Using indentation to automatically begin and end itemize environments

Question

This is a question one might answer with "Why the hell would you want to do it?!", sort of an experiment.

What I am looking for is a way to write lists with many levels of nestings (notes for school) in a "natural" way. Previously, I used an elaborate system of Pandoc, LaTeX and Makefiles to generate notes with occasional LaTeX snippets, however, I would like a more integrated workflow (for example, embedding pseudocode in a codebox environment in a Pandoc/Markdown file is quite challenging, as Markdown was designed with HTML in mind).

If I make + an active character, like

\makeatletter
\mathchardef\@my@mathplus=\mathcode`+
\catcode`+=\active
\def{+}{\ifmmode\@my@mathplus\else\item\fi}
\mateatother

I can write itemizes like

\begin{itemize}
+ one
+ two
+ three
\end{itemize}

however, I would like to get rid of \begin{itemize} and \end{itemize} and write notes like

+ one
    + subitem one
    + subitem two
+ two
+ three

resulting in the nested list, neatly rendered.

Is there a way to achieve this, or I should consider some kind of pre-procession instead? (Creative and mildly insane answers are appreciated.)

Answer 1

Insane answer to an insane question ;)

\documentclass{article}

\def\+{+}

\makeatletter
\catcode`\ =12\let\@nl@space= \catcode`\ =10
\newcount\@nl@rlevel
\newcount\@nl@llevel
\@nl@llevel=-1

\def\@nl{%
  \catcode`\ =12
  \global\@nl@rlevel=0
  \futurelet\@nl@store\@nl@%
}
\def\@nl@gobble#1{\futurelet\@nl@store\@nl@}
\def\@nl@enditemize{
  \ifnum\the\@nl@rlevel<\the\@nl@llevel%
    \end{itemize}%
    \egroup%
    \expandafter\@nl@enditemize%
  \else%
    \ifnum\the\@nl@rlevel=\the\@nl@llevel\else%
       \errmessage{Error: inconsistent identation}
    \fi%
  \fi%
}
\def\@nl@{%
  \ifx\@nl@store\@nl@space%
    \global\advance\@nl@rlevel by 1
    \expandafter\@nl@gobble%
  \else%
    \catcode`\ =10
    \ifx\@nl@store+%
      \ifnum\the\@nl@rlevel>\the\@nl@llevel%
        \bgroup%
        \@nl@llevel=\the\@nl@rlevel
        \begin{itemize}%
      \fi%
      \@nl@enditemize%
      \item \expandafter\expandafter\expandafter\@gobble%
    \else%
      \ifx\@nl@store\@nl%
        \global\@nl@rlevel=-1\relax\@nl@enditemize\par
      \else\space\fi%
    \fi%
  \fi%
}

\catcode`\^^M=\active%
\AtBeginDocument{%
  \catcode`\^^M=\active%
  \let^^M=\@nl%
}%
\catcode`\^^M=5
\makeatother

\begin{document}
Some sample text.

 + foo
 + bar
   + a
   + b
     + c. A very long line
       split into multiple lines.
 + hehe

Just like nothing happened. Ha!

\+ escaped starting plus

\[\lim_{x\to\infty}\frac1x = 0\]
\end{document}

Answer 2

You may need to redefine ^^M to get this feature. That will be somewhat complicated.

As a suggestion, you may use ++ for a subitem, and +++ for a subsubitem. That's much easier:

\documentclass{article}

\makeatletter
\catcode`\+\active
\def\itemX{\@ifnextchar+{\subitemX}{\item}}
\def\subitemX#1{\@ifnextchar+{\subsubitemX}{\subitem}}
\def\subsubitemX#1{\subsubitem}
\def+{\ifmmode\string+\else\expandafter\itemX\fi}
\@makeother\+
\makeatother

\newenvironment{easylist}{\trivlist\item
  \def\item{\par\noindent\textbullet\enspace}%
  \def\subitem{\par\noindent\quad\textasteriskcentered\enspace\ignorespaces}%
  \def\subsubitem{\par\noindent\qquad-\enspace\ignorespaces}%
  \catcode`\+\active
}{\endtrivlist}

\begin{document}

\begin{easylist}
+ foo $a+b$
+ foo 
++ bar
++ bar
+++ baz
+++ baz
+ foo
\end{easylist}

\end{document}

See also: nicetext package; pandoc tool.

Answer 3

This is a bit different rather use two commands, one to narrow the paragraph and another to widen it. I have used \i, \w. The first one to make the paragraph narrower (i.e indent but lose the dotless i and the second the \w for wider.

\documentclass{article}
\usepackage{lipsum}
\parskip12pt
\def\narrower{\advance\leftskip by\parindent
\advance\rightskip by\parindent}
\def\wider{\advance\leftskip by-\parindent
\advance\rightskip by-\parindent}


\def\w{\wider}
\def\i{\narrower}

\begin{document}
   \lipsum[1]
   \i \lipsum[2] 
   \i \lipsum[2]
   \i \lipsum[3]
   \w \lipsum[4]
   \w \lipsum[5]
   \w \lipsum[6]
\end{document}

Advantages, never type more than is necessary! Style to suit!