Preformatted text using ConTeXt's \startlines

Question

This is a follow-up question to

• ConTeXt equivalent to LaTeX's \linebreak with full justification
• Include original page images for side-by-side comparison + SyncTex

I am using the results of pdf text extraction to generate source files for reproducing a (poublic domain) book from its pdf, while trying to retain as much of the original layout as possible, especially linebreaks. Latex has worked fine for this so far, but I'm looking into ConTeXt as an alternative, partly out of curiousity.

So far I've manually inserted \linebreak (in LaTeX) or \break (in ConTeXt) to force the line-breaking algorithm to adhere to the original line breaks, but reading through the documentation I've come across \startlines which suggest I don't have to litter the text with so much cruft.

The documentation describes the inbetween argument as:

command that is expanded and then placed between each line and the next

But the following MWE doesn't behave as expected:

\starttext
\setuplines[inbetween=\break]
\startlines
First line --- should be fully justified
Secondline --- should be fully justified
\stoplines
\stoptext

In fact, substituting some marker test as the inbetween argument suggests that the inbetween code is inserted only on paragraph breaks:

\starttext
\setuplines[inbetween=FOO]
\startlines
First line
Secondline
\stoplines
\stoptext

An additional concern is that the contents may contain multiple paragraphs, so that a blank line (now appended with a \break) should not force a new page.

Edit 1: Ideally I should be able to use something like:

\setupindenting[yes,0.1in]
\startX
first line of first paragraph, whose end becomes a tex linebreak
second line of first paragraph, whose end becomes a tex linebreak
third line, left unjustified.
% parbreak
first line of second paragraph...
\stopX

Similar to the way HTML's <pre> tag works.

Answer 1

As explained in Metafox's answer to your previous question, you can use the paragraph alignment to achieve this effect. For example,

\showframe % to visualize the page
\starttext
\setuplines[align=paragraph]

\startlines
First line --- should be fully justified
Secondline --- should be fully justified
\stoplines

\stoptext

gives

Edit Here is one way to get \startlines to obey \parindent (rather fake \parindent).

\setuppapersize[A5]

\definemeasure[indent][20pt]

\setupindenting[\measure{indent},yes]

\define\ResetIndent
    {\let\Indent\FirstIndent}

\define\FirstIndent
    {\hskip\measure{indent}%
     \let\Indent\relax}

\let\Indent\FirstIndent

\setuplines
  [
    align=paragraph, 
    command={\Indent},
    inbetween={\blank\ResetIndent},
    option=packed,
  ]


\starttext
\input knuth

\startlines
Thus, I came to the conclusion that the designer of a 
new system must not only be the implementer and first 
large--scale user; the designer should also write the first 
user manual. \crlf

The separation of any of these four components would
have hurt TEX significantly. If I had not participated fully
in all these activities, literally hundreds of improvements
would never have been made, because I would never have
thought of them or perceived why they were important. \crlf

But a system cannot be successful if it is too strongly
influenced by a single person. Once the initial design is
complete and fairly robust, the real test begins as people
with many different viewpoints undertake their own exper-
iments. \crlf
\stoplines

\stoptext

Some comments:

• I use \definemeasure and \measure{..} to use the same value for both setting indentation and using indentation in the lines environment.

• option=packed ensures that there is no page break inside the lines environment. So you will have to Add each page in a separate lines environment.

• I have manually added \crlf on the last line of each paragraph. If you don't want to manually add \crlf at the end of each paragraph, then one option is to capture the content of the environment as a buffer, and algorithmically add \crlf at the end of each paragraph. See, for example, this example in the wiki.

Answer 2

Following up on @Aditya LuaTeX reference, here is an alternative solution based on lua:

\setuppapersize[A5]
\setupindenting[0.2in,yes]

% Create an environment that stores everything 
% between \startpreformatted and \stoppreformatted 
% in a buffer named 'preformatted'.
\def\startpreformatted
{\dostartbuffer
    [preformatted]
    [startpreformatted]
    [stoppreformatted]}

% On closing the preformatted environment, call the LuaTeX
% function preformatted(), and pass it the contents of 
% the buffer called 'preformatted'
\def\stoppreformatted
{\ctxlua
    {userdata.preformatted(buffers.getcontent('preformatted'))}}

\startluacode
userdata = userdata or {}

function userdata.isemptyline(line)
    return nil ~= string.find(line,'^ *$' )
end

function userdata.preformatted(content)
    local lines    = string.splitlines(content)
    for i=1,#lines do
        if i < #lines then
            if (not userdata.isemptyline(lines[i+1])) and (not userdata.isemptyline(lines[i])) then 
                lines[i] = lines[i]  .. "\\break"    
            end     
        -- else do something special for last line
        end  
    end

    content = table.concat(lines,'\n') 
    context(content) 
end
\stopluacode

\starttext
\startpreformatted 
Thus, I came to the conclusion that the designer of a 
new system must not only be the implementer and first 
large--scale user; the designer should also write the first 
user manual.  

The separation of any of these four components would
have hurt TEX significantly. If I had not participated fully
in all these activities, literally hundreds of improvements
would never have been made, because I would never have
thought of them or perceived why they were important. 
\stoppreformatted  

\stoptext

Result: