An expandable 'character scanning' command?

Question

Please consider the following MWE, for a "character scanning" command "\looper" which utilizes \@tfor:

\documentclass{minimal}

\usepackage{trace}

\makeatletter
\newcommand\looper[1]{%
% \def\looper#1{%
\typeout{#1}%
\@tfor\next:=#1 \do{\typeout{\next}%
\next%
}
}
\makeatother

\begin{document}

\looper{TEST1}
%% terminal typeout:
% TEST1
% T
% E
% S
% T
% 1


%\typeout{\looper{Test2}}
%% terminal typeout:
% ! Undefined control sequence.
% \next ->\@nil

\edef\tmptest{TEST3}
\looper{\tmptest}
%% terminal typeout:
% TEST3
% TEST3

% \traceon
\edef\outtest{\looper{\tmptest}}
%% terminal typeout:
% ! Undefined control sequence.
% \next ->\@nil

\end{document}

Here the \looper{TEST1} behavior is as I want it, but I'd like also to somehow use \looper{\tmptest} (where \edef\tmptest{TEST3}) and get (in terminal):

\looper{\tmptest}
%% terminal typeout:
% TEST3
% T
% E
% S
% T
% 3

However, the function doesn't really have to expand the argument on its own, as long as I can do something in the call itself, like \looper{\detokenize{\tmptest}} (which I otherwise cannot use here, as it doesn't "unpack" characters)

Furthermore, the function "returns" \next for each character scanned, and thus I'd like to be able to write something like

\edef\outtest{\looper{\tmptest}}

... and get the contents of \outtest to again be "TEST3" - except here, they would be the result of concatenation of \next of the \looper routing (thus, I could instead return -\next instead of \next, and get "-T-E-S-T-3" with a function like this).

Is there something like this built-in already - or can anyone point to a resource that contains a similar function?

---

EDIT: Just a note, probably this one doesn't expand because \@tfor uses nonexpandable (?) commands (\@nil) - I get this if I activate \traceon:

% with \traceon:
{into \tracingonline=1}
{\edef}

\looper #1->\typeout {#1}\@tfor \next :=#1 \do {\typeout {\next }\next }
#1<-\tmptest

\typeout #1->\begingroup \set@display@protect \immediate \write \@unused {#1}\e
ndgroup
#1<-\tmptest

\set@display@protect ->\let \protect \string
{\string}

\tmptest ->TEST3

\@tfor #1:=->\@tf@r #1
#1<-\next

\@tf@r #1#2\do #3->\def \@fortmp {#2}\ifx \@fortmp \space \else \@tforloop #2\@
nil \@nil \@@ #1{#3}\fi
#1<-\next
#2<- \tmptest
#3<-\typeout {\next }\next

\@fortmp -> \tmptest

\tmptest ->TEST3

\tmptest ->TEST3
{\ifx}
{false}

\@tforloop #1#2\@@ #3#4->\def #3{#1}\ifx #3\@nnil \expandafter \@fornoop \else
#4\relax \expandafter \@tforloop \fi #2\@@ #3{#4}
#1<-\tmptest
#2<- \@nil \@nil
#3<-\next
#4<-\typeout {\next }\next

\next ->\@nil
{undefined}
! Undefined control sequence.
\next ->\@nil

Answer 1

As noted in comments you can not make expandable assignments, but here is one possible definition of an expandable loop that just puts [] around each token:

\def\expandloop#1{\xloop#1\relax}
\def\xloop#1{%
  \ifx\relax#1\else[#1]\expandafter\xloop\fi}

\edef\zzzz{\expandloop{TEST1}}

\show\zzzz

\bye

running this produces:

$ tex loop
This is TeX, Version 3.1415926 (TeX Live 2011/Cygwin)
(./loop.tex
> \zzzz=macro:
->[T][E][S][T][1].
l.8 \show\zzzz

Answer 2

In LaTeX3 we have a loop called \tl_map_function:nN which seems to fulfil your requirements. That can of course be recoded in primitives, which for ease of reference I've done here:

\documentclass{article}

\usepackage{pdftexcmds}


\makeatletter
\long\def\expandableloop#1#2{%
  \expandableloop@aux#1#2\q@recursion@tail
  \break@point
}
\long\def\expandableloop@aux#1#2{%
  \break@if@quark@tail{#2}%
  #1{#2}%
  \expandableloop@aux#1%
}
\long\def\break@if@quark@tail#1{%
  \ifnum\pdf@strcmp{\noexpand\q@recursion@tail}{\unexpanded{#1}}=\z@
    \expandafter\break@if@quark@tail@aux
  \fi
}
\long\def\break@if@quark@tail@aux#1\break@point{}

\makeatletter
\newcommand\looper[1]{%
 \expandableloop\detokenize{#1}
}
\makeatother

\begin{document}

\def\tmptest{TEST3}
\edef\outtest{\expandafter\looper\expandafter{\tmptest}}
\show\outtest

\end{document}

Notice that this relies on \pdfstctmp or equivalent functionality: I've loaded the pdftexcmds package to sort out the engine variability here. Notice also that this will not 'accidentally' expand the input, so I've had to do so deliberately using \expandafter.