Workaround how to use expl3 functionality in tikz

Question

What I want to do

I want to create a command \planfigur{<edges>}{<angles>} which draws a triangle using TikZ. \planfigur should have 2 mandatory arguments, which let me specify edges and/or angles (where a = alpha, b = beta and c = gamma) of the triangle, which are drawn in red instead of black color.

E. g. \planfigur{ab}{bc} draws a black triangle, where the edges a and b and the angles beta and gamma are red.

I use expl3 and xparse. I already have read (Is it reasonable to expect an expl3-compatible release of TikZ/PGF in the future?), that there are problems in using expl3 and tikz together (which makes sense).

So I played around and tried to outsmart TikZ. I had the following idea, which unfortunately doesn't work (1:0 for TikZ :-)). The approach is to move all expl3 code into a new command, where I can use \tl_if_in:nnT to check the arguments.

Code

\documentclass{article}

\usepackage{expl3, xparse}

\usepackage{tikz}
\usetikzlibrary{calc}

\ExplSyntaxOn

\tl_new:N \__edu_planfigur_temp

\DeclareDocumentCommand \planfigurLines { m } {

  \tl_clear:N \__edu_planfigur_temp

  \tl_if_in:nnT {#1} {a} {
    \tl_put_right:Nn \__edu_planfigur_temp {\draw (b) -- (c);}
  }

  \tl_if_in:nnT {#1} {b} {
    \tl_put_right:Nn \__edu_planfigur_temp {\draw (a) -- (c);}
  }

  \tl_if_in:nnT {#1} {c} {
    \tl_put_right:Nn \__edu_planfigur_temp {\draw (a) -- (b);}
  }

  \tl_use:N \__edu_planfigur_temp
}

\DeclareExpandableDocumentCommand \planfigurAngles { m } {
  \tl_if_in:nnT {#1} {a} {
    \tl_put_right:Nn \__edu_planfigur_temp {\draw ($(a) + (0:1)$) arc (0:40:1) ($(a) + (20:0.7)$) node {$\alpha$};}
  }
  \tl_if_in:nnT {#1} {b} {
    \tl_put_right:Nn \__edu_planfigur_temp {\draw ($(b) + (120:1)$) arc (120:180:1) ($(b) + (150:0.7)$) node {$\beta$};}
  }
  \tl_if_in:nnT {#1} {c} {
    \tl_put_right:Nn \__edu_planfigur_temp {\draw ($(c) + (220:1)$) arc (220:300:1) ($(c) + (260:0.65)$) node {$\gamma$};}
  }

  \tl_use:N \__edu_planfigur_temp
}

\ExplSyntaxOff

\DeclareExpandableDocumentCommand \planfigur { m m } {
  \begin{tikzpicture}[scale=0.65, line join=round, thick]

    \coordinate (a) at (0,0);
    \coordinate (b) at (5,0);
    \coordinate (c) at (3.37, 2.83);

    \draw (a) -- node[below] {$c$} (b) -- node[above right] {$a$} (c) -- node[above left] {$b$} (a) -- cycle;

    \draw (a) node[left] {$A$};
    \draw (b) node[right] {$B$};
    \draw (c) node[above] {$C$};

    \begin{scope}[color=red, fill=red!25, very thick]
      \planfigurLines{#1}
      \planfigurAngles{#2}
    \end{scope}

\end{tikzpicture}
}

\begin{document}

\planfigur{ac}{a}

\end{document}

Error

! Package pgf Error: No shape named 0:1 is known.

See the pgf package documentation for explanation. Type H for immediate help. ...
l.70 \planfigur{ac}{a}

Answer 1

One of the things \ExplSyntaxOn does is giving the colon category code 11 (letter) so it can be used as part of macro names. In coordinates TikZ however expects a colon with category code 12 (other).

The easiest way: you can use the predefined \c_colon_str instead of : in TikZ' coordinates.

Original solution before \c_colon_str was available:

One solution that works is defining a token list that contains only a : with category code 12

\tl_const:Nx \c_edu_colon_tl { \token_to_str:N : } 

and then use said token list \c_edu_colon_tl instead of : in TikZ' coordinates.

---

BTW: your token list \__edu_planfigur_temp misses the recommended ending for a token list: _tl. It also misses the recommended start \l or \g which denotes if it is local or global. I'd rename it into \l__edu_planfigur_temp_tl.

In expl3 variables generally should start either with \l (local), \g (global) or \c (constant) and end with the type of variable (_tl for a token list, _int for an integer and so on).

Answer 2

Two more possibilities

\newcommand*\tikzangle[2]{#1:#2}
…
\ExplSyntaxOn
\draw (0,0) -- (\tikzangle{20}{2cm});
\ExplSyntaxOff

and a more practical one

\begingroup
\lccode`\X=`\:
\lowercase{\endgroup
  \gdef\tikzangle(#1X#2){#1:#2}}
\ExplSyntaxOn
\draw (0,0) -- \tikzangle(20:2cm);
\ExplSyntaxOff

To add the more general one, “easier to see” than cgnieder's

\def\othercolon{:}
\ExplSyntaxOn
\draw (0,0) -- (20\othercolon2cm);
\ExplSyntaxOff