[Xy-pic] xy-pic arrow problem

[Michael Barr]

I don't know what you are doing wrong, but I got an error message (not the 
same error message, but an error message) when running the same file.  I 
once learned all the xy syntax needed to program a front end, diagxy, but 
have since forgot it.  But the following runs perfectly:

\documentclass{article}
  \pdfoutput0
  \usepackage[matrix]{xy}
  \input diagxy
\begin{document}

  % $\xymatrix{R\ar@{-->}[r] B}$
  $\bfig \morphism/-->/[A`B;]\efig$
\end{document}

You can even more simply use

$R\to/-->/B$

Probably you will find the first arrow longer than you want and the second 
shorter, but either can be adjusted easily.

Diagxy is available from CTAN.

Michael