Execute a macro given in a key-value in expl3

Question

I have defined the macro \getdocvar that retrieves a value from a property list given it is key. It has an optional argument which is a list of key-values. At the moment only the transform key is defined. I want to store in this key a macro to be applied to the value retrieved by \getdocvar.

   \keys_define:nn { docvar/get }
   {
     transform .tl_set:N = \l_docvar_tranformation_tl,
   }

The name of the macro is stored as a token list.

   \cs_new_protected:Npn \docvar_get_properties:n #1
   {
     \group_begin:
     \keys_set:nn { docvar/get } { #1 }
     \group_end:
     }

Now, if \getdocvar has no optional argument, the value of the prop is retrieved, but I have no idea how to apply a macro to #2 based on the name.

\DeclareDocumentCommand \getdocvar {o m}{
  \IfNoValueTF { #1 }
  {
    \prop_get:NVN \g_docvar_values_prop #2 \g_tmpa_tl
    \tl_use:N \g_tmpa_tl
  }
  {
    \docvar_get_properties:n {#1}
    %%% MISSIn CODE to cronstruct a macro from transform key
  }

Any hit to how this may be done.

The full sequence of using this macro is

\definedocvar{title}
\setdocvar{title}{This is my title}
\getdocvar{title} 
% This should print:
This is my title
\getdocvar[transform=MakeUppercase]{title}
% This should print:
\MakeUppercase{This is my title}

Answer 1

If I understand correctly your aim, this might do:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\prop_new:N \g_docvar_values_prop
\tl_new:N \l__docvar_temp_tl

\NewDocumentCommand{\setdocvar}{mm}
 {
  \prop_gput:Nnn \g_docvar_values_prop { #1 } { #2 }
 }

\NewDocumentCommand{\getdocvar}{O{}m}
 {
  \group_begin:
  \keys_set:nn { docvar/get } { #1 }
  \prop_get:NnN \g_docvar_values_prop { #2 } \l__docvar_temp_tl
  \__docvar_transform:VV \l_docvar_transform_tl \l__docvar_temp_tl
  \group_end:
 }

\cs_new_protected:Nn \__docvar_transform:nn
 {
  #1{#2}
 }
\cs_generate_variant:Nn \__docvar_transform:nn { VV }

\keys_define:nn { docvar/get }
 {
  transform .tl_set:N = \l_docvar_transform_tl,
  transform .initial:n = \use:n,
 }

\ExplSyntaxOff

\setdocvar{title}{This is my title}

\begin{document}

\getdocvar{title} 

\getdocvar[transform=\MakeUppercase]{title}

\end{document}

The value to transform should be a single argument macro.

The following is easier, though:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\prop_new:N \g_docvar_values_prop
\tl_new:N \l__docvar_temp_tl

\NewDocumentCommand{\setdocvar}{mm}
 {
  \prop_gput:Nnn \g_docvar_values_prop { #1 } { #2 }
 }

\NewDocumentCommand{\getdocvar}{O{\use:n}m}
 {
  \prop_get:NnN \g_docvar_values_prop { #2 } \l__docvar_temp_tl
  \exp_args:NV #1 \l__docvar_temp_tl
 }

\ExplSyntaxOff

\setdocvar{title}{This is my title}

\begin{document}

\getdocvar{title}

\getdocvar[\MakeUppercase]{title}

\end{document}