Getting the first line of an environment

Question

as the title suggests, I'm looking for a way to get the first (printable) line in an environment. My first naive attempt (ignoring the printable requirement) was something like this:

\newcommand*{\theline}{}
\newcommand*{\getline}[1]{%
  \renewcommand*{\theline}{#1}%
  #1%
}

\newenvironment{anewenvironment}{%
  \getline%
}{}

However, afterwards \theline is only the first character. Eg.

\begin{anewenvironment}
  Test
\end{anewenvironment}
Line: \theline

prints:

Test
Line: T

when the required output is:

Test
Line: Test

I suppose this indicates a fundamental misunderstanding about how TeX/LaTeX handles arguments on my part, so any ideas or more broad clarification on how arguments are handled would be appreciated. Thanks.

P.S. I understand that the concept of a "line" is rather vague — "paragraph" might be a better term in retrospect, though "X words" would also fit my use case.

EDIT: The aim of this is to find a way to automatically get incipits for poetry or blocks of text. See this issue on PoetryTeX for more info.

EDIT 2: Looks like none of these solutions are very good for what I need, however, they did answer my question so I've marked an answer.

Answer 1

If you like to scan for more than one token you need to tell TeX so. \def\x#1{...} will scan exactly one token. While \def\x#1\\{...} will scan until a \\ is reached (and raise an error if no such thing is present.)

So to scan one paragraph we do \def\x#1\par{...} and hope that somewhen a \par occurs.

\documentclass{report}

\begin{document}


\def\getline#1\par{%
\gdef\theline{#1}#1%
}

\newenvironment{anewenvironment}{%
  \getline%
}{}

\begin{anewenvironment}
Test

New par

\end{anewenvironment}
Line: \theline

\end{document}

Scans the 'first line'. Will raise error if there is no par in the environment.

Output will be

Test new par
Line: Test

Answer 2

Here's a possibility that however can easily break if there are nested environments or the first \par (blank line) is hidden by braces.

\documentclass{report}
\usepackage{environ}

\NewEnviron{anewenvironment}
 {\expandafter\getpar\BODY\par\end\BODY}
\long\def\getpar#1\par#2\end{\gdef\theline{#1}}

\begin{document}

\begin{anewenvironment}
Testa

New par

\end{anewenvironment}
Line: \theline

\begin{anewenvironment}
Testb
\end{anewenvironment}
Line: \theline
\end{document}