In LaTeX3, how do I peek ahead for an active character?

Question

LaTeX3 defines the \peek_catcode_ignore_spaces:NTF which allows me to look ahead at the next token in the stream (ignoring spaces) and test its catcode against those of a slew of standard tokens. So I can do:

\peek_catcode_ignore_spaces:NTF \c_math_toggle_token
{
Uh-oh,~ upcoming~ math!
}
{
Phew!~ Just~ text.
}

to see if what's next is a math shift.

How do I do the same but for an active character? There's no \c_active_token that I can test against. I've tried making one by emulating the code that makes the other tokens, but my attempts have failed dismally. There's a \c_catcode_active_tl that contains an active token, but it contains it with an \exp_not:N so even doing

\exp_last_unbraced:NV \peek_catcode_ignore_spaces:NTF \c_catcode_token_tl

doesn't work.

Yet there is a \token_if_active:NTF so I presume I could cobble together some variant of the peek code that tested for an active character by effectively copying the code that defines \peek_catcode_stuff and swapping the tests. But before I attempt that, I thought I'd ask if there's something I'm missing.

So, is there a straightforward way to see if the next character is active?

Answer 1

You're almost there. \exp_last_unbraced:NV expands \c_catcode_active_tl only once, then

\exp_last_unbraced:NV \peek_catcode_ignore_spaces:NTF \c_catcode_active_tl
  { true } { false }

becomes

\peek_catcode_ignore_spaces:NTF \exp_not:N % <- test token
  * % <- true branch
  { true } % <- false branch
  { false } % <- leftover

then things go horribly wrong :)

Since the active token in \c_catcode_active_tl is preceded by \exp_not:N, you can safely x-expand it to expose the actual token:

\ExplSyntaxOn
\cs_new_protected:Npx \test
  {
    \peek_catcode_ignore_spaces:NTF \c_catcode_active_tl
      { \exp_not:N \__test_aux:Nn \c_true_bool }
      { \exp_not:N \__test_aux:Nn \c_false_bool }
  }
\cs_new_protected:Npn \__test_aux:Nn #1 #2
  { \iow_term:x { \bool_if:NF #1 { NOT~an~ } active~token:~\tl_to_str:n {#2} } }
\ExplSyntaxOff

\test ~
\test a
\test \stop
\test \relax
\test {ab}

\stop

The test above prints:

active token: ~
non-active token: a
non-active token: \stop
non-active token: \relax
non-active token: ab

P.S.: There is no \c_catcode_active_token because doing \let \c_catcode_active_token <actual active token> would make \c_catcode_active_token a regular macro with the same definition as the <actual active token>, which would be of no use in this case.

Answer 2

An alternative way to extract the active character token from \c_catcode_active_tl is to use \exp_args:Ne:

\documentclass{article}


\ExplSyntaxOn
\NewDocumentCommand\zzMath{}{
\peek_catcode_ignore_spaces:NTF \c_math_toggle_token
{
Uh-oh,~ upcoming~ math!
}
{
Phew!~ Just~ text.
}
}

\NewDocumentCommand\zzActive{}{
\exp_args:Ne\peek_catcode_ignore_spaces:NTF \c_catcode_active_tl 
{
Uh-oh,~ upcoming~ active!
}
{
Phew!~ Just~ text.
}
}

\ExplSyntaxOff
\begin{document}

[\zzMath abc] [\zzMath $x=2$] [\zzMath ~zz]


[\zzActive abc] [\zzActive $x=2$] [\zzActive ~zz]

\end{document}