mjhjmtu

As stated in the interface3 document (Part IX, 1. Creating and initializing sequences) seq_set_split: does not preserve spaces. Wishing to scan the text contained in a _tl variable into a sequence item by item for further processing (to be parsed for example) it would be advantageous to have spaces preserved. The MWE demonstrates that this can be achieved by introducing hard spaces, but is there a way to make do without the ~s?

documentclassarticle
% RN. 11 Nov 2018
%=======================
usepackage[check-declarations,log-functions]expl3
usepackagexparse
%-----------------------
ExplSyntaxOn
tl_new:N l_rn_auxOne_tl
seq_new:N l_rn_auxOne_seq

NewDocumentCommandmyScanTextm
 
 tl_set:Nn l_rn_auxOne_tl #1
 textbftl~variable:~tl_use:N l_rn_auxOne_tl\
 seq_set_split:NnV l_rn_auxOne_seq l_rn_auxOne_tl
 textbfseq~variable:~seq_use:Nn l_rn_auxOne_seq ,\
 % myExperminent
ExplSyntaxOff
%-----------------------
begindocument
 verb+myScanTextThe quick brown fox jumps over the lazy dog.+\ 
 myScanTextThe quick brown fox jumps over the lazy dog.

 verb+myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.+\
 myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.
enddocument

Use a slower routine first splitting at spaces.

documentclassarticle
usepackage[margin=1cm]geometry
usepackagexparse

ExplSyntaxOn
tl_new:N l_rn_auxOne_tl
tl_new:N l_rn_auxTwo_tl
seq_new:N l_rn_auxOne_seq
seq_new:N l_rn_auxTwo_seq

NewDocumentCommandmyScanTextm
 
 tl_set:Nn l_rn_auxOne_tl #1
 seq_clear:N l_rn_auxOne_seq
 % split at spaces
 seq_set_split:Nnn l_rn_auxTwo_seq ~ #1 
 % the first item is special, pop it out and split it
 seq_pop_left:NN l_rn_auxTwo_seq l_rn_auxTwo_tl
 tl_map_function:NN l_rn_auxTwo_tl __rn_add:n
 % now do the other items, reinserting the space before them
 seq_map_inline:Nn l_rn_auxTwo_seq
 
 __rn_add:n ~ 
 tl_map_function:nN ##1 __rn_add:n
 
 % print the data
 textbftl~variable:~tl_use:N l_rn_auxOne_tl\
 textbfseq~variable:~seq_use:Nn l_rn_auxOne_seq ,
 
cs_new_protected:Nn __rn_add:n
 
 seq_put_right:Nn l_rn_auxOne_seq #1 
 
ExplSyntaxOff
%-----------------------
begindocument

setlengthparindent0pt % just for the example

myScanTextThe quick brown fox jumps over the lazy dog.

myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.

enddocument

Alternative method: first replace spaces with a private token and then split with tl_map_function:NN

documentclassarticle
usepackage[margin=1cm]geometry
usepackagexparse

ExplSyntaxOn
tl_new:N l_rn_auxOne_tl
seq_new:N l_rn_auxOne_seq

NewDocumentCommandmyScanTextm
 
 seq_clear:N l_rn_auxOne_seq
 tl_set:Nn l_rn_auxOne_tl #1
 tl_replace_all:Nnn l_rn_auxOne_tl ~ __rn_space: 
 tl_map_function:NN l_rn_auxOne_tl __rn_add:n
 % print the data
 textbfinput:~#1\
 textbfseq~variable:~seq_use:Nn l_rn_auxOne_seq ,
 
cs_new_protected:Nn __rn_add:n
 
 tl_if_eq:nnTF #1 __rn_space: 
 
 seq_put_right:Nn l_rn_auxOne_seq ~ 
 
 
 seq_put_right:Nn l_rn_auxOne_seq #1 
 
 
cs_new_protected:Nn __rn_space: 
ExplSyntaxOff
%-----------------------
begindocument

setlengthparindent0pt % just for the example

myScanTextThe quick brown fox jumps over the lazy dog.

myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.

enddocument

You could use str_map_inline:Nn and append to the seq. The problem is that catcodes are all 12 then.

documentclassarticle
% RN. 11 Nov 2018
%=======================
usepackage[check-declarations,log-functions]expl3
usepackagexparse
%-----------------------
ExplSyntaxOn
tl_new:N l_rn_auxOne_tl
seq_new:N l_rn_auxOne_seq

NewDocumentCommandmyScanTextm
 
 tl_set:Nn l_rn_auxOne_tl #1
 textbftl~variable:~tl_use:N l_rn_auxOne_tl\
 seq_clear:N l_rn_auxOne_seq
 str_map_inline:Nn l_rn_auxOne_tl seq_put_right:Nn l_rn_auxOne_seq ##1 
 textbfseq~variable:~seq_use:Nn l_rn_auxOne_seq ,\
 % myExperminent
ExplSyntaxOff
%-----------------------
begindocument
 verb+myScanTextThe quick brown fox jumps over the lazy dog.+\ 
 myScanTextThe quick brown fox jumps over the lazy dog.

 verb+myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.+\
 myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.
enddocument

You could pre-process the input using regex_replace_all:nnN to replace all spaces with space. (I first tried replacing spaces with ~ and ~ but this did not work so well.) Doing this your MWE produces:

Here is the code:

documentclassarticle
% RN. 11 Nov 2018
%=======================
usepackage[check-declarations,log-functions]expl3
usepackagexparse
%-----------------------
ExplSyntaxOn
tl_new:N l_rn_auxOne_tl
seq_new:N l_rn_auxOne_seq

NewDocumentCommandmyScanTextm
 
 tl_set:Nn l_rn_auxOne_tl #1
 regex_replace_all:nnN s+ cspace l_rn_auxOne_tl
 textbftl~variable:~tl_use:N l_rn_auxOne_tl\
 seq_set_split:NnV l_rn_auxOne_seq l_rn_auxOne_tl
 textbfseq~variable:~seq_use:Nn l_rn_auxOne_seq ,\
 % myExperminent
ExplSyntaxOff
%-----------------------
begindocument
 verb+myScanTextThe quick brown fox jumps over the lazy dog.+\
 myScanTextThe quick brown fox jumps over the lazy dog.

 verb+myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.+\
 myScanTextThe~quick~brown~fox~jumps~over~the~lazy~dog.
enddocument

Note that this replaces repeated spaces with a single space, which I'm guessing is probably what you'd really want.