Can't implement new command in latex (under beamer)

Question

I'm trying to create a new command that will ease on reference placement under beamer. This is an example file:

\documentclass[bigger]{beamer}
\usepackage[absolute,overlay]{textpos}
\tolerance=1000

\newcommand{\myref}[3]{
    \begin{textblock*}{100mm}(0.07\textwidth,.93\textheight)
      {\footnotesize #1, {\emph #2}, #3}
    \end{textblock*}}


\begin{document}

\begin{frame}
\myref{must, write, something}
\end{frame}

\end{document}

When I try to run it through pdflatex I get:

tionary/translator-theorem-dictionary-English.dict) (./error.nav)
! Argument of \end has an extra }.
<inserted text> 
                \par 
l.14 \end{frame}

Runaway argument?
! Paragraph ended before \end was complete.
<to be read again> 
                   \par 
l.14 \end{frame}


! LaTeX Error: \begin{beamer@framepauses} on input line 14 ended by \end{docume
nt}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.17 \end{document}

(./error.aux)
! You can't use `\end' in internal vertical mode.
\enddocument ... \endgroup \deadcycles \z@ \@@end 

l.17 \end{document}


! LaTeX Error: \begin{beamer@framepauses} on input line 14 ended by \end{docume
nt}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...                                              

l.17 \end{document}

)
! Emergency stop.
<*> \nonstopmode\input error.tex

!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on error.log.
[yotam@myhost Res.Prop.Exam]$ 

I couldn't find my missing } can you? *note that my tex file is named Error.tex

Answer 1

The problem here is that the command is defined with three arguments but only one is give. Two possible solutions are available: use three arguments or use one and parse the comma-separated input. The first case is just

\myref{must}{write}{something}

For the second case, I would use xparse

\documentclass[bigger]{beamer}
\usepackage{xparse}
\usepackage[absolute,overlay]{textpos}
\tolerance=1000

\NewDocumentCommand{\myref}{>{\SplitArgument{2}{,}}m}{%
  \myrefaux#1%
}
\newcommand{\myrefaux}[3]{%
    \begin{textblock*}{100mm}(0.07\textwidth,.93\textheight)
      {\footnotesize #1, {\em #2}, #3}
    \end{textblock*}
}


\begin{document}

\begin{frame}
\myref{must, write, something}
\end{frame}

\end{document}

There are many ways of setting up a comma-separated list parser, but this one is pretty easy. The auxiliary function will get three arguments in all cases, even if you mess up and forget a comma or something similar.

Answer 2

As Andrew Stacey mentioned in his comment you must write \myref{must}{write}{something}. If you want the syntax: \myref{must, write, something} you can do the following:

\newcommand{\myref}[1]{\myrefI#1\nil}
\def\myrefI#1,#2,#3\nil{%
    \begin{textblock*}{100mm}(0.07\textwidth,.93\textheight)
      {\footnotesize #1, {\emph #2}, #3}
    \end{textblock*}%
}

The complete MWE:

\documentclass[bigger]{beamer}
\usepackage[absolute,overlay]{textpos}
\tolerance=1000

\newcommand{\myref}[1]{\myrefI#1\nil}
\def\myrefI#1,#2,#3\nil{%
    \begin{textblock*}{100mm}(0.07\textwidth,.93\textheight)
      {\footnotesize #1, {\emph #2}, #3}
    \end{textblock*}%
}


\begin{document}

\begin{frame}
\myref{must, write, something}





\end{frame}

\end{document}