LaTeX3 version of expandafter-ifx-csname?

Question

I currently have this code from a previous question (link here)

\NewDocumentCommand \IfFontTF {mmmmmm}
{%
  \begingroup
  \fontencoding{#1}\fontfamily{#2}\fontseries{#3}\fontshape{#4}\try@load@fontshape
  \expandafter\ifx\csname\curr@fontshape\endcsname\relax
    #6%
  \else
    #5%
  \fi
  \endgroup
}

How to write the same functionality in expl3, in particular the line: \expandafter\ifx\csname\curr@fontshape\endcsname\relax

I am not sure which function to use in interface3?

Answer 1

[expl3 error corrected thanks to Skillman. Output is unchanged.]

Here's a version which is technically¹ expl3^2,3,4,5:

\documentclass{article}
% ateb: https://tex.stackexchange.com/a/727927/
% ateb: https://tex.stackexchange.com/a/728868/
\usepackage{nfssext-cfr}
\makeatletter
\ExplSyntaxOn
\NewDocumentCommand \IfFontTF {mmmmmm}
{
  \group_begin:
  \fontencoding{#1}\fontfamily{#2}\fontseries{#3}\fontshape{#4}\try@load@fontshape
  % sylwad Skillman: https://tex.stackexchange.com/questions/729130/problem-with-conditional-tf-version-working-but-not-the-p-version?noredirect=1#comment1815533_729130
  \exp_args:Nc \if_meaning:w { \curr@fontshape } \relax
    \group_end:
    #6
  \else:
    \fontencoding{#1}\fontfamily{#2}\fontseries{#3}\fontshape{#4}\selectfont
    \if_cs_exist:w  exfs@fake@\curr@fontshape \cs_end:
      \group_end:
      #6
    \else:
      \group_end:
      #5
    \fi:
  \fi:
}
\NewDocumentCommand \FontIfExists {ommmm}
{
  \group_begin:
  \fontencoding{#2}\fontfamily{#3}\fontseries{#4}\fontshape{#5}\try@load@fontshape
  % sylwad Skillman: https://tex.stackexchange.com/questions/729130/problem-with-conditional-tf-version-working-but-not-the-p-version?noredirect=1#comment1815533_729130
  \exp_args:Nc \if_meaning:w { \curr@fontshape } \relax
    \group_end:
    \IfValueT{#1}{#1}
  \else:
    \fontencoding{#2}\fontfamily{#3}\fontseries{#4}\fontshape{#5}\selectfont
    \if_cs_exist:w  exfs@fake@\curr@fontshape \cs_end:
      \group_end:
      \IfValueT{#1}{#1}
    \else:
      \group_end:
      \fontencoding{#2}\fontfamily{#3}\fontseries{#4}\fontshape{#5}\selectfont
    \fi:
  \fi:
}
\cs_new_eq:NN \ShowFont\curr@fontshape
\makeatother
\ExplSyntaxOff
\begin{document}
\IfFontTF{OT1}{cmr}{m}{sc}{OT1/cmr/m/sc exists}{OT1/cmr/m/sc is non-existent.}

\IfFontTF{T1}{clm}{bx}{n}{T1/clm/bx/n exists}{T1/clm/bx/n is non-existent.}

\IfFontTF{T1}{clm}{bz}{n}{T1/clm/bz/n exists}{T1/clm/bz/n is non-existent.}

\IfFontTF{T1}{clmx}{bx}{n}{T1/clmx/bx/n exists}{T1/clmx/bx/n is non-existent.}

\IfFontTF{T1}{clm}{bx}{blurb}{T1/clm/bx/blurb exists}{T1/clm/bx/blurb is non-existent.}

\IfFontTF{OT1}{cmr}{m}{blurb}{YES}{NO}

\FontIfExists{OT1}{cmr}{m}{sc}
\ShowFont

\FontIfExists{T1}{clm}{bx}{n}
\ShowFont

\FontIfExists{T1}{clmx}{bx}{n}
\ShowFont

\FontIfExists[Oh, no!]{OT1}{cmr}{m}{sc}
\ShowFont

\FontIfExists[Oh, dear!]{T1}{clm}{bx}{n}
\ShowFont

\FontIfExists[Oops!]{T1}{clmx}{bx}{n}
\ShowFont


\FontIfExists[egreg says `no']{OT1}{cmr}{m}{blurb}
\ShowFont

\thispagestyle{empty}
\end{document}

Proof in pudding:

_{¹Note that I do not claim it is in the spirit of the enterprise, but only that the replacement code technically qualifies.}

_{²Except for font macros and \relax.}

_{³In case wipet ever reads this: you need not say it. I concede your point in advance.}

_{⁴Do not try this at home without professional supervision. This code is not intended for domestic use.}

_{⁵There is a 99% chance I will delete this later. The other 1% involves Virgin Media and/or Acts of Fate.}

Answer 2

Note that your example code tries to test font existence based on old method how LaTeX works with old 8bit fonts (it means .fd files, short and unclear filenames, .tfm files, .vf files, .map files, .enc files, obscure encodings and other complications). Today, we are using Unicode fonts in Unicode TeX engines. We can forget the ancient complications. We are asking for existence the font directly by font filename of font name using \font primitive while \suppressfontnotfounderror=1. Simply try to use \font TeX primitive and then ask if the result is \nullfont.

For example, you can implement a macro \isfont similar like in OpTeX by:

\initunifonts % or another initializing of Unicode `\font` primitive

\def\isfont#1{%
   \begingroup
       \suppressfontnotfounderror=1
       \font\testfont={#1}\_relax
       \ifx\testfont\nullfont \endgroup \expandafter \unless
       \else \endgroup \_fi
}

\isfont{TeXGyreTermes-Bold}\iftrue yes \else no \fi % prints yes

\isfont{TeXGyreTomos-Bold}\iftrue  yes \else no \fi % prints no

\bye

And to the question from the title of this thread: Of course, there is expl3 alternative but also we have primitive equivalent of \expandafter\ifx\csname which sounds \ifcsname (available in all today's TeX engines because it is from eTeX). The usage is

\ifcsname name of control sequence\endcsname is defined \else isn't defined \fi