[NTG-context] How to make something appear to the right of every paragraph?

[Max Chernoff]

Hi Joel,
> When I use the code given, it compiles and displays fine. But when I
> try replacing \everypar with \EveryPar, it halts during compiling

It looks like \EveryPar is a macro and not a token list.

> These both work great, but do that for the whole document? Is there a
> way to restrict it to only apply the lines to some parts of the file,
> not every single paragraph?

Option 1:

   \newif\ifprintlines
   
   \EveryPar{%
       \ifprintlines%
           \vbox to 0pt{%
               \dorecurse{3}{%
                   \rlap{%
                       \hskip\dimexpr\hsize+1em%
                       \vrule height 0.4pt width 3cm%
                       \relax%
                   }%
               }%
           }%
       \fi%
   }
   
   \parskip=\baselineskip
   
   \starttext
       \printlinestrue
       One line paragraph
   
       Two line paragraph \\
       Two line paragraph
   
       Three line paragraph \\
       Three line paragraph \\
       Three line paragraph
       \printlinesfalse
   
       Four line paragraph \\
       Four line paragraph \\
       Four line paragraph \\
       Four line paragraph
   
       \samplefile{bryson}
   
       \printlinestrue
       \samplefile{knuth}
   \stoptext

Option 2:

   \startluacode
       -- Constants
       local RULE_OFFSET    = tex.sp "1em"
       local RULE_THICKNESS = tex.sp "0.4pt"
       local RULE_LENGTH    = tex.sp "3cm"
   
       local show_lines = true
   
       -- Callback
       function userdata.lines(head)
           if status.output_active or
              tex.nest.ptr > 1 or
              not show_lines
           then
               return head
           end
   
           local i = 0
           for n in node.traverseid(node.id "hlist", head) do
               i = i + 1
               if i > 3 then
                   break
               end
   
               local offset = node.new "glue"
               offset.width = RULE_OFFSET
               node.slide(n.list).next = offset
   
               local rule = node.new "rule"
               rule.width = RULE_LENGTH
               rule.height = RULE_THICKNESS
               rule.depth = 0
               offset.next = rule
           end
   
           return head
       end
   
       nodes.tasks.appendaction(
           "finalizers",
           "after",
           "userdata.lines"
       )
   
       interfaces.implement {
           name      = "showlines",
           public    = true,
           arguments = { "boolean" },
           actions   = function(x)
               show_lines = x
           end,
       }
   \stopluacode
   
   \parskip=\baselineskip
   
   \starttext
       \showlines true
       One line paragraph
   
       Two line paragraph \\
       Two line paragraph
   
       Three line paragraph \\
       Three line paragraph \\
       Three line paragraph
   
       \showlines false
       Four line paragraph \\
       Four line paragraph \\
       Four line paragraph \\
       Four line paragraph
   
       \samplefile{bryson}
   
       \showlines true
       \samplefile{knuth}
   \stoptext

These are both kind of hacky though. I'd recommend that you use Hans's
solution instead:

   \starttext
   
   \def\StartHack
     {\margintext
       [location=right,style=]
       {\thinrules[n=3]}}
   
   \def\StopHack
     {\par \ifnum\prevgraf<3
        \blank[\the\numexpr4-\prevgraf\relax*line]
      \fi}
   
   \StartHack \input tufte \StopHack
   
   \StartHack test test \StopHack
   
   \StartHack test test \StopHack
   
   \stoptext

Thanks,
-- Max