Position margin notes relative to left margin in plain TeX

Question

I want to add margin notes in the Tufte style to my Plain TeX document. (I'm using eplain, if that matters.) I know that if I were using LaTeX I could just use tufte-latex, but as a thorough contrarian I enjoy coding these things myself.

I'm almost there but my current macro will place text relative to the current location when I invoke it, instead of the margin. In my MWE, the note is placed smack dab in the middle of things. But if you move the invocation of \sidenote around, the side note may appear closer to the left margin. How can I reposition my sidenote text relative to the left (or right) margin, instead of the current location? (I used the left margin in my MWE.)

The thing I really do like about my macro is that the baselines of the first line of the side note and the main body text to which it refers align. I want to keep that. And any other suggestions about my macro are welcome.

MWE:

\input eplain
\leftmargin=2in

\font\sidenotefont=      pxi at8.5pt

\long\def\sidenote#1{\llap{\smash{\vtop{%
\parindent=0pt\hsize=1.5in\rightskip=.87in\parfillskip=0pt\leftskip=-1in plus1fil%
\baselineskip=10pt\sidenotefont #1}}}}


When we are working at a difficult task and strive after a good thing, we are
fighting a righteous battle, the direct reward of which is that we are kept from
\sidenote{This note has been brought to you by the letter pi and
the number e. Please make a note of it. The reason for its existence is the
great lengths I went to do things.}much evil. As we advance in life it becomes more and more difficult, but in
fighting the difficulties the inmost strength of the heart is developed. Indeed
life is a have to defend and protect ourselves, and with a cheerful and brace
spirit we must battle; we plan and calculate in order to make progress.

\bye

Answer 1

If lines don't have unusual depth, this could be a starting point:

\input eplain
\leftmargin=2in

\font\mainfont=pxr at 10pt
\font\sidenotefont=pxi at 8.5pt

\long\def\sidenote#1{%
  \vadjust{\llap{\smash{\vtop{%
    \parindent=0pt
    \hsize=1.7in
    \parfillskip=0pt
    \leftskip=0pt plus1fil
    \baselineskip=10pt\sidenotefont\vglue-\ht\strutbox #1}}\kern1em}}}

\mainfont

When we are working at a difficult task and strive after a good thing, we   
are fighting a righteous battle, the direct reward of which is that we are 
kept from \sidenote{This note has been brought to you by the letter pi and 
the number e. Please make a note of it. The reason for its existence is the 
great lengths I went to do things.}much evil. As we advance in life it becomes 
more and more difficult, but in fighting the difficulties the inmost strength 
of the heart is developed. Indeed life is a have to defend and protect ourselves, 
and with a cheerful and brace spirit we must battle; we plan and calculate in 
order to make progress.

\bye

The key is \vadjust that adds the material to the enclosing vertical list.

Answer 2

After reading the appropriate exercise in the TeXBook (14.28), here is my \strut-based solution. Thanks to @Mafra for suggesting the exercise.

\def\strutdepth{\dp\strutbox}
\def\simplesidenote#1{%
  \strut\vadjust{\kern-\strutdepth\lsidenote{#1}}}
\def\lsidenote#1{\vtop to \strutdepth{%
                  \baselineskip\strutdepth%
                  \vss\llap{\vtop to 0pt{%
                      \leftskip=0pt plus1fill \rightskip=8pt%
                      \parfillskip=0pt \parindent=0pt%
                      \hsize=1.7in%
                      \eightpoint\it #1}}\null}}

Note that \eightpoint is just a macro to adjust the font size.

Answer 3

I want point out that there are problems in the previous answers and give more simple solution.

The egerg's answer includes the ad-hoc correction \vglue-\ht\strutbox. The vertical positioning of the margin note text depends on the depth of the line in the paragraph and on the height of the first line in the note text in his solution. You can try to remove all letters and characters with non-zero descenders from the second line of the paragraph in the example (i.e. g, p, comma, etc.) and you can see that the marginal note position will be bad. This bug is malicious because the code \vglue-\ht\strutbox seems that there is some reason for this. But it is not true. The \vskip-8.5pt does the same work and don't remove the accidental position of the marginal note.

The EfForEffort's answer is bad, because the \baselineskip is set to small value in the \vtop resulting the lines without equidistant baseline: the lines are leaning each against other with small vertical space 1pt which is default from the \lineskip register.

The Knuth's answer to exercise 14.28 from TeXbook (print star to the left margin using \marginstar macro):

\def\strutdepth{\dp\strutbox}
\def\marginalstar{\strut\vadjust{\kern-\strutdepth\specialstar}}
\def\specialstar{\vtop to \strutdepth{
   \baselineskip\strutdepth
   \vss\llap{* }\null}}

is overcomplicated because there is \vtop to\strutdepth followed by setting \baselineskip=\strutdepth followed by \null terminator in the \vtop. Much more simple is this solution of the 14.28 excercise:

\def\marginalstar{\strut\vadjust{\kern-\dp\strutbox\smash{\llap{* }}\kern\dp\strutbox}}

If you need more lines in the margin note:

\let\mnotefont=\tenit
\def\mnote#1{\strut\vadjust{\kern-\dp\strutbox\mnoteA{#1}\kern\dp\strutbox}}
\def\mnoteA#1{\smash{\llap{\hbox{\vtop{\mnoteB#1}\kern1em}}}}
\def\mnoteB{\hsize=1.7in \parindent=0pt \leftskip=0pt plus1fill \mnotefont}
\hoffset=1.5in \advance\hsize by-\hoffset

When we are working at a difficult task and strive after a good thing, we
are fighting a righteous battle, the direct reward of which is that we are
kept from%
\mnote{This note has been brought to you by the letter pi and the number e.
       Please make a note of it. The reason for its existence is the great
       lengths I went to do things.}
much evil. As we advance in life it becomes more and
more difficult, but in fighting the difficulties the inmost strength of the
heart is developed. Indeed life is a have to defend and protect ourselves,
and with a cheerful and brace spirit we must battle; we plan and calculate
in order to make progress.

\bye

If you are using OPmac:

\input opmac
\fixmnotes\left \mnotesize=1.7in \def\mnotehook{\typosize[8/10]\it}
\margins/1 letter (2.5,1,1,1)in

When we are working at a difficult task and strive after a good thing, we
are fighting a righteous battle, the direct reward of which is that we are
kept from%
\mnote{This note has been brought to you by the letter pi and the number e.
       Please make a note of it. The reason for its existence is the great
       lengths I went to do things.}
much evil. As we advance in life it becomes more and
more difficult, but in fighting the difficulties the inmost strength of the
heart is developed. Indeed life is a have to defend and protect ourselves,
and with a cheerful and brace spirit we must battle; we plan and calculate
in order to make progress.

\bye

Answer 4

There is a great macro written by Petr Olsak from the Czech Republic, which perhaps beats even eplain. Olsak is notorious for prefering plainTeX over LaTeX, and he wrote this macro in order to offer the power of LaTeX to plainTeX users. The name of the file (opmac.tex) actually stand for Olsak's Plaintex MACros.

http://petr.olsak.net/opmac-e.html

Documentation is available in English, too. After installing and \input opmac you can just type \mnote{your text in margin} and it will place it accordingly, in the left margin on odd pages, in the right margins on even ones, unless you have reason to have them on the same side of all pages. Everything can be setup, as you'll learn in the documentation.