How to distinguish "no value" from "empty value" when setting `l3keys`?

Question

I'm trying to create an option with l3keys such that, if it receives a value, even if an empty one, it gets added to a property list, but if it receives no value, it gets removed from the property list.

However, I don't seem to be able do distinguish key from key= in a .code:n "action". I know that .default:n captures that situation, but it cannot receive a code block, just a value. I also know that I can distinguish the situation in \keyval_parse:nnn, but I'd like this behavior in some options among the whole set, and using \keyval_parse:nnn for the whole thing would become complicated.

The MWE illustrates the situation:

\documentclass{article}

\ExplSyntaxOn
\prop_new:N \l__myprops_prop
\keys_define:nn { options }
  {
    mykey .code:n =
      {
        \tl_if_novalue:nTF {#1}
          { \prop_remove:Nn \l__myprops_prop { mykey } }
          { \prop_put:Nnn \l__myprops_prop { mykey } {#1} }
        \prop_show:N \l__myprops_prop
      } ,
    mykey2 .code:n =
      {
        \tl_if_empty:nTF {#1}
          { \prop_remove:Nn \l__myprops_prop { mykey2 } }
          { \prop_put:Nnn \l__myprops_prop { mykey2 } {#1} }
        \prop_show:N \l__myprops_prop
      }
  }
\ExplSyntaxOff

\begin{document}

\ExplSyntaxOn
\keys_set:nn { options } { mykey }
\keys_set:nn { options } { mykey= }
\keys_set:nn { options } { mykey=val }
\prop_clear:N \l__myprops_prop
\keys_set:nn { options } { mykey2 }
\keys_set:nn { options } { mykey2= }
\keys_set:nn { options } { mykey2=val }
\ExplSyntaxOff

\end{document}

The log for this document contains:

The property list \l__myprops_prop contains the pairs (without outer braces):
>  {mykey}  =>  {}.
<recently read> }
                 
l.27 \keys_set:nn { options } { mykey }
                                       
The property list \l__myprops_prop contains the pairs (without outer braces):
>  {mykey}  =>  {}.
<recently read> }
                 
l.28 \keys_set:nn { options } { mykey= }
                                        
The property list \l__myprops_prop contains the pairs (without outer braces):
>  {mykey}  =>  {val}.
<recently read> }
                 
l.29 \keys_set:nn { options } { mykey=val }
                                           
The property list \l__myprops_prop is empty
> .
<recently read> }
                 
l.31 \keys_set:nn { options } { mykey2 }
                                        
The property list \l__myprops_prop is empty
> .
<recently read> }
                 
l.32 \keys_set:nn { options } { mykey2= }
                                         
The property list \l__myprops_prop contains the pairs (without outer braces):
>  {mykey2}  =>  {val}.
<recently read> }
                 
l.33 \keys_set:nn { options } { mykey2=val }

Which shows that neither key nor key= pass the "no value" test, and both key and key= pass the emptiness test. Unfortunately, I cannot use emptiness to "signal" the removal, since "empty" is a valid value for the key in question. Is indeed \keyval_parse:nnn the only choice here?

Answer 1

This builds on the same basic idea as @Phelype's answer, namely use \c_novalue_tl as a marker, but instead of wrapping \keyval_parse:nnn around \keys_set:nn this uses the .default:x handler to set the default value to \c_novalue_tl:

\documentclass{article}

\ExplSyntaxOn
\prop_new:N \l__myprops_prop
\cs_new_protected:Npn \__myprops_handle:n #1
  {
    \tl_if_novalue:nTF {#1}
      { \prop_remove:NV \l__myprops_prop \l_keys_key_str }
      { \prop_put:NVn   \l__myprops_prop \l_keys_key_str {#1} }
    \prop_show:N \l__myprops_prop
  }
\keys_define:nn { options }
  {
     mykey  .code:n = \__myprops_handle:n {#1}
    ,mykey  .default:x = \c_novalue_tl
    ,mykey2 .code:n = \__myprops_handle:n {#1}
    ,mykey2 .default:x = \c_novalue_tl
  }
\ExplSyntaxOff

\begin{document}

\ExplSyntaxOn
\keys_set:nn { options } { mykey }
\keys_set:nn { options } { mykey= }
\keys_set:nn { options } { mykey=val }
\prop_clear:N \l__myprops_prop
\keys_set:nn { options } { mykey2 }
\keys_set:nn { options } { mykey2= }
\keys_set:nn { options } { mykey2=val }
\ExplSyntaxOff

\end{document}

Answer 2

There is no built-in way to distinguish mykey from mykey={} when it's declared with the .code:n handler. Internally these cases are different, and there is a boolean variable that keeps track of that, but all using private variables from l3keys.

What you can do using the available interfaces is use \keyval_parse:nnn, as you suggested, to differentiate both cases, and then when mykey is used, turn that into mykey = \c_novalue_tl, then the code in your key can detect that using \tl_if_novalue:nTF. This makes the parsing a bit slower than just using \keys_set:nn because some common bookkeeping code in \keys_set:nn is executed more often, but that's likely minimal.

The example below implements \gusbrs_keys_set:nn, which passes \c_novalue_tl to a key if no value is given:

\documentclass{article}

\ExplSyntaxOn
\cs_new_protected:Npn \gusbrs_keys_set:nn #1 #2
  {
    \keyval_parse:nnn
      { \__gusbrs_keys_set:nn {#1} }
      { \__gusbrs_keys_set:nnn {#1} }
        {#2}
  }
\cs_new_protected:Npn \__gusbrs_keys_set:nn #1 #2
  { \use:x { \keys_set:nn {#1} { #2 = \c_novalue_tl } } }
\cs_new_protected:Npn \__gusbrs_keys_set:nnn #1 #2 #3
  { \keys_set:nn {#1} { #2 = #3 } }
\ExplSyntaxOff


\ExplSyntaxOn
\prop_new:N \l__myprops_prop
\keys_define:nn { options }
  {
    mykey .code:n =
      {
        \tl_if_novalue:nTF {#1}
          { \prop_remove:Nn \l__myprops_prop { mykey } }
          { \prop_put:Nnn \l__myprops_prop { mykey } {#1} }
        \prop_show:N \l__myprops_prop
      } ,
  }
\ExplSyntaxOff

\begin{document}

\ExplSyntaxOn
\gusbrs_keys_set:nn { options } { mykey }
\gusbrs_keys_set:nn { options } { mykey= }
\gusbrs_keys_set:nn { options } { mykey=val }
\ExplSyntaxOff

\end{document}

This is printed on the terminal:

The property list \l__myprops_prop is empty
> .
<recently read> }
                 
l.35 \gusbrs_keys_set:nn { options } { mykey }
                                              
? 
The property list \l__myprops_prop contains the pairs (without outer braces):
>  {mykey}  =>  {}.
<recently read> }
                 
l.36 \gusbrs_keys_set:nn { options } { mykey= }
                                               
? 
The property list \l__myprops_prop contains the pairs (without outer braces):
>  {mykey}  =>  {val}.
<recently read> }
                 
l.37 \gusbrs_keys_set:nn { options } { mykey=val }
                                                  
?