LaTeX3: expansion of argument to function

Question

I would like to define functions which extract the PDF object number from a PDF object reference, such as 12 from 12 0 R.

I defined two versions of such a function, a low level one and a high level one. The low level function accepts a PDF object reference as a literal string, such as the one given above and extracts the object number 12. This one I got working.

The high level one, however, which accepts a reference as a token list variable unfortunately fails. It should first expand the token list variable and pass it to the low level function. Could someone please find the error in the high level function?

Here is the code I wrote:

\documentclass{article}
\usepackage{expl3}

\begin{document}

\ExplSyntaxOn
\tl_new:Nn\oref{12~0~R} %define some object reference

\cs_new:Npn \lolevel:n #1~0~R {#1}
\cs_new:Nn \hilevel:x {\lolevel:n #1}

Ref:~\oref\par
Num:~\lolevel:n 12~0~R\par  %this works!
%Num:~\hilevel:x \oref      %doesn't :(
\ExplSyntaxOff

\end{document}

Answer 1

There are a few misunderstandings in your code on how argument specifications work, one of which actually causes the issue.

The practical issue is that the definition of \hilevel:x does not do any expansion. Simply giving the function an x argument does not 'magically' carry out expansion. You need to define the expansion either using \cs_generate_variant:Nn (automatic) or within the function itself.

The conceptual misunderstandings are on how you tell what kind of argument things are. First, \lowlevel:n is not a n function: this is a w argument as it's delimited. Secondly, I'd probably go for an o or V expansion here, as you almost certainly don't want to do an x-type expansion. (Every o or V function should have an n equivalent, and in the same way every c function needs an N 'parent'.)

One approach would therefore be

\cs_new:Npn \lolevel:w #1~0~R {#1}
\cs_new:Npn \hilevel:n #1 { \lolevel:w #1 }
\cs_generate_variant:Nn \hilevel:n { V }

or alternatively

\cs_new:Npn \lolevel:w #1~0~R {#1}
\cs_new:Npn \hilevel:n #1 { \lolevel:w #1 }
\cs_new:Npn \hilevel:o #1 { \exp_after:wN \lolevel:w #1 }

optimising the o variant for speed by hand.