What's the difference between the various methods for producing for loops?

Question

I've been wondering about the various methods available for looping over a comma separated list and the like. In particular, I was wondering about their various strengths and weaknesses. In other words, I was wondering about such things as the following (but not solely limited to this list):

1. whether or not they're expandable,
2. how they handle empty items,
3. how they handle extraneous leading and trailing spaces
4. whether you can use \def/\edef or need to use \gdef/\xdef to save information from within the loop for later use.

In no particular order, here's a list of looping methods I'm familiar with. In the following list \current@item represents a macro, taking one argument, for formatting the current item in iteration.

Using commands from the 2ekernal:

%% \@for
\def\@for@myloop#1{%%
  \@for \x:=#1 \do{\current@item \x}}

%% \@tfor (not a comma separated list--probably shouldn't be here)
\def\@tfor@myloop#1{%%
  \@tfor \x:=#1 \do{\if,\x\relax\else\current@item \x\fi}}

Using etoolbox package

%% `etoolbox`:  need to be careful whether passed a macro:
%%  in that case expansion may be necessary to that the 
%%  delimiters are visible to `\forcsvlist`
\def\etoolbox@myloop#1{%%
  \expandafter\forcsvlist
  \expandafter\current@item
  \expandafter{#1}}

Using pgffor package

%% pgffor
\def\pgffor@myloop#1{%%
  \foreach \x in {#1} {\current@item \x}}

A homebrew method:

%% version a la `ae`
\def\@ae@myloop#1,#2\@nil{%%
  \current@item{#1}%%
  \expandafter\ifx\expandafter\relax\detokenize{#2}\relax\else
    \@ae@myloop#2\@nil
   \fi
}
\def\ae@myloop#1{%%
  \@ae@myloop#1,\@nil
}

Then there are also the various expl3 methods such as (there are quite a few, so this is hardly exhaustive):

\tl_map_inline:nn
\tl_map_function:Nn
\clist_map_inline:Nn
\seq_map_inline:Nn

Here is a MWE illustrating the results of each of these (including one LaTeX3 version):

\documentclass{article}
\usepackage[margin=0.5in,paperheight=15in]{geometry}
\usepackage{xparse}
\usepackage{etoolbox}
\usepackage{pgffor}
\usepackage{enumitem}
\makeatletter

\def\my@item@count{0}
\def\step@my@counter{\xdef\my@item@count{\number\numexpr\my@item@count+1\relax}}
%% \rules to emphasize how spaces are seen and treated!
\def\current@item#1{\step@my@counter\item \rule{0.4pt}{2ex}#1\rule{0.4pt}{2ex}}

%% `etoolbox`
\def\etoolbox@myloop#1{%%
  \forcsvlist \current@item {#1}}
  %% but if passed a macro, then it  first needs to be expanded so the delimiters are visible to `\forcsvlist`.  You'll need to write
  %% \expandafter\forcsvlist \expandafter\current@item \expandafter{#1}}

%% \@for
\def\@for@myloop#1{%%
  \@for \x:=#1 \do{\current@item \x}}

%% \@tfor
\def\@tfor@myloop#1{%%
  \@tfor \x:=#1 \do{\current@item \x}}

%% pgffor
\def\pgffor@myloop#1{%%
  \foreach \x in {#1} {\current@item \x}}

%% version a la `ae`
\def\@ae@myloop#1,#2\@nil{%%
  \current@item{#1}%%
  \expandafter\ifx\expandafter\relax\detokenize{#2}\relax\else
    \@ae@myloop#2\@nil
   \fi
}
\def\ae@myloop#1{%%
  \@ae@myloop#1,\@nil
}

\def\listoffruit#1#2#3{%%
  \def\my@item@count{0}%%
  \noindent
  List of type \texttt{#1}: \parbox[t]{3in}{\raggedright#3}
  \begin{itemize}[topsep=4pt,itemsep=2pt]
    \csname#1@myloop\endcsname{#2}%%
  \end{itemize}
  Total number of bulleted items: \my@item@count
  \par \vspace{2ex}\hrule\par \vspace{2ex}
  }

\ExplSyntaxOn
\NewDocumentCommand{\expl@myloop}{ m }
{
  \clist_map_inline:nn{#1}{\current@item {##1}}
}
\ExplSyntaxOff

\makeatother
\def\apples{apples}
\def\bananas{bananas}
\def\cherries{cherries}

\begin{document}

\listoffruit{etoolbox}{apples,, oranges, bananas ,cherries}{Ignores leading spaces and empty items. Trailing spaces not ignored.}

\listoffruit{@for}{apples,, oranges, bananas ,cherries}{Empty lines and trailing or leading spaces not ignored.  Something goes on with last item in list.}

\listoffruit{@tfor}{\apples,{} {oranges}\bananas\cherries}{Spaces ignored, all other token respected, bracketed tokens treated as one.}

\listoffruit{pgffor}{apples,, oranges, bananas ,cherries}{Ignores leading spaces, empty items and trailing spaces not ignored.}

\listoffruit{ae}{apples,, oranges, bananas ,cherries}{Leading or trailing spaces not ignored.  Empty items not ignored.}

\listoffruit{expl}{apples,, oranges, bananas ,cherries}{Trailing or leading spaces ignored.   Empty items ignored.}

\end{document}

Issues I'm aware of:

• I believe \@for is not expandable,
• \cslist_map_inline:nn is expandable but with limitations: i.e., it is not expandable in an f-type argument.
• pgffor's \foreach loop in executed within a group. So you need to use \gdef or \xdeg to save information from within the group for later use. I've not fully explored which others of the loops presented here have a similar short-coming (not sure that's the right choice of word). I have no idea whether \foreach is expandable or not.
• Some methods handle their lists rather nicely whether passed explicitly or implicitly via a macro. For example, pgffor's \foreach knows what do to with a list passed via a macro. etoolbox's \forcsvlist needs that macro to first be expanded: hence the reason my first illustration of a \forcslist is as complicated as it is.

So what I'm interested in here is:

1. Responses which address the strengths and weaknesses of the different approaches presented here as itemized at the beginning of this post, but not necessarily limited to those suggestions since I may be unaware of other important issues.. For example, I'm not really sure which are expandable.
2. Responses which introduce other methods for iterating over a list of items along with their known weaknesses and strengths.
3. Responses which can illustrate realistic examples for which one would want to use such a loop in an expandable context
4. Reponses which illustrate how one can save information gathered from within the group for later use.

Answer 1

I propose a different definition of \current@item

\def\current@item#1{%
  \stepcounter{item@count}
  \item $|$#1$|$\ $|$\texttt{\detokenize{#1}}$|$%
}

so the output also shows what's really passed as its argument. Also I changed the complicated management of \my@item@count with a simple counter. I don't comment about \@tfor, which is a different tool not designed for comma separated lists.

1. etoolbox: \forcsvlist doesn't remove trailing spaces; your definition is too complicated, because

   %% `etoolbox`
   \def\etoolbox@myloop#1{%%
     \forcsvlist\current@item{#1}}
   

   suffices. Items are passed explicitly.

2. \@for is the basic tool defined in the LaTeX kernel. Leading and trailing spaces are not removed. Items are passed as \x, the control sequence used after \@for.

3. \foreach doesn't remove trailing spaces. Items are passed as \x, the control sequence used after \foreach.

4. ae is basically like \@for, although it works by expansion. Leading and trailing spaces are not removed. Items are passed explicitly.

5. expl removes leading and trailing spaces, but also empty items; items are passed explicitly.

\documentclass{article}
\usepackage[margin=0.5in,paperheight=15in]{geometry}
\usepackage{xparse}
\usepackage{etoolbox}
\usepackage{pgffor}
\usepackage{enumitem}
\makeatletter

\newcounter{item@count}
%% \rules to emphasize how spaces are seen and treated!
\def\current@item#1{%
  \stepcounter{item@count}
  \item $|$#1$|$\ $|$\texttt{\detokenize{#1}}$|$%
}

%% `etoolbox`
\def\etoolbox@myloop#1{%%
  \forcsvlist\current@item{#1}}

%% \@for
\def\@for@myloop#1{%%
  \@for \x:=#1\do{\current@item \x}}

%% pgffor
\def\pgffor@myloop#1{%%
  \foreach \x in {#1} {\typeout{pgf:\x}\current@item \x}}

%% version a la `ae`
\def\@ae@myloop#1,#2\@nil{%%
  \current@item{#1}%%
  \if\relax\detokenize{#2}\relax\else
    \@ae@myloop#2\@nil
   \fi
}
\def\ae@myloop#1{%%
  \@ae@myloop#1,\@nil
}

\def\listoffruit#1#2{%%
  \setcounter{item@count}{0}%%
  \noindent
  List of type \texttt{#1}
  \begin{itemize}[topsep=4pt,itemsep=2pt]
    \csname#1@myloop\endcsname{#2}%%
  \end{itemize}
  Total number of bulleted items: \arabic{item@count}%
  \par \vspace{2ex}\hrule\par \vspace{2ex}
  }

\ExplSyntaxOn
\NewDocumentCommand{\expl@myloop}{ m }
{
  \clist_map_inline:nn{#1}{\current@item {##1}}
}
\ExplSyntaxOff

\makeatother

\begin{document}

\listoffruit{etoolbox}{apples,, oranges, bananas ,cherries}

\listoffruit{@for}{apples,, oranges, bananas ,cherries}

\listoffruit{pgffor}{apples,, oranges, bananas ,cherries}

\listoffruit{ae}{apples,, oranges, bananas ,cherries}

\listoffruit{expl}{apples,, oranges, bananas ,cherries}

\end{document}

I'd add another expl3 loop, that also considers empty items:

\ExplSyntaxOn
\NewDocumentCommand{\seq@myloop}{ m }
 {
  \seq_set_split:Nnn \l_tmpa_seq { , } { #1 }
  \seq_map_inline:Nn \l_tmpa_seq { \current@item {##1} }
 }
\ExplSyntaxOff

(of course, using \l_tmpa_seq is discouraged, better allocate a new variable).

The comparison should leave no doubt. The only place where \foreach is superior is in its treatment of “incomplete lists”, such as 1,2,...,20.

Answer 2

LuaLaTeX:

\documentclass{standalone}
\usepackage{tikz}
\usepackage{filecontents}
\begin{document}
    \newcommand{\drawCircle}[1]{\tikz \draw (0,0) circle (#1);}
    \begin{filecontents*}{testlua.lua}
        for i=1,10 do
            tex.print("\\drawCircle{",0.1*i,"}")
        end
    \end{filecontents*}
    \directlua{dofile('testlua.lua')}
\end{document}