gbranden pushed a commit to branch master
in repository groff.
commit beb5be8127a0fcd26f85851bd5d6eabac11d92ba
Author: G. Branden Robinson <[email protected]>
AuthorDate: Mon Mar 7 12:29:47 2022 +1100
doc/groff.texi: Revise discussion of `\B' escape.
Clarify that it interpolates a value. Add example of usage. Help
reader avoid having to leap forward to a discussion of register formats.
Clarify: it is not the "entire expression" that needs to be surrounded
by parentheses to permit use of spaces, just the (sub)expression that
contains them.
Add example of (formally) complex numeric expressions.
---
doc/groff.texi | 40 +++++++++++++++++++++++++++++++++-------
1 file changed, 33 insertions(+), 7 deletions(-)
diff --git a/doc/groff.texi b/doc/groff.texi
index dd96bdd3..280a9838 100644
--- a/doc/groff.texi
+++ b/doc/groff.texi
@@ -5950,24 +5950,50 @@ In the foregoing example, we've used the @code{\Z}
escape sequence
(@pxref{Page Motions}) to restore the drawing position after formatting
@samp{C}, then moved vertically to the first text baseline on the page.
-@codequotebacktick off
-@codequoteundirected off
-
@Defesc {\\B, @code{'}, anything, @code{'}}
@cindex numeric expression, valid
@cindex valid numeric expression
-Return@tie{}1 if @var{anything} is a valid numeric expression; or@tie{}0
-if @var{anything} is empty or not a valid numeric expression.
+Interpolate@tie{}1 if @var{anything} is a valid numeric expression,
+and@tie{}0 otherwise. You might use this escape sequence along with the
+@code{if} request to filter out invalid macro or string arguments.
+@xref{Conditionals and Loops}.
@endDefesc
+@Example
+.\" Indent by amount given in first argument; assume ens.
+.de Indent
+. if \B'\\$1' .in (n;\\$1)
+..
+@endExample
+
A register interpolation used as an operand in an expression must have
-an Arabic format; see @ref{Assigning Register Formats}.
+an Arabic format; luckily, this is the default. @xref{Assigning
+Register Formats}.
@cindex space characters, in expressions
@cindex expressions, and space characters
Due to the way arguments are parsed, spaces are not allowed in
-expressions unless the entire expression is surrounded by parentheses.
+expressions unless the (sub)expression containing them is surrounded by
+parentheses.
@xref{Request and Macro Arguments}, and @ref{Conditionals and Loops}.
+@Example
+.nf
+.nr a 1+2 + 2+1
+\na
+ @error{} expected numeric expression, got a space
+ @result{} 3
+.nr a 1+(2 + 2)+1
+\na
+ @result{} 6
+@endExample
+
+The @code{nr} request (@pxref{Setting Registers}) expects up to two
+numeric expressions as arguments; a bare @samp{+} does not qualify, so
+our first attempt got a warning.
+
+@codequotebacktick off
+@codequoteundirected off
+
@c =====================================================================
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