[Douglas McIlroy]

Branden,

I like these intro paragraphs and the elision of the ominous
adjective, "advanced".

>  An environment is created for the footnote so that it is set at a smaller typeface.

To avoid implying that one must create an environment with each
footnote and to illustrate that environments are collections of
settings, you might say something like this

> Switch to a previously created footnote environment with a smaller typeface and distinctive line length.

Doug

On Fri, Jan 21, 2022 at 9:34 PM Branden Robinson
<g.branden.robinson at gmail.com> wrote:
>
> On Tue, Jan 11, 2022 at 1:25 PM Lyndon Nerenberg (VE7TFX/VE6BBM)
> <lyndon at orthanc.ca> wrote:
> > A lot of people get turned off by how troff markup can often look like
> > line noise.  That's true, but if you spend the time to actually learn
> > the syntax (and it's really not that hard), you can't help but be
> > overwhelmed by the beauty of its self-consistency.  Although after
> > three decades I still can't wrap my head around traps and diversions
> > :-P
>
> It seems like a lot of people get stuck on the dread trio of traps,
> diversions, and environments.  Some old groff documentation did not, I
> think, help matters very much by characterizing them as "advanced" and
> particularly by comparing diversions to pointers in C.  I've been
> rewriting a lot of groff's documentation over the past five years.
>
> Here's my attempt to introduce these 3 concepts in the groff(7) page of
> the forthcoming 1.23 release.  Let me know how I can improve it.  (I
> retiterate that it's just an introduction--there is much more detail
> about all three later in the page and in groff's Texinfo manual, much of
> which has parallel content to its man pages.)
>
>        A further few language elements arise as page layouts become more
>        sophisticated and demanding.  Environments collect formatting
>        parameters like line length and typeface.  A diversion stores
>        formatted output for later use.  A trap is a condition on the
>        input or output, tested automatically by the formatter, that is
>        associated with a macro, causing it to be called when that
>        condition is fulfilled.
>
>        Footnote support often exercises all three of the foregoing
>        features.  A simple implementation might work as follows.  A pair
>        of macros is defined: one starts a footnote and the other ends
>        it.  The author calls the first macro where a footnote marker is
>        desired.  The macro establishes a diversion so that the footnote
>        text is collected at the place in the body text where its
>        corresponding marker appears.  An environment is created for the
>        footnote so that it is set at a smaller typeface.  The footnote
>        text is formatted in the diversion using that environment, but it
>        does not yet appear in the output.  The document author calls the
>        footnote end macro, which returns to the previous environment and
>        ends the diversion.  Later, after much more body text in the
>        document, a trap, set a small distance above the page bottom, is
>        sprung.  The macro called by the trap draws a line across the
>        page and emits the stored diversion.  Thus, the footnote is
>        rendered.
>
> Regards,
> Branden
> Foo
>
> On Tue, Jan 11, 2022 at 1:25 PM Lyndon Nerenberg (VE7TFX/VE6BBM) <lyndon at orthanc.ca> wrote:
>>
>> Dan Cross writes:
>>
>> > This is interesting; I've always felt like I could pick out troff pretty
>> > readily; I agree that TeX has a certain "look" to it (at least by default),
>> > but I always felt the same about troff as well.
>>
>> My guess this is more about how ms(7) does page layout.  I can spot
>> those documents from a mile away :-)
>>
>> But older versions of troff can often be spotted by how box corners
>> don't always line up properly.
>>
>> A lot of people get turned off by how troff markup can often look
>> like line noise.  That's true, but if you spend the time to actually
>> learn the syntax (and it's really not that hard), you can't help
>> but be overwhelmed by the beauty of its self-consistency.  Although
>> after three decades I still can't wrap my head around traps and
>> diversions :-P
>>
>> --lyndon