Generate l3keys meta key from subkey

Question

I’ve created a simple package with options settable via l3keys2e. I was able to figure out how to make the syntax

\usepackage[format = international]{phone}

work, but I’d like to make the syntax

\usepackage[international]{phone}

be a synonym. The method I’m using in the code below works, but is very repetitive, especially when I’ve got more formatting options. Is there a simpler way?

Here’s the code:

\RequirePackage{expl3, l3keys2e, xparse}
\ProvidesExplPackage
    {phone}{2013/06/19}{0.1}{Format phone numbers}

\cs_new_nopar:Npn \phone_international_fmt:NNN  #1#2#3
  { +1 - #1 - #2 - #3 }

\cs_new_nopar:Npn \phone_parendash_fmt:NNN  #1#2#3
  { (#1) \nobreakspace #2 - #3 }

\keys_define:nn { phone }
  {
    format .choice_code:n =
      {
        \cs_new_eq:Nc \phone_fmt:NNN 
          { phone_ \tl_use:N \l_keys_choice_tl _fmt:NNN }
      },
    format .generate_choices:n =
      {
        international,
        parendash,
      },
% These next lines will become repetitive if I have many more formatting options
    international .meta:n = { format = international },
    parendash     .meta:n = { format = parendash     },
  }

\ProcessKeysOptions { phone }

\NewDocumentCommand \phone
  { >{ \SplitArgument {2} {-} } m }
  { \phone_fmt:NNN #1 }

\endinput

A minimal example using this package looks like this:

\documentclass{article}

% \usepackage[parendash]{phone}
\usepackage[format = parendash]{phone}

\begin{document}

\phone{212-555-1212}

\end{document}

This yields the desired result

(212) 555-1212

Answer 1

You could use an unknown key handler for this whereby you pass any unknown key back into the system as format = <unknown key> (a similar system is used by TikZ to recognise colours and node shapes). Here's an example which uses that. It gives a warning if there isn't a corresponding format.

I have to use a helper command to feed the unknown key back into the key handler since if I just did format / unknown .code:n = { \keys_set:nn { phone} { format = #1 } } then the #1 would be \l_keys_key_tl and L3 would be extremely careful and not expand the \l_keys_key_tl until the last minute, by which time it would have changed its value from the unknown key to format. The helper macro allows me to use the value of \l_keys_key_tl instead of the token list variable.

I also fixed the NNN to nnn. Because I wanted to set a default, the setter inside the key handler is now \cs_set_eq:Nc instead of \cs_new_eq:Nc.

\documentclass{article}
%\url{http://tex.stackexchange.com/q/120102/86}
\usepackage{filecontents}

\begin{filecontents}{phone.sty}
\RequirePackage{expl3, l3keys2e, xparse}
\ProvidesExplPackage
    {phone}{2013/06/19}{0.1}{Format phone numbers}

\cs_new_nopar:Npn \phone_international_fmt:nnn  #1#2#3
  { +1 - #1 - #2 - #3 }

\cs_new_nopar:Npn \phone_parendash_fmt:nnn  #1#2#3
  { (#1) \nobreakspace #2 - #3 }

% Set a default
\cs_set_eq:NN \phone_fmt:nnn \phone_parendash_fmt:nnn

\msg_new:nnnn {phone} {no~ format} {Format~ `#1'~ is~ not~ recognised~ by~ this~ package,~ using~ the~ default~ `parendash'~ instead.} {}

\keys_define:nn { phone }
  {
    format .choice:,
    format .choice_code:n =
      {
        \cs_set_eq:Nc \phone_fmt:nnn 
        { phone_ \tl_use:N \l_keys_choice_tl _fmt:nnn }
      },
    format .generate_choices:n =
      {
        international,
        parendash,
      },
      format / unknown .code:n = {
        \msg_warning:nnn {phone} {no~ format} {#1}
      },
    unknown .code:n = {
      \phone_unknown_key:V \l_keys_key_tl
    },
  }

\cs_new:Npn \phone_unknown_key:n #1
{
  \keys_set:nn { phone} { format = #1 }
}

\cs_generate_variant:Nn \phone_unknown_key:n {V}


\ProcessKeysOptions { phone }

\NewDocumentCommand \phone
  { >{ \SplitArgument {2} {-} } m }
  { \phone_fmt:nnn #1 }

\endinput
\end{filecontents}
 \usepackage[british]{phone}

\begin{document}

\phone{212-555-1212}

\end{document}

Answer 2

The approach you've taken looks correct. Internally, the .meta:n property is creating a standard key which is more-or-less equivalent to

key-name-1 .code:n = { \keys_set:nn { module } { key-name-2 = #1 } }

If you have a large number of these meta keys to create, you might want to use a comma-list mapping

\clist_map_inline:nn { key-one , key-two , key-three ... }
  { \keys_define:nn { mymodule } { #1 .meta:n = { format = #1 } }

but for just a couple this is probably making your code more not less complex.