Keycommand package - \commandkey{mykey} doesn't expand

Question

UPDATED, see edit history for completeness

I'm trying out the keycommand package but it's not working as expected. There seems to be some incompatibility with tables, but I've read and re-read the documentation and I can't figure out what it is.

I've built a minimal example of what I'm doing. It's using longtable, since those seem to work a little better than regular ones and since that is what I need it to work with (for now):

\documentclass{article}
\usepackage{longtable} 
\usepackage{keycommand}

\newkeycommand\mycommand[first=one,second=2][1]{%
#1 \\    
First: \commandkey{first} & second: \commandkey{second}%
}

\begin{document}
\begin{longtable}{ll}
\mycommand{hello} \\
\mycommand[first=1,second=two]{world!}
\end{longtable}
\end{document}

The problem is that instead of outputting the parameter values, it outputs the parameter names:

EXPECTED OUTPUT                      ACTUAL OUTPUT
***************                      *************
hello                                hello
First: one    second: 2              first: first        second: second
world!                               world!
First: 1      second: two            first: first        second: second

Why does it do this? What can I do about it?

Answer 1

This seems to be a bad bug in keycommands. The assignments performed to provide a value when you call \commandkey are local; the \\ in the definition ends the group where these assignments are made and keycommands loses track of what the values should be, so it prints the key name.

You can use a different (and reliable) method for obtaining the same result.

\documentclass{article}
\usepackage{longtable} 
\usepackage{xparse}

\ExplSyntaxOn
% Set up the keys
\keys_define:nn { lycken-mycommand }
 {
  first  .tl_gset:N = \g_lycken_first_tl,
  second .tl_gset:N = \g_lycken_second_tl,
 }

\NewDocumentCommand{\mycommand}{ O{} m }
 {
  \keys_set:nn { lycken-mycommand }
   {
    % provide the default values
    first = first,
    second = 2,
    #1 % evaluate the keys in the optional argument
   }
  #2 \\    
  First:~\g_lycken_first_tl & second:~\g_lycken_second_tl
 }
\ExplSyntaxOff

\begin{document}
\begin{longtable}{ll}
\mycommand{hello} \\
\mycommand[first=1,second=two]{world!}
\end{longtable}
\end{document}

Answer 2

This is trivial and possible without any global assignments (any assignments for that matter) using expkv-cs (the following uses its Hash variant, as that is closer in syntax to keycommand, even though the example only needs two keys and it would be faster to set it up with the Split variant).

^{Disclaimer: I'm the author of expkv}

\documentclass{article}
\usepackage{longtable} 

\usepackage{expkv-cs}
% grabbing the argument as an optional argument
\newcommand\mycommand[1][]{\mycommandKV{#1}}
% parsing the keys "first" and "second"
\ekvcHashAndForward\mycommandKV\mycommandOUT{first=one,second=2}
% output
% #1 contains the parsed keys accessible via \ekvcValue
% #2 the mandatory argument
\newcommand\mycommandOUT[2]
  {%
    #2 \\
    First: \ekvcValue{first}{#1} & second: \ekvcValue{second}{#1}%
  }

\begin{document}
\begin{longtable}{ll}
\mycommand{hello} \\
\mycommand[first=1,second=two]{world!}
\end{longtable}
\end{document}