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From: ogawa@orion.arc.nasa.gov (Arthur Ogawa)
Newsgroups: comp.text.tex
Subject: Re: Strange result in macros
Summary: Difference between delimited and undelimited args
Keywords: macro, delimited, undelimited
Message-ID: <1991Jan25.084958.27482@news.arc.nasa.gov>
Date: 25 Jan 91 08:49:58 GMT
References: <1991Jan24.095405.8868@uni-paderborn.de>
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Lines: 27

The macro \foo, defined as 

\long\def\foo#1
{Hurrah #1 Hurrah}

Has a delimited argument: the delimiter is the whitespace immediately
preceeding the left brace. In your document, 

Now I test \foo{Why} and \gnat{Why not?} and everything's ok.

But watch out for the difference to \foo{Me}-too and \gnat{You}-never.

Your first instance traces to:

\foo #1 ->Hurrah #1 Hurrah
#1<-Why

In the second, it traces to 

\foo #1 ->Hurrah #1 Hurrah
#1<-{Me}-too

This behaviour is easy to understand if you keep in mind that the argument to
\foo ends at the first whitespace character---unlike the definition of \gnat.
This is an attribute of TeX (see chapter 20 of the TeXbook), not a peculiarity
of LaTeX. Keep at it, your experimentation will pay off richly in a deeper 
understanding of TeX!