Cfrgtkky

This is a demo

documentclass{article}

usepackage{soul}

makeatletter

newcommand{mydef}[2]{%

    expandafterdefcsname my#1endcsname {#2}

}

newcommand{mygetdef}[1]{%

    ifcsname my#1endcsname csname my#1endcsname else #1 not defined fi%

}

makeatother



deftest{some text to complete}

mydef{xxx}{some text to complete}



begin{document}

ultest                 % it works

%%%% if uncomment, the below one does not work, an error is reported 

% ulmygetdef{xxx}

end{document}

As the comment says, if ulmygetdef{xxx} takes place, there comes a error:

! Argument of SOUL@@ has an extra }.

UPDATE:

I switch to package ulem where uline can underline fbox.
Everything works (see MWE below).
Then I try to make colorful box.
However, I find that once I include package xcolor or tcolorbox (as the comment in MWE), some weird error comes up immediately even before I actually use them in the document.

documentclass{article}

%%% uncomment either of the packages below, it reports errors.

%usepackage{xcolor}

%usepackage{tcolorbox}



usepackage[normalem]{ulem}

makeatletter

newcommand{mydef}[2]{%

    expandafterdefcsname my#1endcsname {#2}

}

newcommand{mygetdef}[1]{%

    ifcsname my#1endcsname csname my#1endcsname else protectfbox{#1} fi%

}



deftest{some text to complete}

mydef{xxx}{some text to complete}



begin{document}

uline{test}



edefmytmp{mygetdef{xxx}}

uline{mytmp}



edefmytmp{mygetdef{yyy}}

uline{mytmp}



end{document}

After all my efforts, I can not figure out why and solve it.
So I updated my original problem.

Try

edeftmp{mygetdef{xxx}}

ultmp

The mygetdef needs to be expanded before being passed to ul.

documentclass{article}

usepackage{soul}

makeatletter

newcommand{mydef}[2]{%

    expandafterdefcsname my#1endcsname {#2}

}

newcommand{mygetdef}[1]{%

    ifcsname my#1endcsname csname my#1endcsname else #1 not defined fi%

}

makeatother



deftest{some text to complete}

mydef{xxx}{some text to complete}



begin{document}

ultest                 % it works

%%%% if uncomment, the below one does not work, an error is reported 

% ulmygetdef{xxx}



edeftmp{mygetdef{xxx}}

ultmp

end{document}

SUPPLEMENTAL NOTE TO OP

Not really to answer OP question, but in reply to some of his comments to this answer:

documentclass{article}

usepackage{soul}

makeatletter

newcommand{mydef}[2]{%

    expandafterdefcsname my#1endcsname {#2}

}

newcommand{mygetdef}[1]{%

    ifcsname my#1endcsname csname my#1endcsname else

#1 not defined fi%

}

newcommand{myulgetdef}[1]{%

    ifcsname my#1endcsname expandafterul{csname my#1endcsname}%

  elsefbox{#1 not defined} fi%

}

newcommand{myxgetdef}[1]{%

    ifcsname my#1endcsname expandaftercsname my#1endcsname%

  elseprotectprotectprotectfbox{#1 not defined} fi%

}

newcommandexul[1]{edeftmp{#1}ultmp}

makeatother



deftest{some text to complete}

mydef{xxx}{some text to complete}



begin{document}

ultest                 % it works



exul{mygetdef{xxx}}



1. mygetdef{yyy}



2. myulgetdef{yyy}



3. myxgetdef{yyy}



4. exul{mygetdef{yyy}}



5. exul{myxgetdef{yyy}}% HOWEVER, fbox IS NOT UNDERLINED



%6. exul{myulgetdef{yyy}}% FAILS; CANNOT ul{fbox{...}}



end{document}

The command ul is defined as SOUL@ulsetupSOUL@. The first command sets up some commands that will be later delivered after a group has started and is of no concern here.

The macro SOUL@ is defined as

% soul.sty, line 99:

defSOUL@{%

    futureletSOUL@@SOUL@expand

}

which means that the following token is looked up and stored in SOUL@@; then SOUL@expand is executed

% soul.sty, line 102:

defSOUL@expand{%

    ifcatbgroupnoexpandSOUL@@

        letSOUL@nSOUL@start

    else

        bgroup

            def\##1##2{def##2{noexpand##2}}%

            theSOUL@cmds

            SOUL@buffer={%

                \TeX\LaTeX\soulomit\mbox\hbox\textregistered

                \slash\textcircled\copyright\S\,\<\>\~%

                \\%

            }%

            def\##1{def##1{noexpand##1}}%

            theSOUL@buffer

            letprotectnoexpand

            xdefSOUL@n##1{noexpandSOUL@start{SOUL@@}}%

        egroup

    fi

    SOUL@n

}

Quite a mouthful! The behavior depends on whether the token stored in SOUL@@ is a left brace or not. In your case it isn't.

The part of the code above which is of primary interest is

            letprotectnoexpand

            xdefSOUL@n##1{noexpandSOUL@start{SOUL@@}}%

Note that SOUL@@ is the token found at the first step, in your case mygetdef. At the end, SOUL@n is executed, which discards the next token (your mygetdef) and does

SOUL@start{<full expansion of mygetdef`>}

The place where the error happens is at the xdef, because mygetdef does not find its argument, but just the closing brace. What you want is that

ulmygetdef{xxx}

becomes

ulmyxxx

You need to do some clever expansion for this, but it becomes very awkward, mainly because of the conditionals. If you do

expandafterulmyxxx

you get

ulifcsname myxxxendcsnamecsname myxxxendcsnameelse xxx not defined fi

With three expandafters

ulcsname myxxxendcsnameelse xxx not defined fi

With seven expandafters

ulmyxxxelse xxx not defined fi

That's it! Let's see what happens with mygetdef{noway} (assuming it's undefined). You'd get

ul noway not defined fi

which is not really what you want, but can be easily fixed.

documentclass{article}

usepackage{soul}



newcommand{mydef}[2]{%

  expandafterdefcsname my#1endcsname{#2}

}

newcommand{mygetdef}[1]{%

  ifcsname my#1endcsname

    csname my#1endcsname

  else

   {}#1 not defined%

  fi

}



deftest{some text to complete}

mydef{xxx}{some text to complete}



begin{document}



ultest



expandafterexpandafterexpandafter

expandafterexpandafterexpandafter

expandafterulmygetdef{xxx}



expandafterexpandafterexpandafter

expandafterexpandafterexpandafter

expandafterulmygetdef{noway}



end{document}

My proposal is to define differently mygetdef, if you plan to use it after soul macros, but not only.

documentclass{article}

usepackage{soul}



makeatletter

newcommand{mydef}[2]{%

  @namedef{my#1}{#2}

}

newcommand{mygetdef}[2][relax]{%

  @ifundefined{my#2}%

    {{}#2 not defined}%

    {expandafter#1csname my#2endcsname}%

}

makeatother



mydef{xxx}{some text to complete}



begin{document}



mygetdef{xxx}



mygetdef[ul]{xxx}



mygetdef[textit]{xxx}



mygetdef[ul]{noway}



end{document}