How concatenate a list of strings?

Question

I want to define a lot of keys in preamble and concatenate inside the document. Like this code:

\documentclass{article}
\usepackage{etoolbox}

%---------------------------------------------------------------------
% my cls files
\providetoggle{firstkey}
\settoggle{firstkey}{true}

\newcommand{\keys}[1]{\newcommand{\mykeys}{#1.}}
\newcommand{\key}{}

\renewcommand{\key}[1]{\iftoggle{firstkey}{#1}{; #1}\settoggle{firstkey}{false}}

%-----------------------------------------------------------------------

\keys{
    \key{aaaaa}
    \key{bbbbb}
    \key{ccccc}
}

\begin{document}

\mykeys

\end{document}

But, this code generate:

aaaaa ; bbbbb ; ccccc .

How can I remove this white spaces to and get:

aaaaa; bbbbb; ccccc.

Answer 1

Add \ignorespaces in the appropriate places.

\documentclass{article}
\usepackage{etoolbox}

%---------------------------------------------------------------------
% my cls files
\providetoggle{firstkey}
\settoggle{firstkey}{true}

\newcommand{\keys}[1]{\newcommand{\mykeys}{\ignorespaces#1.}}
\newcommand{\key}[1]{%
  \iftoggle{firstkey}{#1}{; #1}%
  \settoggle{firstkey}{false}%
  \ignorespaces
}

%-----------------------------------------------------------------------

\keys{
    \key{aaaaa}
    \key{bbbbb}
    \key{ccccc}
}

\begin{document}

X\mykeys X

\end{document}

The X characters are used to show that no spurious space gets inserted.

A different approach with expl3. The user inputs the keys with a more natural syntax and it's a job for expl3 to normalize it. In the example there is a trailing semicolon that would produce an empty key, but it can be easily checked for and removed.

\documentclass{article}
\usepackage{xparse}

%---------------------------------------------------------------------
% my cls files

\ExplSyntaxOn
\NewDocumentCommand{\keys}{m}
 {
  \seq_set_split:Nnn \l_tmpa_seq { ; } { #1 }
  \tl_if_blank:xT { \seq_item:Nn \l_tmpa_seq { -1 } }
   {% user has a trailing semicolon, remove the last (empty) item
    \seq_pop_right:NN \l_tmpa_seq \l_tmpa_tl
   }
  \tl_set:Nx \mykeys { \seq_use:Nn \l_tmpa_seq { ;~ } .}
 }
\prg_generate_conditional_variant:Nnn \tl_if_blank:n { x } { T }
\ExplSyntaxOff

%-----------------------------------------------------------------------

\keys{
  aaaaa;
  bbbbb;
  ccccc;
}

\begin{document}

X\mykeys X

\end{document}

Answer 2

Add a % after each key, since a new line in TeX is a space. See What is the use of percent signs (%) at the end of lines?

\documentclass{article}
\usepackage{etoolbox}

%---------------------------------------------------------------------
% my cls files
\providetoggle{firstkey}
\settoggle{firstkey}{true}

\newcommand{\keys}[1]{\newcommand{\mykeys}{#1.}}
\newcommand{\key}{}

\renewcommand{\key}[1]{\iftoggle{firstkey}{#1}{; #1}\settoggle{firstkey}{false}}

%-----------------------------------------------------------------------

\keys{
    \key{aaaaa}%
    \key{bbbbb}%
    \key{ccccc}%
}

\begin{document}

\mykeys

\end{document}