Problem with \hfill and hyphenation

Question

When I use \hfill and in the previous line the last word is hyphenated, the \hfill doesn't work the same way as usual:

\documentclass[a4paper, 12pt]{book}
\begin{document}

Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings are exceptions   \hfill go to library

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings  \hfill go to library

\end{document}

What should I write to get \hfill work like the first \hfill in the example?

Answer 1

The first solution is a modification of my solution to Bump right-aligned text to next line iff no room

I show also David's solution, in order to see where it can fail.

\documentclass[a4paper, 12pt,draft]{book} % draft for showing the overfull boxes

\newcommand*{\gotoend}[1]{%
  \unskip
  {\nobreak\hfill\penalty50\ \null\nobreak
   \hfill\mbox{#1}%
   \parfillskip=0pt \finalhyphendemerits=0 \par}
}

%% David's version
\newcommand*{\Dgotoend}[1]{\unskip\hspace*{\fill}\mbox{ #1}}


\begin{document}

Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings are exceptions \gotoend{go to library}

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings \gotoend{go to library}

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but AA phonetic spellings are just bigger 
exceptions \gotoend{go to library}

David's version:

Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings are exceptions \Dgotoend{go to library}

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings \Dgotoend{go to library}

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but AA phonetic spellings are just bigger exceptions \Dgotoend{go to library}

\end{document}

Answer 2

The behaviour isn't really related to hypphenation, but I think you just want to avoid the final phrase breaking over a line so:

\documentclass[a4paper, 12pt]{book}
\begin{document}

Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings are exceptions\hspace*{\fill}\mbox{ go to library}

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings\hspace*{\fill}\mbox{ go to library}

\end{document}

You could make a macro out of this (and address egreg's point) by

\documentclass[a4paper, 12pt,draft]{book} % draft for showing the overfull boxes


%% David's version
\newcommand*{\Dgotoend}[1]{\unskip\hfill\penalty9999\hspace*{\fill}\mbox{ #1}}


\begin{document}


David's version:

Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings are exceptions \Dgotoend{go to library}

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but phonetic spellings \Dgotoend{go to library}

AA Spellings attempt to transcribe the sounds of the language into 
alphabetic letters, but AA phonetic spellings are just bigger exceptions \Dgotoend{go to library}

\end{document}