Strict version of \keys_set_groups:nnn

Question

Here's a little code where the key-value allowed by the macro is an unwanted feature : for example, single[both = OK, only = KO] doesn't give any error as I'd hoped. But on the other hand, I would also like to eat this kind of unwanted keys: see the call of \single inside \both. Concretely, I would like to use \single options when calling \both, but also allows more options from \both.

\documentclass{article}

\ExplSyntaxOn

\tl_new:N \l_mbc_both_tl
\tl_new:N \l_mbc_only_tl

\keys_define:nn { strict-mode } {
  both .tl_set:N  = \l_mbc_both_tl,
  both .groups:n  = { single-use },
  only .tl_set:N  = \l_mbc_only_tl,
  only  .groups:n = { both-use },
}


\NewDocumentCommand{\single}{ O{} }{
  \group_begin:
    \keys_set_groups:nnn { strict-mode }
                         { single-use }
                         { #1 }

    Single used...
  \group_end:
}


\NewDocumentCommand{\both}{ O{} }{
  \group_begin:
    \keys_set:nn { strict-mode } { #1 }

    Both used... ~ \single[#1]
  \group_end:
}

\ExplSyntaxOff


\begin{document}

\single[both = OK]

\single[both = OK, only = KO]

\single[after = math KO]

\both[both = OK]

\both[both = OK, only = OK]

\both[after = math OK]

\end{document}

Is there a way of defining sub-keys that are only allowed for one environment?

Answer 1

One approach is to use two paths. Then process the command which should accept additional keys by first processing known keys against one path and then processing the remainder against the other. This way, both commands will generate errors for completely unknown keys (after) and the more restrictive command will also generate errors for keys defined exclusively for the more expansive command (only).

In the code below, I've prevented errors by typesetting an 'unknown key' message, just for the sake of the example. Obviously, you would either redefine this in your actual code or just delete it and use the default messaging.

\documentclass{article}

\ExplSyntaxOn
% 
% internal names should be double-underscored
\tl_new:N \l__mbc_both_tl
\tl_new:N \l__mbc_only_tl

\keys_define:nn { mbc / strict-mode / common } {% try to guarantee unique name space
  both .tl_set:N  = \l__mbc_both_tl,
  unknown .code:n = { % just for testing
    \texttt{\l_keys_key_str}  ~ not ~ recognised!  \par   
  },
}
\keys_define:nn { mbc / strict-mode / exclusive } {% try to guarantee unique name space
  only .tl_set:N  = \l__mbc_only_tl,
}


\NewDocumentCommand {\single}{ O{} }{
  \group_begin:
    \keys_set:nn { mbc / strict-mode / common  } { #1 }
    \texttt{\textbackslash l \_\_ mbc \_ both \_ tl} ~ is ~ \texttt{\l__mbc_both_tl}\par
    \texttt{\textbackslash  l \_\_ mbc \_ only \_ tl} ~ is ~ \texttt{\l__mbc_only_tl}\par
  \group_end:
}


\NewDocumentCommand {\both}{ O{} }{
  \group_begin:
    \keys_set_known:nnN { mbc / strict-mode / exclusive } { #1 } \l_tmpa_tl
    \keys_set:nV { mbc / strict-mode / common } \l_tmpa_tl
    \texttt{\textbackslash l \_\_ mbc \_ both \_ tl} ~ is ~ \texttt{\l__mbc_both_tl}\par
    \texttt{\textbackslash  l \_\_ mbc \_ only \_ tl} ~ is ~ \texttt{\l__mbc_only_tl}\par
  \group_end:
}

\ExplSyntaxOff


\begin{document}

\single[both = OK]

\single[both = OK, only = KO]

\single[after = math KO]

\both[both = OK]

\both[both = OK, only = OK]

\both[after = math OK]

\end{document}