Problem between keys, paths with two macros and pgfkeys

Question

Sorry the code below for my minimal example is a bit long. In this code I defined two macros : the first macro \SC draws semi-circle and the second macro \ARC draws arcs. The first macro uses the second one.

\SC uses some options like radius and diameter. I need to use /SCopt/.search also={/tikz} if I want some keys from tikz.

\ARC uses some options too like through and angle but here only through is used.

My english is not perfect so I don't understand the subtleties of the pgfkeys section in the pfgmanual. My problem is how to manage the keys and the paths of \pgfkeys{/ARCopt/.cd,....

If I do not use

/ARCopt/.unknown/.code     = {\let\searchname=\pgfkeyscurrentname
                                  \pgfkeysalso{\searchname/.try=#1,
                                   /tikz/\searchname/.retry=#1}},

I can't use the key diameter and I get

Error: I do not know the key '/tikz/diameter'.

It's normal this is a key of the first macro \SC but in this case I can use blue and not color=blue with /ARCopt/.search also={/tikz} (thanks to Paul Gaborit)

The question is : it is possible to get the two possibilities : unknown keys in the second macro and keys from pgf/tikz ?

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\makeatletter

\def\SCR(#1,#2){% semi circle with radius defined by 2 pts
    \coordinate (LastPT) at (barycentric cs:#2=1,#1=-2)  ;
    \coordinate (CenterPT) at (#1);
    \coordinate (StartPT) at (#2); 
}
%-----------------------------------------------------
\def\SCD(#1,#2){% semi circle with diameter defined by 2 pts
    \coordinate (CenterPT) at (barycentric cs:#1=1,#2=1)  ;
    \coordinate (StartPT) at (#1);
    \coordinate (LastPT) at (#2);}
%-----------------------------------------------------
\pgfkeys{/SCopt/.cd,
        opt/.is choice,
        opt/diameter/.code    =\def\num{1},
        opt/radius/.code      =\def\num{0},
        opt/.default =radius,
        opt,
        diameter/.style={opt=diameter},
        radius/.style={opt=radius},
        /SCopt/.search also={/tikz}
        } 
%-----------------------------------------------------
\def\SC{\@ifnextchar[{\SCb}{\SCb[]}}    
\def\SCb[#1](#2){% 
\pgfqkeys{/SCopt}{#1}
    \ifcase\num%
        \SCR(#2) % semi circle with radius defined by 2 pts
    \or% 1
        \SCD(#2)  % semi circle with diameter defined by 2 pts
    \fi
    \ARC[#1](CenterPT,StartPT,LastPT)}%
%-----------------------------------------------------
\pgfkeys{/ARCopt/.cd,
        type/.is choice,
        type/through/.code      =   \def\numa{0},
        type/angle/.code        =   \def\numa{1},
        type/.default           =   through,
        type,
        through/.style          =   {type=through},% to avoid "type ="
        angle/.style            =   {type=angle},
        /ARCopt/.unknown/.code     = {\let\searchname=\pgfkeyscurrentname
                                      \pgfkeysalso{\searchname/.try=#1,
                                       /tikz/\searchname/.retry=#1}},
        %/ARCopt/.search also={/tikz},
} 
%-----------------------------------------------------
\def\ARC{\@ifnextchar[{\ARCb}{\ARCb[]}} 
\def\ARCb[#1](#2){% 
\pgfqkeys{/ARCopt}{#1}
    \ifcase\numa%
        \ARCT[#1](#2) % arc with only pts
    \or% 1
        \ARCA[#1](#2)  % arc with 2 pts and one angle
    \fi}

\def\ARCT[#1](#2,#3,#4){%
    \pgfpointdiff{\pgfpointanchor{#2}{center}}{\pgfpointanchor{#3}{center}}
    \pgfmathparse{veclen(\pgf@x,\pgf@y)}
    \let\ra\pgfmathresult                       %it's the radius
    \pgfmathanglebetweenpoints{\pgfpointanchor{#2}{center}}{%
                               \pgfpointanchor{#3}{center}}  %first angle
    \let\fa\pgfmathresult
    \pgfmathanglebetweenpoints{\pgfpointanchor{#2}{center}}{%
                               \pgfpointanchor{#4}{center}}  %last angle
    \let\la\pgfmathresult
    \draw[shift = {(#2)},/ARCopt/.cd,#1] (\fa:\ra pt) arc (\fa:\la:\ra pt);}

\def\ARCA[#1](#2,#3,#4){%
% not defined
}
%-----------------------------------------------------
\makeatother

\begin{document}
\begin{tikzpicture}[point/.style={circle,minimum size=4pt,
    inner sep=0pt,outer sep=0pt,fill=black}]
    \path coordinate[point] (A) at (0,0)
          coordinate[point] (B) at (6,0)        
          coordinate[point] (C) at (4,0);
    \SC[color=red,thick](A,B)
    \SC[blue,thick](A,C)
    \SC[diameter](A,B)
    %\SC[diameter](B,C)
\end{tikzpicture}

\end{document}

Answer 1

Perhaps my original answer is not clear.

You can replace

    /ARCopt/.unknown/.code     = {\let\searchname=\pgfkeyscurrentname
                                  \pgfkeysalso{\searchname/.try=#1,
                                   /tikz/\searchname/.retry=#1}},

by

    /ARCopt/.search also    =   {/SCopt,/tikz},

---

Since in the definition of /SCopt it searches also /tikz, it should be equivalent to say

    /ARCopt/.search also    =   {/SCopt},

But then color=red fails. I do not know why.

On the other hand, if you remove

    /SCopt/.search also={/tikz},

then TeX complains that color unknown key. This is really undesired.

---

---

The argument of `.search also` is a **list** of paths. That is to say, you can `.search also` a hundred paths, including `/pgf`, `/tikz`, `/ARCopt`, `/SCopt`, and whatever you want. In your case, what you need is `/ARCopt/.search also={/SCopt}` because that is where you defined `diameter`.