Hi all,

my goal is a macro \definestates[foo][...,...] which takes a comma
list and creates a monadic macro \foo[n]. \foo[n] should return
either the nth item or, if (n > list length), a default. I
thought the right tools were \processcommalist to generate the
mapping n-><list item>, as well as \processaction to retrieve the
items. But naively I didn’t consider expansion ...

My code so far:

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\unprotect

\def\definestates{\dodoubleempty\do_define_states}

\def\do_define_states[#1][#2]{%
  \expandafter\edef\csname#1_states\endcsname{%
    \nstates0
    %% this is supposed to be expanded so we employ the \raw... variety
    \rawprocesscommalist[#2]\add_one_state%
    unknown=>\dummystate,%
    default=>\dummystate,%
  }%
  \do_do_define_states{#1}%
}

\let\dummystate\empty

\newcount\nstates
\unexpanded\def\add_one_state#1{%
  \advance\nstates\plusone
  \the\nstates=>#1,%
}

\def\do_do_define_states#1{%
  \expandafter\def\csname#1\endcsname
    {\expandafter\dosingleempty\csname do_#1\endcsname}%
  \expandafter\def\csname do_#1\endcsname[##1]{%
    \iffirstargument
      % <[DBG] cmd:#1, state:##1>\par
      \edef\currentstates{\csname#1_states\endcsname}%
      %% At this point \currentstates should yield the whole mapping so
      %% we can use it with \processaction. But ...
      % \show\currentstates
      % \currentstates\par
      %% ... here it still contains
      %%    \nstates 0 \rawprocesscommalist [foo,bar,baz] ...
      %% and the next directive has an empty result:
      \rawprocessaction[##1][\currentstates]%
    \fi
  }%
}

\protect
\starttext

\definestates[mystates][foo,bar,baz]
%% At this point I’d like \mystates_states to contain (literally)
%%   “1=>foo,2=>bar,3=>baz,unknown=>,default=>,”
%% so the following command
\mystates[1]  % -> foo
%% would be equivalent to
%%    \processaction[1][1=>foo,2=>bar,3=>baz,unknown=>,default=>,]
\mystates[2]  % -> bar
\mystates[42] % -> dummy

\stoptext
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My question: How can I use \[raw]processcommalist in an expanded
definition? How can I get \[raw]processcommalist to operate on
the expansion of its second argument? Does this even make sense?
If not, what is the proper way?

I’d be grateful for any advice
Philipp


PS: Yeah, I could use Lua for it but that’s not the question.