usebib: Printing abstract (including \par)

Question

• Inspired by this answer/question, I wanted to use the usebib package to print the abstract.
• The following MWE gives an error and I do not see an obvious reason.
• Watch it, the abstract contains \par.

---

% Based on https://tex.stackexchange.com/questions/73678/
\documentclass{article}
\usepackage{biblatex}
\begin{filecontents}{\jobname.bib}
@misc{key,
  title = {title},
  year = {2000},
  abstract = {abstract \par abstract},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\usepackage[hidelinks]{hyperref}
% Load AFTER hyperref
\usepackage{usebib}
% Use BEFORE \bibinput
\newbibfield{abstract}
\bibinput{\jobname}
\begin{document}
\usebibentry{key}{title}
\usebibentry{key}{abstract}
\end{document}

See the usebib package documentation for explanation.
Type  H <return>  for immediate help.
 ...
l.28 \usebibentry{key}{abstract}
The key you used is wrong or the value to `abstract' has not been set

Answer 1

The command that loads the entries is not \long, sorry.

You can fix it:

\begin{filecontents*}{\jobname.bib}
@misc{key,
  title = {title},
  year = {2000},
  abstract = {abstract \par abstract},
}
\end{filecontents*}
\documentclass{article}
\usepackage{etoolbox}
\usepackage[hidelinks]{hyperref}
% Load AFTER hyperref
\usepackage{usebib}
% Use BEFORE \bibinput
\newbibfield{abstract}
\makeatletter
\patchcmd[\long]{\reuse@extract}{}{}{}{}
\makeatother
\bibinput{\jobname}
\begin{document}
\usebibentry{key}{title}
\usebibentry{key}{abstract}
\end{document}

Note. I removed biblatex because it's not needed. On the other hand, I believe you can extract fields using biblatex methods.

Alternatively, use the experimental expl3 version.

usebib3.sty

\RequirePackage{expl3,xparse,url}
\ProvidesExplPackage{usebib3}{2020/10/11}{v. 0.2}{Use fields read in bib files}
\prop_new:N \l__usebib_temp_prop
\prop_new:N \g__usebib_temp_prop
\seq_new:N \g_usebib_discard_seq
\NewDocumentCommand{\bibinput}{m}
 {
  \clist_map_function:nN { #1 } \usebib_main:n
 }
\NewExpandableDocumentCommand{\usebibentry}{mm}
 {
  \prop_item:cf { g_usebib_entry_#1_prop } { \str_lower_case:n { #2 } }
 }
\NewExpandableDocumentCommand{\usebibentryurl}{O{|}m}
 {
  __usebib_url:nf { #1 } { \prop_item:cn { g_usebib_entry_#2_prop } { url } }
 }
\NewDocumentCommand{\newbibfield}{m}{}
@ifpackageloaded{hyperref}
 {
  \cs_new_protected:Nn __usebib_url:nn
   {
    \url{ #2 }
   }
 }
 {
  \cs_new_protected:Nn __usebib_url:nn
   {
    \tl_rescan:nn { \url #1 #2 #1 }
   }
 }
\cs_generate_variant:Nn __usebib_url:nn { nf }
\cs_generate_variant:Nn \prop_item:Nn { cf }
\NewDocumentCommand{\newbibignore}{m}
 {
  \seq_gput_right:Nx \g_usebib_discard_seq { \str_lower_case:n { #1 } }
 }
\cs_new_protected:Nn \usebib_main:n
 {
  \group_begin:
  \char_set_active_eq:NN @ \usebib_read_entry:w
  \char_set_catcode_active:n { `@ }
  \file_input:n { #1.bib }
  \group_end:
 }
\cs_new_protected:Npn \usebib_read_entry:w #1#
 {
  \seq_if_in:NxTF \g_usebib_discard_seq { \str_lower_case:n { #1 } }
   {
    \use_none:n
   }
   {
    __usebib_read_entry:w
   }
 }
\cs_new_protected:Npn __usebib_read_entry:w
 {
  \group_begin:
  \char_set_catcode_other:n { @ } % may appear in text \char_set_catcode_other:n {% } % can be used in URLs
  \char_set_catcode_other:n { `# } % can be used in fields
  __usebib_read_entry:n
 }
\cs_new_protected:Nn __usebib_read_entry:n
 {
  \prop_gset_from_keyval:Nn \g__usebib_temp_prop { USEBIBREADKEY=#1 }
  \group_end:
  __usebib_save_entry:
 }
\cs_new_protected:Nn __usebib_save_entry:
 {
  \prop_clear:N \l__usebib_temp_prop
  \prop_map_inline:Nn \g__usebib_temp_prop
   {
    \str_if_eq:nnF { ##1 } { USEBIBREADKEY }
     {
      \prop_put:Nxn \l__usebib_temp_prop { \str_lower_case:n { ##1 } } { ##2 }
     }
   }
  \prop_new:c { g_usebib_entry_ \prop_item:Nn \g__usebib_temp_prop { USEBIBREADKEY } prop }
  \prop_gset_eq:cN
   { g_usebib_entry \prop_item:Nn \g__usebib_temp_prop { USEBIBREADKEY } _prop }
   \l__usebib_temp_prop
 }
\cs_generate_variant:Nn \prop_put:Nnn { Nx }

Now you can do

\begin{filecontents*}{\jobname.bib}
@misc{key,
  title = {title},
  year = {2000},
  abstract = {abstract \par abstract},
}
\end{filecontents*}
\documentclass{article}
\usepackage[hidelinks]{hyperref}
% Load AFTER hyperref
\usepackage{usebib3}
% Use BEFORE \bibinput
\newbibfield{abstract}
\bibinput{\jobname}
\begin{document}
\usebibentry{key}{title}
\usebibentry{key}{abstract}
\end{document}