Is there a way to replace an environment with a command?

Question

Under certain conditions (triggered by selection of a package option) I would like to change specific environments into margin notes. However marginnote is not itself an environment, but a command that takes its contents as an argument. So what I want to do is replace

\begin{someenvironment}
...
\end{someenvironment}

with

\marginnote{
...
}

but I clearly cannot do this with something like

\renewenvironment{someenvironment}
    {\marginnote{ }
    { }}

What is the correct way to replace an environment with a command that takes the environment's contents as an argument?

Answer 1

The environ package may be of help here. You define an environment using

\NewEnviron{<env>}{%
  % environment contents
}

and the contents of the environment is available in the macro \BODY. Here's a minimal example:

\documentclass{article}
\usepackage{marginnote}% http://ctan.org/pkg/marginnote
\usepackage{environ}% http://ctan.org/pkg/environ
\NewEnviron{someenvironment}{%
  \marginnote{\BODY}% Place contents of <someenvironment> in a \marginnote
}
\begin{document}
Here is some text with a margin note.
\begin{someenvironment}
This is a margin note.
\end{someenvironment}
\end{document}
​

\NewEnviron has a macro-like look to it, so the usage should be fairly intuitive.

Answer 2

Note also that you can use xparse (now included directly inside LaTeX), recent enough versions provide b or +b arguments (to allow multiple paragraphs):

\documentclass{article}
\usepackage{xparse}
\usepackage{marginnote}% http://ctan.org/pkg/marginnote
\usepackage{environ}% http://ctan.org/pkg/environ
\NewDocumentEnvironment{someenvironment}{+b}{%
  \marginnote{#1}% Place contents of <someenvironment> in a \marginnote
}{}
\begin{document}
Here is some text with a margin note.
\begin{someenvironment}
This is a margin note.
\end{someenvironment}
\end{document}

I also got lucky sometimes by replacing {...} with \bgroup...\egroup, but it seems that it's not working for all functions.