!Missing control sequence inserted. \inaccessible

Question

Tried to found solution by myself but didn't find nothing certain, I'm still not very good in LaTeX. I have code which works well.

\documentclass[14pt]{extreport}
\usepackage[normalem]{ulem}

\newcommand\s{\nobreak \hskip\fill \allowbreak \space \hskip-\fill \relax}

\begin{document}
\hfill\begin{minipage}[t]{0.7\textwidth}
\begin{flushleft}
\obeyspaces\let =\s
Lorem:~\uline{ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\hfill}
\end{flushleft}
\end{minipage}
\end{document}

But after I try to move \obeyspaces\let =\s command to environment header

\documentclass[14pt]{extreport}
\usepackage[normalem]{ulem}

\newcommand\s{\nobreak \hskip\fill \allowbreak \space \hskip-\fill \relax}

\newenvironment{under}
{\obeyspaces\let =\s}
{}

\begin{document}
\hfill\begin{minipage}[t]{0.7\textwidth}
\begin{flushleft}
\begin{under}
Lorem:~\uline{ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\hfill}
\end{under}
\end{flushleft}
\end{minipage}
\end{document} 

I'm getting an error

! Missing control sequence inserted. <inserted text> \inaccessible l.13 \begin{under}

How can I fix it?

Answer 1

You have to use an active space in the instruction

\let =\s

which can be done with the \lowercase trick, see The \lowercase trick. The problem is that in

\newenvironment{under}
  {\obeyspace\let =\s}
  {}

the space is not active which it is in the “expanded” code because there \obeyspaces is executed prior to \let. In the \newenvironment, \obeyspaces is only stored and doesn't act until the environment is called.

\documentclass[14pt]{extreport}
\usepackage[normalem]{ulem}

\newcommand\spaceinunder{\nobreak \hskip\fill \allowbreak \space \hskip-\fill \relax}

\begingroup\lccode`~=` \lowercase{\endgroup
  \newenvironment{under}
  {\obeyspaces\let~=\spaceinunder}
  {}
}

\begin{document}
\hfill\begin{minipage}[t]{0.7\textwidth}
\begin{flushleft}
\begin{under}
Lorem:~\uline{ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\hfill}
\end{under}
\end{flushleft}
\end{minipage}
\end{document} 

A perhaps better definition:

\documentclass[14pt]{extreport}
\usepackage[normalem]{ulem}

\newcommand\spaceinunder{%
  \nobreak\hspace{\fill}\allowbreak\space\hspace{-\fill}\relax
}

\begingroup\lccode`~=` \lowercase{\endgroup
  \newenvironment{under}[1]
  {%
   \begin{flushright}
   \begin{minipage}[t]{#1}\raggedright
   \obeyspaces\let~=\spaceinunder
  }
  {\end{minipage}\end{flushright}}
}

\begin{document}
\begin{under}{0.7\textwidth}
Lorem:~\uline{ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua.\hfill}
\end{under}
\end{document}