Re: Explanations with an EQN User Guide

[G. Branden Robinson]

Hi Doug,

At 2023-05-22T20:04:53-0400, Douglas McIlroy wrote:
> > Basically, [if] a tab occurs within braces, it will be rejected.

This statement of mine was, alas, too general.  A tab within braces will
be rejected (diagnosed and ignored) if the braces are necessary to
disambiguate the input.

> Running eqn standalone, I got identical outputs from these
> two inputs
> 
> .EQ
> a<tab>b
> .EN
> 
> .EQ
> {a<tab>b}
> .EN

So do I.  But I think I know why.

Try the following slightly more complex input:

(Interpret all sequences of more than one space as a tab character.)

.EQ
{ a     b }
.EN
.sp
.EQ
sqrt { a        b }
.EN

Like parentheses in the expression "(2+3)", there was no need to push
anything onto an evaluation stack because you were already at the top
level.  For the same reason, you can't trick eqn into complaining about
the nested tab character with:

.EQ
{ { { { a       b } } } }
.EN

> The tab is passed to groff as \t\,

Yes--but _inside a string definition_ (technically an appendment, a rare
word that I have impressed into service in groff documentation).

> man 7 groff says \t is "uninterpreted",

groff(7) on Git HEAD hasn't said this for a while.  Its mentions of "\t"
are as follows.

    The following escape sequences don't take arguments and thus are
    allowed as delimiters: \space, \%, \|, \^, \{, \}, \', \`, \-, \_,
    \!, \?, \), \/, \,, \&, \:, \~, \0, \a, \c, \d, \e, \E, \p, \r, \t,
    and \u.  However, using them this way is discouraged; they can make
    the input confusing to read.

    The escape sequences \", \#, \$, \*, \?, \a, \e, \n, \t, \g, \V, and
    \newline are interpreted even in copy mode.

    \t  Interpolate a tab in copy mode.

> yet the tab skips to a tab stop set by .ta.

Because once the string containing is interpolated, it transforms into a
"real" tab.

> This leaves me totally confused, because groff apparently ignores \t
> when it doesn't come from eqn.

You can do the same thing eqn is doing.

.ds st foo\tbar
.tc *
\*(st

I changed the tab fill character to an asterisk only to show that
nothing is up my sleeve.  This works the same for me in DWB 3.3,
Heirloom Doctools, and groff.

Regards,
Branden

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