Re: eqn(7) "delim" is undocumented (maybe)

[Ingo Schwarze <schwarze@usta.de>]

Hi Anthony,

Anthony J. Bentley wrote on Sun, Dec 08, 2019 at 01:42:37AM -0700:

> I can't find anyplace that documents eqn(7)'s "delim" statement

Brian W. Kernighan and Lorinda L. Cherry:
Typesetting Mathematics - User's Guide (Second Edition)
August 15, 1978
page 8, chapter 19: Shorthand for In-line Equations

> (even groff's eqn(1) doesn't really).

I think you have a point.

> This is widely used in the X11 manuals
> and others for inline math.

Definitely.

> I gave a shot at documenting it in the diff below... of course, open to
> improvements in both correctness and clarity.
> 
> As an aside, I expected these two examples to render the same, but they
> don't. Should they look the same, or am I misunderstanding how eqn(7)
> works?
> 
> .\" "delim off sin(<pi>)"
> .EQ
> delim $$
> delim off
> .EN
> $sin { pi }$

On first sight, that looks like a bug in mandoc.
I need to have a closer look.


Regarding your documentation patch, i'd like to do it slightly
differently.

 - Let's avoid the term "inline equation".  Conventionally, an
   equation is called "inline" when it appears in the middle of
   an *output* line.  The opposite is a displayed equation.
   But here, we are talking about an equation in the middle of
   a (text) *input* line.
   In mdoc(7) and man(7), .EQ/.EN vs. $$ has nothing to do with
   inline vs. displayed.  Both input forms create inline equations
   by default.  Both forms can be wrapped in display macros, for
   example .Bd -unfilled, to achieve a display.
   Admittelly, Kernighan/Cherry also talked about "in-line equations".
   But that happened only because they focussed on the context of
   the -ms macros, and for -ms, .EQ/.EN does indeed generate a
   display by default.  However, even in that context, they were
   careful enough to not call $$ an "in-line equation" outright,
   but rather a "shorthand for in-line equations".

 - The beginning of the eqn(7) manual page is a mess.  The second
   and third paragraphs say about the same, and the non-standard
   .Sh macro between them is completely useless.  So let's delete
   the section header, combine the two paragraphs, and add the
   new information to it.

 - I don't see the point in ".D1 Cm delim Ar param".
   It says nothing new: the fact the "delim" needs to take an
   argument is already obvious from the verb "set".

 - But i would like to stress the conventional form "delim $$"
   by giving an explicit example.

OK?
  Ingo


Index: eqn.7
===================================================================
RCS file: /cvs/src/share/man/man7/eqn.7,v
retrieving revision 1.11
diff -u -p -r1.11 eqn.7
--- eqn.7	23 Apr 2019 17:52:12 -0000	1.11
+++ eqn.7	7 Jan 2020 18:27:00 -0000
@@ -44,28 +44,16 @@ specification (see
 .Sx SEE ALSO
 for references).
 .Pp
-Equations within
-.Xr mdoc 7
-or
-.Xr man 7
-documents are enclosed by the standalone
-.Sq \&.EQ
-and
-.Sq \&.EN
-tags.
-Equations are multi-line blocks consisting of formulas and control
-statements.
-.Sh EQUATION STRUCTURE
-Each equation is bracketed by
-.Sq \&.EQ
-and
-.Sq \&.EN
-strings.
-.Em Note :
-these are not the same as
-.Xr roff 7
-macros, and may only be invoked as
-.Sq \&.EQ .
+An equation starts with an input line containing exactly the characters
+.Sq \&.EQ ,
+may contain multiple input lines, and ends with an input line
+containing exactly the characters
+.Sq \&.EN .
+Equivalently, an equation can be given in the middle of a single
+text input line by surrounding it with the equation delimiters
+defined with the
+.Cm delim
+statement.
 .Pp
 The equation grammar is as follows, where quoted strings are
 case-sensitive literals in the input:
@@ -178,6 +166,25 @@ statement is a synonym for
 while
 .Cm tdefine
 is discarded.
+.It Cm delim
+This statement takes a string argument consisting of two bytes,
+to be used as the opening and closing delimiters for equations
+in the middle of text input lines.
+Conventionally, the dollar sign is used for both delimiters,
+as follows:
+.Bd -literal -offset indent
+\&.EQ
+delim $$
+\&.EN
+An equation like $sin pi = 0$ can now be entered
+in the middle of a text input line.
+.Ed
+.Pp
+The special statement
+.Cm delim off
+temporarily disables previously declared delimiters and
+.Cm delim on
+reenables them.
 .It Cm gfont
 Set the default font of subsequent output.
 Its syntax is as follows:
--
 To unsubscribe send an email to tech+unsubscribe@mandoc.bsd.lv