Arguments to function not losing braces

Question

This MWE is boiled down so much the usefulness of the code is lost. However, the code nevertheless still exhibits the problem I'm encountering.

\documentclass{article}
\usepackage{pgffor}
\usepackage{etoolbox}

\makeatletter
\newcommand\aetest[1]{%%
  \foreach \myl  in {#1}
    {%%
      \expandafter\ae@parse@line\myl
    }%% 
}

%% read the next character and                
%% branch off if its "*"                      
\def\ae@parse@line{%%
  \@ifnextchar*
  {\s@ae@parse@line}
  {\@ae@parse@line[]}}

%% what to do if there is no "*"              
\def\@ae@parse@line[#1]#2#3{%%
  \fbox{Case #2}\hspace*{0.5em}%%
  \def\ae@tmp{#3}%%
  \typeout{==[base case][#2]==>\expandonce\ae@tmp}%%
  \@ae@parse@content#3\@nil
}

%% If there's  "*" the command now            
%% expects three arguments with the argument  
%% #3 dressed up like an optional argument.   
%% After processing, pass arguments back to   
%% "\@ae@parse@line"                          
\def\s@ae@parse@line*#1#2[#3]{%%
  \def\ae@tmp{#2}%%
  \typeout{==[star case]___==>\expandonce\ae@tmp}%%
  \@ae@parse@line[#3]{#1}{#2}}

%% the "content" argument                     
%% may start with "[" which                   
%% signals that the "content"                 
%% should go in a `minipage`.                 
\def\@ae@parse@content{%%
  \@ifnextchar[%%]
  {\@ae@parse@mp}
  {\@@ae@parse@content}}

%% formatting for "non-minipage" case         
\def\@@ae@parse@content#1\@nil{%%
  \textbf{#1}\par}

%% formatting for "minipage" case             
\def\@ae@parse@mp[#1]#2\@nil{%%
  \fbox{\begin{minipage}[t]{#1}
    #2
  \end{minipage}}\par}

\makeatother

\begin{document}

\aetest{
  {A}{formatted correctly},
 *{B} {formatted correctly}[],
 *{C} {[4in] spurious space, and is \textbf{not} correct formatted}[],
 *{D}{[4in] no spurious space and correctly formatted}[],
  {E} {[2in] spurious space but still correctly formatted},
  {F}{[3in] no spurious space, formatted correctly},
  {G} {done as expected}}

\end{document}

producing

What I'm curious about is why does case C not get correctly formatted, but case E does. It seems to me that, if they both suffer from the same spurious space, then I would expect them to be handled similarly.

Looking at the log file shows that in cases B and C the space is parsed as part of the argument and so the braces are not lost. But why do the braces then disappear is cases E and G?

Here's the relevant portion of the log file. The ==> points to the content of the passed argument and shows the spurious space and the braces.

==[base case][A]==>formatted correctly
==[star case]___==> {formatted correctly}
==[base case][B]==> {formatted correctly}
==[star case]___==> {[4in] spurious space, and is \textbf {not} correct formatted}
==[base case][C]==> {[4in] spurious space, and is \textbf {not} correct formatted}
==[star case]___==>[4in] no spurious space and correctly formatted
==[base case][D]==>[4in] no spurious space and correctly formatted
==[base case][E]==>[2in] spurious space but still correctly formatted
==[base case][F]==>[3in] no spurious space, formatted correctly
==[base case][G]==>done as expected

Answer 1

Argument #2 in \s@ae@parse@line is delimited (by [), so TeX doesn't skip spaces when looking for it. And, in case C, the initial space prevents the brace stripping.

Use two steps:

%% If there's  "*" the command now
%% expects three arguments with the argument
%% #3 dressed up like an optional argument.
%% After processing, pass arguments back to
%% "\@ae@parse@line"
\def\s@ae@parse@line*#1#2{%
  \s@ae@parse@line@aux{#1}{#2}%
}
\def\s@ae@parse@line@aux#1#2[#3]{%
  \def\ae@tmp{#2}%%
  \typeout{==[star case]___==>\expandonce\ae@tmp}%%
  \@ae@parse@line[#3]{#1}{#2}}

In this way argument #2 will lose the braces correctly and you won't have problems of spaces because TeX ignores them when looking for undelimited arguments.

I didn't change anything else, so the image doesn't tell the truth. ;-)

Anyway, you'll be getting the same if you leave a space before the trailing optional argument like in

 *{C} {[4in] spurious space, and is \textbf{not} correct formatted} []% <-- a space!

Answer 2

Following @egreg 's lead, here's what I came up with to handle noxious, spurious white space creeping in:

\def\s@ae@parse@line*#1#2{%%
  \def\ae@tmp@a{#1}%%
  \def\ae@tmp@b{#2}%%
  \s@ae@parse@line@aux}

\def\s@ae@parse@line@aux{%%
  \@ifnextchar[%]
  {\@s@ae@parse@line@aux}
  {\@s@ae@parse@line@aux[]}}

\def\@s@ae@parse@line@aux[#1]{%%
  \def\ae@tmp@c{#1}%%
  \edef\ae@tmp{[\expandonce\ae@tmp@c]{\expandonce\ae@tmp@a}{\expandonce\ae@tmp@b}}%%
  \expandafter\@ae@parse@line\ae@tmp}

As egreg suggested, I handle the optional argument by defining an auxiliary macro. But as pointed out, white space could still interfere with the expect parsing if it creeps in before the [. So, I handle this by saving the first two arguments to macros, then I use two auxiliary macros. The first tests whether there's a following [ (\@ifnextchar very conveniently gobbles up whitespace for me). Then this third argument is also saved to a macro. I then build the arguments for \@ae@parse@line using expansion tricks.