\documentclass{article}
\def\eatone#1{}
\def\eatnone#1{#1}
\newcommand{\onearg}[1]{(onearg: #1)}
\newcommand{\twoarg}[2]{(twoarg: #1 ; #2)}
\newcommand{\twoargopt}[2][missing]{(twoargopt: #1 ; #2)}
\def\textwoarg#1#2{(textwoarg: #1 ; #2)}
\begin{document}
\section{Testing}
Testing raw tex macros:
Expect inline math: xx\eatone$$a+b\eatone$$xx
Expect display math: xx\eatnone$$a+b\eatnone$$xx
Expect 2 empty maths: xx$ $a+b$ $xx
Testing latex macros
Expect (onearg: one) \onearg{one}
Expect (twoarg: one two) \twoarg{one}{two}
Expect (twoargopt: one ; two) \twoargopt[one]{two}
Expect (twoargopt: missing ; two)\twoargopt{two}
Testing whitespace:
Expect (twoarg: a ; b) \twoarg
    {a}   
    {b}   
Expect (textwoarg: a ; b) \textwoarg
    {a}   
    {b}   
Expect (twoargopt: a ; b) \twoargopt
    [a]
    {b}
\expandafter\def\expandafter\baz\eatone##1{(#1)}
Expecting (beep) : \baz{beep}
\expandafter\def\expandafter\bloop\expandafter{\baz{foo}}
Expecting (foo) : \bloop
%\expandafter\def\expandafter\blorp\expandafter{\textrm{foo}}
%\blorp
%\section{Error inside arg \stoopid undefined macro}
\def\inside#1{[[#1]]}
\def\outside{\inside{x}}
\def\deftoo#1{{\def\inside##1{(#1(##1)#1)}\outside}}
Expect [[x]] : \outside
Expect (y(x)y) : \deftoo{y}
Expect [[x]] : \outside
\def\now#1{NOW #1}
\def\later#1{LATER #1}
\edef\expanded{\now{a} \later{b} \noexpand\later{c}}
\def\later#1{MUCHLATER #1}
Expect NOW a LATER b MUCHLATER c: \expanded
\end{document}