pgfkeys strips braces from argument

Question

This is most likely a dupe of pgfkeys overeager in stripping away my braces

If I pass {[[To]], [[From]]} to a macro as the value of an optional argument using pgfkeys, the outermost bracket is stripped. This doesn't happen if I pass it as the single mandatory argument.

I've been using pgfkeys for a while. It generally works well for me, and has the features I need, which may not be the case for other similar key packages. So I was wondering, assuming this is the issue as described in that other question, and not user error, can anyone suggest a workaround? Adding extra spaces to the \normalmacro argument doesn't help. They all get stripped away.

For background, this is for use with LuaTeX, to pass arrays from TeX to Lua, but Lua is not needed to illustrate the issue.

Since an extra pair of braces makes no difference to the printed output here, the difference can only be seen in the \typeout output to the terminal and console.

\documentclass[12pt]{scrartcl}
\usepackage{pgfkeys}

\newcommand\colnames{}

\pgfkeys
{
  /foo/.is family, /foo,
  default/.style =
  {
    colnames,
  },
  colnames/.estore in = \colnames,
  colnames/.default = {},
}
\begin{document}

\NewDocumentCommand{\usingpgfkeys}{O{}}
{
  \pgfkeys{/foo, default, #1}%                                                                                                      
  \typeout{USINGPGFKEYS RETURNS \colnames}\colnames
}
 
\usingpgfkeys[colnames={[[`To`]], [[`From`]]}]

\NewDocumentCommand{\normalmacro}{m}{\typeout{NORMALMACRO RETURNS #1}#1}

\normalmacro{{[[`To`]], [[`From`]]}}

\end{document}

Answer 1

pgfkeys is pretty unpredictable in brace stripping, imho. It depends on the used key handlers (if I remember correctly, been a while since I took a thorough look).

Compare the following (which uses expkv as an example for a parser that predictably strips braces -- of course you could use another one, the ones I'm aware of are: expkv, kvsetkeys, and l3keys).

In case of your pgfkeys usage, you'd have to put four sets of braces around your value so that one survives, but if you put a space in front of the first braces, you need only three, or if you put a space after the first brace, you also need four. And if you're daring and put a space before and after the first brace you can't get the input you want.

\documentclass[12pt]{scrartcl}
\usepackage{pgfkeys}

\newcommand\colnames

\pgfkeys
{
  /foo/.is family, /foo,
  default/.style =
  {
    colnames,
  },
  colnames/.estore in = \colnames,
  colnames/.default = {},
}

% more or less identical definition to your pgfkeys using expkv-def
\usepackage{expkv-def}
\ekvdefinekeys{foo}
  {
     store   colnames = \colnames
    ,default colnames = {}
    ,meta    default  = {colnames}
    ,default default  = {}
  }
\NewDocumentCommand \usingexpkv { O{} }
  {%
    \ekvset{foo}{default, #1}%
    \typeout{BAR RETURNS \colnames}\colnames
  }
% similar macro definition using expkv-cs
\usepackage{expkv-cs}
\ekvcSplit\usingexpkvcs
  {
    colnames = {}
  }
  {\typeout{BARR RETURNS #1}#1}

\begin{document}

\NewDocumentCommand{\usingpgfkeys}{O{}}
{
  \pgfkeys{/foo, default, #1}%                                                                                                      
  \typeout{FEE RETURNS \colnames}\colnames
}

\usingexpkv[colnames={{[[`To`]], [[`From`]]}}]

\usingexpkvcs{colnames={{[[`To`]], [[`From`]]}}}
 
\usingpgfkeys[colnames={{{{[[`To`]], [[`From`]]}}}}]

\NewDocumentCommand{\normalmacro}{m}{\typeout{BAZ REURNS #1}#1}

\normalmacro{{[[`To`]], [[`From`]]}}

\end{document}

Answer 2

Making pgfkeys strict in space trimming and brace stripping for keys and values may need more effort than expected, thus I guess a full fix won't appear in recent future. Perhaps it should start with writing tests.

Here's a trick you can use for keys, but currently restricted to your specific usage (/.estore in). A more powerful hence general scheme is possible I guess.

This trick requires user to add a mark command before normal value, and macro author to trim that command in key definition.

\documentclass[12pt]{scrartcl}
\usepackage{pgfkeys}

\newcommand\colnames{}

\pgfkeys
{
  /foo/.is family, /foo,
  colnames/.estore in = \colnames,
  colnames/.default = {},
}
\begin{document}
\ttfamily

\NewDocumentCommand{\usingpgfkeys}{O{}}
{%
  \pgfkeys{/foo, colnames, #1}% `colnames` is used to init \colnames
  \detokenize\expandafter{\colnames}%
}
\newcommand\mymark{}
 
using pgfkeys:
% use `colnames={\mymark<normal value>}` to disable further brace stripping
\usingpgfkeys[colnames={\mymark{[[`To`]], [[`From`]]}}]


\NewDocumentCommand{\normalmacro}{m}{\detokenize{#1}}

normal macro:
\normalmacro{{[[`To`]], [[`From`]]}}
\end{document}