Error in TikZ using defined \coordinates with \draw to [in= , out=] syntax

Question

Dear fellow LaTeX/TikZ users. I'm clearly misunderstanding something about how TikZ handles coordinate definitions. I am trying to draw a Bezier curve between prededfined points, but TikZ complains as soon as the target point is a defined coordinate (here (B)) instead of a literal coordinate (here (10,10)) the compile fails with the following error:

! Package PGF Math Error: Unknown function `up' (in 'up').

See the PGF Math package documentation for explanation.
Type  H <return>  for immediate help.
...                                              

l.16   \draw (A) to [out=down,in=up] (B)
;
? 

I tried to google this but can't seem to find this specific error. What am I doing wrong?

Here is a minimal working example:

\documentclass[]{article}

\usepackage{tikz}
% EDIT: This is for workaround
\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
  \coordinate (A) at (0,1);
  \coordinate (B) at (10,10);

  % This will work
  \draw (0,1) to [out=down,in=up] (10,10);
  % So will this
  \draw (A)   to [out=down,in=up] (10,10);  
  % And this
  \draw (A) -- (B);

  % But these wont
  % \draw (A) to [out=down,in=up] (B);
  % \draw (0,1) to [out=down,in=up] (B);

  % EDIT: Added the following workaround:
  \draw ($(A)$) to [out=down,in=up] ($(B)$);
\end{tikzpicture}
\end{document} 

EDIT

I found a workaround, but it made me even more confused. Using \tikzlibrary{calc} and replacing (B) with ($(B)$) removes the problem, but I don't understand why. The ($...$) enviroment for calc allows calculations for points, but how/why is ($(B)$) different from (B)?

Answer 1

The problem is that if you use a named coordinate (or a node) as target, tikz will use the in value as anchor. But there is no up anchor in a node. This means that in=90 (which will set a target B.90) or in=north (which will set a target B.north) will work, but not in=up as B.up doesn't exist.

Your work-around works as in this case the target is converted in an absolute coordinate (0.0pt,28.45274pt) and tikz doesn't try to use the in value as anchor.

\documentclass[]{article}

\usepackage{tikz}

\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
  \coordinate (A) at (0,1);
  \coordinate (B) at (10,10);

  \draw (A) to [out=down,in=90] (B);
  \draw (A) to [out=down,in=north] (B);
  %won't work 
 % \draw (A) to [out=down,in=up] (B);

  \draw (A) to [out=down,in=up] ($(B)$);


\end{tikzpicture}
\end{document}