\message: add symbol in front of new lines even if there is a line break

Question

If I do:

\message{^^J! my very long message that will be cut on many lines because LaTeX for some weird reasons like to manually cut the lines instead of letting the terminal do it^^J}

I will see something like this in the log:

! my very long message that will be cut on many lines because LaTeX for some we
ird reasons like to manually cut the lines instead of letting the terminal do i
t

But I'd like to prepend ! to all the lines, because TeXstudio will only print lines starting with a ! as errors. How could I automatically add a symbol when a new line is created in the log?

MWE

\documentclass[]{article}
\begin{document}
\message{! my very long message that will be cut on many lines because LaTeX for some weird reasons like to manually cut the lines instead of letting the terminal do it}
\end{document}

EDIT: to clarify, I'd like the log to contain:

! my very long message that will be cut on many lines because LaTeX for some we
! ird reasons like to manually cut the lines instead of letting the terminal do
! it

(or even better if it could break at word boundaries, but hey, lets not ask too much)

Answer 1

Here, I introduce \xmessage, which takes an extended message and breaks it at the first space that exceeds a set limit (here 4o characters). Each new line begins with a ! character.

Note that any given command sequence in the \xmessage only counts as a single character.

\documentclass[]{article}
\usepackage{tokcycle}
\newcounter{charcount}
\makeatletter
\newcommand\xmessage[1]{\bgroup%
  \aftertokcycle{\typeout{! \the\cytoks}}%
  \tokcycle{\addcytoks{##1}\stepcounter{charcount}}
  {\processtoks{##1}}
  {\addcytoks{##1}\stepcounter{charcount}}
  {\addcytoks{##1}\stepcounter{charcount}\splitline}{#1}%
  \egroup}
\newcommand\splitline{%
  \ifnum\value{charcount}>40\typeout{! \the\cytoks}%
  \setcounter{charcount}{0}\cytoks{}\fi}
\begin{document}
\xmessage{my very long message that will be cut on many lines because LaTeX for some weird reasons like to manually cut the lines instead of letting the terminal do it}
\end{document}

Answer 2

Split the input (possibly after full expansion, with the *-variant) at spaces and then prepare the message computing the total length of the line and taking the decision to break if the max_print_line limit is exceeded.

\documentclass{article}
\ExplSyntaxOn
% get the current length of error lines
\sys_get_shell:nnN { kpsewhich ~ -var-value=error_line } {} \l_tmpa_tl
\int_const:Nn \c__tobiasbora_prefixmessage_limit_int { \l_tmpa_tl }
\NewDocumentCommand{\prefixmessage}{sO{!}m}
 {
  \IfBooleanTF { #1 }
   {
    \tobiasbora_prefixmessage:ne { #2 } { #3 }
   }
   {
    \tobiasbora_prefixmessage:nn { #2 } { #3 }
   }
 }
\seq_new:N \l__tobiasbora_prefixmessage_text_seq
\str_new:N \l__tobiasbora_prefixmessage_text_str
\int_new:N \l__tobiasbora_prefixmessage_len_int
\cs_new_protected:Nn \tobiasbora_prefixmessage:nn
 {
  \seq_set_split:Nne \l__tobiasbora_prefixmessage_text_seq { ~ } { \tl_to_str:n { #2 } }
  % start up
  \str_set:Nn \l__tobiasbora_prefixmessage_text_str { #1 }
  \int_zero:N \l__tobiasbora_prefixmessage_len_int
  % prepare the string to output
  \seq_map_inline:Nn \l__tobiasbora_prefixmessage_text_seq
   {
    \int_add:Nn \l__tobiasbora_prefixmessage_len_int { \str_count:n { ##1 } + 1 }
    \int_compare:nNnTF
     { \l__tobiasbora_prefixmessage_len_int }
     >
     { \c__tobiasbora_prefixmessage_limit_int - \str_count:n { #1 } }
     {
      \str_put_right:Nn \l__tobiasbora_prefixmessage_text_str { ^^J #1 ~ ##1 }
      \int_set:Nn \l__tobiasbora_prefixmessage_len_int { \str_count:n { ##1 } + 1 }
     }
     {
      \str_put_right:Nn \l__tobiasbora_prefixmessage_text_str { ~ ##1 }
     }
   }
  % output
  \iow_term:e { ^^J \l__tobiasbora_prefixmessage_text_str ^^J }
 }
\cs_generate_variant:Nn \tobiasbora_prefixmessage:nn { ne }
\ExplSyntaxOff
\prefixmessage{my very long message that will be cut on many 
  lines because \LaTeX for some weird reasons like to manually
  cut the lines instead of letting the terminal do it}
\def\foo{(something in the middle)}
\prefixmessage*[>>>]{my very long message that will be cut on many 
  lines because LaTeX \foo\space for some weird reasons like to manually
  cut the lines instead of letting the terminal do it}
\stop

Output on the terminal and in the log file

! my very long message that will be cut on many lines because \LaTeX for some
! weird reasons like to manually cut the lines instead of letting the terminal
! do it
>>> my very long message that will be cut on many lines because LaTeX
>>> (something in the middle) for some weird reasons like to manually cut the
>>> lines instead of letting the terminal do it

Answer 3

Another version with LaTeX3: lines are separated by \robExtPrefixLogMessage (if you use it in your code, choose other names not to collude with my CTAN library), and will prefix it with \robExtPrefixLogMessage.

\def\robExtMessageWithPrefixNumberLines{^^J}%
\cs_new:Nn \robExt__message_with_prefix:n {
  \str_set:Nn \l_tmpa_str {#1}  
  \int_compare:nNnTF {\str_count:N \l_tmpa_str + \str_count:N \robExtPrefixLogMessage} > {78} {
    \typeout{\robExtPrefixLogMessage \str_range:Nnn \l_tmpa_str {1} {78 - \str_count:N \robExtPrefixLogMessage}\robExtMessageWithPrefixNumberLines}
    \robExt__message_with_prefix:x {\str_range:Nnn \l_tmpa_str {78 - \str_count:N \robExtPrefixLogMessage + 1} {-1}}
  }{
    \typeout{\robExtPrefixLogMessage \l_tmpa_str \robExtMessageWithPrefixNumberLines}
  }
}
\cs_generate_variant:Nn \robExt__message_with_prefix:n { x }