List Parsing the ascii space character

Question

I have been using the list parsing functionality from etoolbox and I have a weird use case in which I want a space separated list.

\usepackage{etoolbox}
\DeclareListParser*{\symbolListParser}{<symbol for space character>}
\newcommand{\processSymbolList}[1]{
    \symbolListParser{}{#1}
}

I am unsure where to even start looking. I have poked around for a while looking at documentation about lists, documentation about space, and documentation about ascii characters to no avail.

so if someone could either tell me how to do it, where to look or that it isn't possible I would really appreciate it.

UPDATE: I did find this but I was looking for something a bit cleaner like what the OP wanted

UPDATE2: Thanks to Tobi for the answer. I've accepted it. below is a more general version of spacelist that behaves more like the list parser

\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{ \spacelist }{ mm }{
    \seq_set_split:Nnn \l_tmpa_seq { ~ } { #2 }
    \seq_map_inline:Nn \l_tmpa_seq {
        #1{##1}
    }
}

\ExplSyntaxOff

% to use you would write it like:
\spacelist{\fbox}{Boxes and Spaces in a List}

Answer 1

Here’s an approach using expl3

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand{ \spacelist }{ m }{
   \seq_set_split:Nnn \l_tmpa_seq { ~ } { #1 }
   \seq_map_inline:Nn \l_tmpa_seq {
      \fbox { ##1 }
   }
}

\ExplSyntaxOff

\begin{document}
List: \spacelist{Boxes and Spaces in a List}
\end{document}

The code uses so called sequences which is generated by splitting the Argument #1of \spacelist at spaces, which are ~ in the new syntax*, while normal spaces are ignored. The sequins ist stored in a local, temporarily variable named \l_tempa_seq and with \seq_map_inline we can iterate through all items of the sequence. The current item is given as argument code #1, here it is ##1 because the mapping is nested inside a definition.

You may replace the \fbox part to fit your needs …

---

* new syntax means the syntax introduced with LaTeX3 which is currently available as the expl3packed, which is loaded by xparse. To find out more look at the manual of expl3 or source3.

Answer 2

The listofitems package can provide it, in either rendered or detokenized form:

\documentclass{article}
\usepackage{lmodern}
\usepackage[T1]{fontenc}
\usepackage{listofitems}
\newcommand\spacelist[2][]{
  \setsepchar{ }%
  \readlist\mylist{#2}%
  \showitems#1\mylist%
}
\begin{document}
List: \spacelist{Boxes and \textbf{Spaces} in a List}

Tokens: \spacelist[*]{Boxes and \textbf{Spaces} in a List}
\end{document}

Answer 3

You can write in two lines of TeX macros a no-package approach for your goal.

\documentclass{article}

\catcode`z 3
\makeatletter
\newcommand\spacelist[1]{\spacelist@boxit #1 {z} }%
\long\def\spacelist@boxit #1 {\ifx z#1\relax\else
         \fbox{#1}\expandafter\spacelist@boxit\fi}
\catcode`z 11
\makeatother

\begin{document}
List: \spacelist{Boxes and Spaces in a List}
\end{document}

Admission of guilt: the method is fragile if the parsed list contains \else or \fi tokens.

Here is more robust way still with simple means. But argument should not contain catcode 3 letter Z...

\documentclass{article}
\usepackage[T1]{fontenc}

\catcode`Z 3
\makeatletter
\newcommand\spacelist[1]{\spacelist@getone{}#1 Z }%
\long\def\spacelist@getone #1 {\spacelist@check #1.Z\spacelist@check{#1}}%
\long\def\spacelist@check  #1Z#2\spacelist@check#3%
        {\if\relax\detokenize{#1}\relax
         \expandafter\@gobbletwo % abort parse
         \else
%
% #3 contains the searched for item, but with an empty brace pair
% added, which serves to prevent brace removal in processing
% so I am showing here how to remove it with \expandafter/\@gobble
% initial braces are not lost.
% 
         \fbox{\detokenize\expandafter{\@gobble#3}}%
         \fi
         \spacelist@getone{}}%
\catcode`Z 11
\makeatother

\begin{document}
List: \spacelist{Boxes {and Spaces} in a {List with \if, \else, \end tokens}}

\end{document}