LaTeX: An extension to easylist, where an arbitrary symbol can be used as list starters

tceasylist.sty

%
% tceasylist.sty
%
%% package declaratoin
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{tceasylist}[2014/09/15 v0.2]

%% preparations
\def\tcel@pkgname{tceasylist}
\def\tcel@error{\PackageError\tcel@pkgname}
\def\tcel@warn{\PackageWarningNoLine\tcel@pkgname}
\newif\iftcel@ok
\newif\iftcel@unicode
\providecommand*\bxDebug[1]{}

%---------------------------------------  helpers

%% unique toens
\def\tcel@mt{\tcel@mt@}
\def\tcel@mark{\tcel@mark@}
\def\tcel@end{\tcel@end@}

%% Set switch tcel@unicode, which means whether Unicode codespace
% is available.
\begingroup
  \def\tcel@check#1#2{%
    \edef\tcel@tmpa{\string#1}%
    \edef\tcel@tmpb{\meaning#1}%
    \ifx\tcel@tmpa\tcel@tmpb #2\fi
  }
  \tcel@check\XeTeXversion{\global\tcel@unicodetrue}
  \tcel@check\luatexversion{\global\tcel@unicodetrue}
\endgroup

%% \tcel@bs : a backslash
\edef\tcel@bs{\expandafter\@gobble\string\\}

%% \tcel@detokenize\CS
% Update a token-list macro \CS with the detokenized list.
\def\tcel@detokenize#1{%
  \expandafter\tcel@detokenize@a\meaning#1\tcel@end#1%
}
\def\tcel@detokenize@a#1->#2\tcel@end#3{\def#3{#2}}

%% \tcel@check@single{<arg>}\CSa\CSb
% If <arg> is a single char then call \CSa with that char;
% if <arg> is a single cs then call \CSb with that cs;
% otherwise do nothing.
\def\tcel@check@single#1#2#3{%
  \def\tcel@tmpa{#1}%
  \let\tcel@next\tcel@tmpa \tcel@detokenize\tcel@next
  \def\tcel@tmpb{#2}%
  \def\tcel@tmpc{#3}%
  \expandafter\tcel@check@single@a\tcel@next\relax\relax\tcel@end
}
\def\tcel@check@single@a#1#2#3\tcel@end{%
  \if#1\tcel@bs % starting with a cs
    \expandafter\tcel@check@single@b\tcel@tmpa\tcel@end
  \else\ifx#1\relax % no-op
  \else\ifx#2\relax % single char
    \tcel@tmpb#1%
  \fi\fi\fi
}
\def\tcel@check@single@b#1#2\tcel@end{%
  \ifx\tcel@mt#2\tcel@mt % single cs
    \tcel@tmpc#1%
  \fi
}

%% \tcel@check@hexnum{<arg>}
% Parse <arg> as a hex number and set the result to \@tempcnta;
% if failed the result is negative.
\def\tcel@check@hexnum#1{%
  \def\tcel@tmpa{#1}\tcel@detokenize\tcel@tmpa
  \ifx\tcel@tmpa\@empty\else
    \edef\tcel@next{%
      \uppercase{\noexpand\tcel@check@hexnum@a{\tcel@tmpa}}}%
    \tcel@next
  \fi
}
\def\tcel@check@hexnum@a#1{%
  \afterassignment\tcel@check@hexnum@b
   \@tempcnta\string"0#1\relax
}
\def\tcel@check@hexnum@b#1\relax{%
  \ifx\tcel@mt#1\tcel@mt\else
    \@tempcnta\m@ne
  \fi
}

%% \tcel@get@char@tokens{<number>}
% Make char token with the given code value and assign them
% to token-list macros; \tcel@char@A holds an active char
% and \tcel@char@N holds a non-active char.
\def\tcel@get@char@tokens#1{%
  \lccode`\~=#1\relax \lccode`\:=#1\relax
  \lowercase{\def\tcel@char@A{~}\def\tcel@char@N{:}}%
}

%--------------------------------------- option handling

%% \tcel@promise
% Process that will be done at the end of the package.
\let\tcel@promise\relax

%% \tcel@check@istart{<string>}
% Check an option string, and if it is of the form 'item=X' then
% update \tcel@promise with an invocation of \SetItemStarter.
\def\tcel@check@istart#1{%
  \tcel@okfalse
  \tcel@check@istart@a#1\tcel@mark item=\tcel@mark\tcel@end
}
\def\tcel@check@istart@a#1item=#2\tcel@mark#3\tcel@end{%
  \ifx\tcel@mt#1\tcel@mt
    \tcel@check@single{#2}\tcel@check@istart@b\tcel@check@istart@b
    \iftcel@ok\else
      \tcel@error
       {Option value of 'item' must be a single character}%
       \@ehc
      \tcel@oktrue % not pass the option anyway
    \fi
  \fi
}
\def\tcel@check@istart@b#1{%
  \tcel@oktrue
  \def\tcel@promise{\SetItemStarter{#1}}%
}

%% option handler
\DeclareOption{snowman}{%
  \def\tcel@promise{\SetItemStarter{2603}}%
}
\DeclareOption*{%
  \expandafter\tcel@decl@opt@a\expandafter{\CurrentOption}%
}
\def\tcel@decl@opt@a#1{%
  \tcel@check@istart{#1}%
  \iftcel@ok\else
    \PassOptionsToPackage{#1}{easylist}%
  \fi
}

%% process it
\ProcessOptions\relax

%% load the package
\RequirePackage{easylist}

%--------------------------------------- patch to easylist

%% Check consistency
\def\tcel@next{%
  \endgroup\par}
\ifx\tcel@next\endeasylist\else
  \tcel@error
   {Cannot patch to easylist code}
   {Package loading is aborted.}
\expandafter\endinput\fi\relax

%% Redefine '\easylist'
\tcel@okfalse
\begingroup
  \def\tcel@patch{%
    \expandafter\tcel@patch@a\easylist\tcel@mark
     \begingroup\tcel@mark\tcel@end
  }
  \def\tcel@patch@a#1\begingroup#2\tcel@mark#3\tcel@end{%
    \ifx\tcel@mt#3\tcel@mt\else
      \aftergroup\tcel@oktrue
      \gdef\tcel@reset@item@starter{#1}%
      \gdef\easylist{%
        \let\SetItemStarter\tcel@invalid@SetItemStarter
        \tcel@reset@item@starter
        \begingroup #2}%
    \fi
  }
  \tcel@patch
\endgroup
\iftcel@ok\else
  \tcel@error
   {Cannot patch to easylist code}
   {Package loading is aborted.}
\expandafter\endinput\fi\relax

%% Patch to '\elCreateItem' (not mandatory)
\ifx\detokenize\@undefined\else
  \begingroup
    % messages after replacement
    \def\tcel@msga{%
      Too many item-start tokens.}
    \def\tcel@msgb{%
      You can't use more than \the\el@CounterTotal\space
      item-start tokens}
    % patch process
    \edef\tcel@patch{%
      \noexpand\tcel@patch@a
      \detokenize{Too many:.:You can't:'s:}%
      {\detokenize\expandafter{\elCreateItem}}}
    \def\tcel@patch@a#1:#2:#3:#4:#5{%
      \def\tcel@next##1#1##2#2##3#3##4#4##5\tcel@mark{%
        \tcel@patch@c{##1\tcel@msga##3\tcel@msgb##5}}%
      \tcel@next#5\tcel@mark#1#2#3#4\tcel@mark\tcel@end}
    \def\tcel@patch@c#1#2\tcel@end{%
      \ifx\tcel@mt#2\tcel@mt
        \tcel@warn{Cannot apply patch}%
      \else
        \edef\tcel@next{\noexpand\tcel@patch@d{#1}}%
        \tcel@next
      \fi}
    \def\tcel@patch@d#1{%
      \scantokens{\toks@{#1}}%
      \edef\elCreateItem{\the\toks@}}
    \tcel@patch
  \endgroup
\fi

%%<+>\ItemStarter
% The item-starter function, which is normally assigned to
% 'the active char' in easylist.
\newcommand*\ItemStarter{%
  \futurelet\elNextToken\elCreateItem
}

%%<+>\ItemStarterUpdated\CS
% Must be called when the specification of item-starter token
% is updated, where the argument \CS is a macro that activates
% the token specified.
\newcommand*\ItemStarterUpdated[1]{%
  \global\let\tcel@reset@item@starter#1%
}

%% \tcel@invalid@SetItemStarter
% In easylist environments, \SetItemStarter is overwritten by
% this macro.
\def\tcel@invalid@SetItemStarter#1{%
  \tcel@error
   {Cannot change item starter here}
   \@eha
}

%--------------------------------------- all done

%% \tcel@g@char : codepoint of item-starter char
\let\tcel@g@char\relax
%% \tcel@char : for local use
\let\tcel@char\relax
%% \tcel@cmd : item-starter command
\let\tcel@g@cmd\relax

%%<*>\SetItemStarter{<some>}
\newcommand*\SetItemStarter[1]{%
  \tcel@okfalse \let\tcel@char\relax
  \tcel@check@single{#1}\tcel@set@is@char\tcel@set@is@cmd
  \iftcel@ok\else
    \tcel@check@hexnum{#1}%
    \ifnum\@tempcnta<\z@
      \tcel@error
       {Invalid argument given to \string\SetItemStarter}%
       \@ehc
    \else\ifnum
     \iftcel@unicode "10FFFF \else "FF \fi
     <\@tempcnta
      \tcel@error
       {The code value given is too big}%
       \@ehc
    \else
      \chardef\tcel@char\@tempcnta
    \fi\fi
  \fi
  \ifx\tcel@char\relax\else
    \tcel@okfalse
    \ifnum`\#=\tcel@char \tcel@oktrue \fi
    \ifnum`\@=\tcel@char \tcel@oktrue \fi
    \ifnum`\&=\tcel@char \tcel@oktrue \fi
    \ifnum\tcel@char>"7F
      \ifnum\catcode\tcel@char=11 \tcel@oktrue \fi
      \ifnum\catcode\tcel@char=12 \tcel@oktrue \fi
    \fi
    \tcel@get@char@tokens\tcel@char
    \iftcel@ok
      \expandafter\tcel@set@is@char@a\tcel@char@A
    \else
      \tcel@error
       {Cannot use '\tcel@char@N' as item starter}%
       \@eha
    \fi
  \fi
}
\def\tcel@set@is@char#1{%
  \chardef\tcel@char`#1\relax
  \tcel@oktrue
}
\def\tcel@set@is@char@a#1{%
  \global\let\tcel@g@char\tcel@char
  \global\let\Activate\tcel@Activate@char
  \global\let\Deactivate\tcel@Deactivate@char
  \def\tcel@next{%
    \catcode\tcel@g@char\active
    \let#1\ItemStarter}%
  \ItemStarterUpdated\tcel@next
}
\def\tcel@set@is@cmd#1{%
  \ifx#1\tcel@ltx@star
    \gdef\tcel@g@cmd{#1}%
    \global\let\Activate\relax
    \global\let\Deactivate\relax
    \def\tcel@next{\let#1\tcel@switch@star}%
    \ItemStarterUpdated\tcel@next
  \else
    \@ifdefinable{#1}{%
      \gdef\tcel@g@cmd{#1}%
      \global\let\Activate\relax
      \global\let\Deactivate\relax
      \def\tcel@next{\let#1\ItemStarter}%
      \ItemStarterUpdated\tcel@next
    }%
  \fi
  \tcel@oktrue
}
\def\tcel@ltx@star{%
  \discretionary{\thinspace\the\textfont2\char2}{}{}%
}
\def\tcel@switch@star{%
  \ifmmode \tcel@ltx@star \expandafter\@gobble \fi
  \ItemStarter
}

%% \tcel@Activate@char
\let\tcel@deactivate\relax
\def\tcel@Activate@char{%
  \ifx\tcel@deactivate\relax
    \edef\tcel@deactivate{%
      \catcode\tcel@g@char=\the\catcode\tcel@g@char\relax}%
  \fi
  \catcode\tcel@g@char\active
}
\def\tcel@Deactivate@char{%
  \tcel@deactivate
  \let\tcel@deactivate\relax
}

%--------------------------------------- all done
%% do the promise
\tcel@promise
%% done
\endinput
%% EOF

test-1.tex

% XeLaTeX document, encoded in UTF-8
\documentclass[a4paper]{article}
\usepackage[snowman]{tceasylist}% '☃'  starts a list item
\begin{document}
\begin{easylist}
☃ Blah.
☃ Blahblah.
☃☃ Blah, blahblah blah blah.
☃☃☃ Blah.
☃☃ Blahblah, `blah' blah?
☃☃☃ Blahblahblah\ldots.
☃ Blah, \emph{blahblah blah} blah blahblah.
\end{easylist}
\end{document}

test-2.tex

% XeLaTeX document, encoded in UTF-8
\documentclass[a4paper]{article}
\usepackage[item=☺]{tceasylist}% '☺' starts a list item
\begin{document}
\begin{easylist}
☺ Blah.
☺ Blahblah.
☺☺ Blah, blahblah blah blah.
☺☺☺ Blah.
☺☺ Blahblah, `blah' blah?
☺☺☺ Blahblahblah\ldots.
☺ Blah, \emph{blahblah blah} blah blahblah.
\end{easylist}
\end{document}