No space no caps tokenlist

Question

This took me longer than I'd like to accomplish what should have been a very simple task. I needed a way to turn a tokenlist lowercase and remove the spaces. But str_lowercase:N doesn't exist! And trying to generate the variant failed. Any combination I tried of \MakeLowercase and \tl_remove_all:Nn didn't play nice together. I ended up using two helper functions to get the job done. The solution works so far. I am making this question two fold, for one to document my solution and two, to ask if there is a better way, preferably using expl3 only.

% this function exists to simplify the call for the other, reducing the number of arguments to 1.
\cs_new_protected:Npn \tl_lowercase:N #1
    { \bob_tl_lowercase:No #1 #1 }
% this function and its generated varient cause arguement 2 to expand once 
% so it can be accepted by \str_lowercase:n, then sets it to itself
\cs_new_protected:Npn \bob_tl_lowercase:Nn #1#2
    { \tl_set:Nx #1 { \str_lowercase:n {#2} } }
\cs_generate_variant:Nn \bob_tl_lowercase:Nn { No }
% just a example use case function
\tl_new:N \itemfilename
\NewDocumentCommand{\getfilename}{s+m} {
    \tl_set:Nn \itemfilename {#2}
    \tl_remove_all:Nn \itemfilename {~}
    \tl_lowercase:N \itemfilename
    \tl_show:N \itemfilename
}

Answer 1

There is \str_lowercase:n and it's fully expandable, but (as Joseph rightly suggests) there's the better \str_foldcase:n.

\str_new:N \l_bob_itemfilename_str
\NewDocumentCommand{\getfilename}{m}
 {
  \str_set:Nx \l_bob_itemfilename_str { \str_foldcase:n { #1 } }
  \str_replace_all:Nnn \l_bob_itemfilename_str { ~ } { }
  \str_show:N \l_bob_itemfilename_str
 }
\getfilename{SaMPleName WITH upperCase and SPACES}

The console shows

> \l_bob_itemfilename_str=samplenamewithuppercaseandspaces.

An improved version with the possibility to manage an already built token list or string:

\documentclass{article}
\ExplSyntaxOn
\NewDocumentCommand{\getfilename}{sm}
 {
  \IfBooleanTF { #1 }
   {
    \bob_filename_get:V #2
   }
   {
    \bob_filename_get:n { #2 }
   }
 }
\str_new:N \l_bob_filename_item_str
\cs_new_protected:Nn \bob_filename_get:n
 {
  \str_set:Nx \l_bob_filename_item_str { \str_foldcase:n { #1 } }
  \str_replace_all:Nnn \l_bob_filename_item_str { ~ } { }
  \str_show:N \l_bob_filename_item_str
 }
\cs_generate_variant:Nn \bob_filename_get:n { V }
\getfilename{SaMPleName WITH upperCase and SPACES}
\def\test{SaMPleName WITH upperCase and SPACES}
\getfilename*{\test}

In either case the console will have

> \l_bob_filename_item_str=samplenamewithuppercaseandspaces.

Answer 2

Having looked around I think that this is the proper answer to the question.

 \tl_set:Nx \itemfilename { \str_foldcase:V \itemfilename }

A important piece of information is with \str_foldcase "The result of this process is left in the input stream." This also seems true for str_lowercase even though it is not said in the manual.

A bit of more careful reading in the manual shows that "the [str_lowercase:n] functions should not be used for caseless comparisons".

However I was also successful with the following.

\cs_generate_variant:Nn \str_lowercase:n {V}
\tl_set:Nx \itemfilename { \str_lowercase:V \itemfilename }.

Answer 3

Here is a way without expl3...rather, it uses a token cycle to remove spaces and direct characters to the macro \explowercase.

Here it is set up to pre-expand its argument, so that it works with a string, or a \def of a string.

\documentclass{article}
\def\explowerchar#1{%
  \ifcase\numexpr`#1-`A\relax
   a\or b\or c\or d\or e\or f\or g\or h\or i\or j\or k\or l\or m\or
   n\or o\or p\or q\or r\or s\or t\or u\or v\or w\or x\or y\or z\else
   #1\fi
}
\usepackage{tokcycle}
\Characterdirective{\addcytoks[x]{\explowerchar{#1}}}
\Spacedirective{}
\newcommand\getfilename[1]{\expandedtokcyclexpress{#1}\the\cytoks}
\begin{document}
\getfilename{SaMPleName WITH upperCase and SPACES}
\def\test{SaMPleName WITH upperCase and SPACES}
\getfilename{\test}
\end{document}