how to space after LaTeX3 commands

Question

I am trying to define a message that includes a computed number.

\RequirePackage{expl3}
\ExplSyntaxOn

\int_set:Nn \l_tmpa_int { 42 }
\msg_new:nnn { package } { message } {
  The~number~ \int_use:N \l_tmpa_int ~is~too~large
}
\msg_error:nn { package } { message }

This results in

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! package error: "message"
! 
! The number 42is too large
! 
! See the package documentation for further information.
! 
! Type <return> to continue.
!...............................................  

Note the missing space after the number. To insert it I could

• make the number an argument, passed when the message is issued. This would take care of the problem in messages, but I suppose it to affect all output material.
• Replacing the space after the number with \c_space_tl also works. But this does not look like something \c_space_tl was intended for. Is it even save to use \c_space_tl or does it have to be wrapped in \tl_use:N?

Is there a better or even recommended way to insert a space after such a LaTeX3 command?

Answer 1

It appears that the integer control sequence \l_tmpa_int is gobbling the explicit spaces after it.

To prevent that behavior, I do think \c_space_tl is a good option. It's definition from my copy of expl3-code.tex is

\tl_const:Nn \c_space_tl { ~ }

So the token list is just a space character wrapped in a control sequence. After all of the gobbling of the space characters following the control sequence is finished, that ~ is placed into the input stream, and mission accomplished.

The variable \c_space_tl is usable on its own since, to quote the API documentation, "it is possible to use a <tl var> directly without an accessor function." I've never run into a situation where \tl_use:N was required (not to say it never is) and think it may exist mainly to generate the extremely useful variant \tl_use:c.

Another option is to use the counter as the argument to a function that will output the value of the counter. Three that come to mind are

• \int_use:c{l_tmpa_int}
• \int_eval:n{\l_tmpa_int}
• \int_to_arabic:n{\l_tmpa_int}

With the last one being my favorite due to its extremely explicit nature about what it's doing.

---

Side note: To prevent this in normal text, I'd append an empty group {} at the end of the sequence, but that appears to generate a literal {} in this context, which is no good.

Answer 2

The fact that spaces disappear after a control sequence is unfortunate but unavoidable (this is the same as when TeX ignores spaces in a document after a command).

In a message, use \ (a backslash followed by a space) to produce a space. You can of course use \c_space_tl, but that is longer.

So taking your example, you can do:

\RequirePackage{expl3}
\ExplSyntaxOn

\int_set:Nn \l_tmpa_int { 42 }
\msg_new:nnn { package } { message } {
  The~number~ \int_use:N \l_tmpa_int \ is~too~large
}
\msg_error:nn { package } { message }

I would advise however to pass the value of \l_tmpa_int as an argument to the message:

\RequirePackage{expl3}
\ExplSyntaxOn

\int_set:Nn \l_tmpa_int { 42 }
\msg_new:nnn { package } { message } {
  The~number~#1~is~too~large
}
\msg_error:nnx { package } { message } { \int_use:N \l_tmpa_int }

This way there is no worry about spaces, and if some day you want to provide different choices of languages for your error message that will let you separate a bit better the message text from the implementation detail of using \l_tmpa_int.