Macro delimiter

Question

I'd like to have a macro to split up another macro content, with field separator being a space. So:

\def\mytext{foo bar}
\def\secondof#1 #2{#2}
\secondof\mytext

What? What do you mean: "runaway argument"? Oh right, I guess \mytext counts as only one argument and you're expecting two. Ok, so

\expandafter\secondof\mytext

and indeed, the result is "bar", just as I was hoping for.

But alas, then comes something rather surprising:

\def\firstof#1 #2{#1}
\expandafter\firstof\mytext

"fooar"? You got to be kidding me! How on earth did you—dear and beloved TeX—ever come to such a ridiculous decision? You did so well with the \secondof, after all. What gives?

(just to make this a complete MWE, I'm going to add a \bye here)

Answer 1

\firstof foo bar will simply take foo as first argument and b as second one. So the ar is left afterwards and will by typeset as ordinary text. For that reason you got foo + ar = fooar as result.

If you don't want this behaviour, you need to add an end delimiter, \nil or \@nil is often used for that purpose:

\def\firstof#1 #2\nil{#1}
\expandafter\firstof\mytext\nil

Now #1 will get foo and #2 will get all the rest until \nil, i.e. bar.