By applying the #{-notation, you can define macros whose last argument is delimited by an opening brace. Unlike with other argument delimiters that get removed when gathering arguments, TeX will leave a delimiting opening brace in place.
(Actually with TeX-releases prior to TeX Version 3.141592653—things were changed in the 2021 TeX-TuneUp—the mechanism isn't restricted to opening brace character tokens. With TeX-releases prior to TeX Version 3.141592653 you can use any token whose category code is 1 at definition time. Could as well be #\WeIrd after \let\WeIrd={ .)
Delimited arguments can be empty.
Therefore for obtaining a control sequence token from a set of character tokens that form the name of the control sequence token in question both for defining and for calling that control sequence token, you can (by applying the #{-notation) invent a single control sequence \name which processes a brace delimited argument trailed by an undelimited argument (which is nested in braces). After having TeX fetch the arguments, you can have TeX whirl them around and apply \csname..\endcsname to the argument supplied inside braces. The name of the control sequence token in question can contain space tokens as well.
\makeatletter
%
\newcommand\name{}%
\long\def\name#1#{\UD@innername{#1}}%
%
\newcommand\UD@innername[2]{%
\expandafter\UD@exchange\expandafter{\csname#2\endcsname}{#1}%
}%
%
\newcommand\UD@exchange[2]{#2#1}%
%
\makeatother
\name foo{bar} → expansion step 1:
\UD@innername{foo}{bar} → expansion step 2:
\expandafter\UD@exchange\expandafter{\csname bar\endcsname}{foo} → expansion step 3:
\UD@exchange{\bar}{foo} → expansion step 4:
foo\bar .
In expansion contexts you would need four \expandafter-chains for obtaining the result.
As \romannumeral does not produce any token when encountering a non-positive number, you can add a bit of \romannumeral-expansion in order to reduce the amount of \expandafter-chains.
Either do \romannumeral\name0 foo{bar}. This way only one \expandafter-chain hitting the \romannumeral-token is needed.
Or have the \romannumeral-expansion "hardcoded" within the definition - this way two \expandafter-chains are needed. The first one for obtaining the topl-level-expansion of \name. The second one for inducing \romannumeral-expansion.
\makeatletter
%
\newcommand\name{}%
\long\def\name#1#{\romannumeral0\UD@innername{#1}}%
%
\newcommand\UD@innername[2]{%
\expandafter\UD@exchange\expandafter{\csname#2\endcsname}{ #1}%
}%
%
\newcommand\UD@exchange[2]{#2#1}%
%
\makeatother
With such a macro you are not bound to specific definition commands:
\name{foo} → \foo .
\name\newcommand{foo} → \newcommand\foo .
\name\DeclareRobustCommand{foo} → \DeclareRobustCommand\foo .
\name\global\long\outer\def{foo} → \global\long\outer\def\foo .
\name\expandafter{foo}\bar → \expandafter\foo\bar .
\name\let{foo}=\bar → \let\foo=\bar .
\name\string{foo} → \string\foo .
\name\meaning{foo} → \meaning\foo .
You can as well use such a macro for defining/calling macros whose names
contain spaces:
\name{foo } → \foo␣ .
\name\newcommand{foo } → \newcommand\foo␣ .
\name\DeclareRobustCommand{foo } → \DeclareRobustCommand\foo␣ .
\name\global\long\outer\def{foo } → \global\long\outer\def\foo␣ .
\name\expandafter{foo }\bar → \expandafter\foo␣\bar .
\name\let{foo }=\bar → \let\foo␣=\bar .
\name\string{foo } → \string\foo␣ .
\name\meaning{foo } → \meaning\foo␣ .
You can also nest the calls of \name:
Example 1:
\name\name\expandafter{f o o }{b a r }
Processing the first \name yields:
\name\expandafter\f␣o␣o␣{b a r } .
Processing the second \name yields:
\expandafter\f␣o␣o␣\b␣a␣r␣ .
(Analogously: \name\name\let{f o o }={b a r } → \let\f␣o␣o␣=\b␣a␣r␣.)
Example 2:
\name\name\name\expandafter\expandafter\expandafter{f o o }\expandafter{b a r }{c r a z y }
Processing the first \name yields:
\name\name\expandafter\expandafter\expandafter\f␣o␣o␣\expandafter{b a r }{c r a z y } .
Processing the second \name yields:
\name\expandafter\expandafter\expandafter\f␣o␣o␣\expandafter\b␣a␣r␣{c r a z y } .
Processing the third \name yields:
\expandafter\expandafter\expandafter\f␣o␣o␣\expandafter\b␣a␣r␣\c␣r␣a␣z␣y␣ .
Example 3:
In expansion contexts you can use \romannumeral-expansion in order to keep things going.
\romannumeral\name\name\name0 \expandafter\expandafter\expandafter{f o o }\expandafter{b a r }{c r a z y }
\romannumeral keeps expanding until it has found some number. In the end it will find the number0 while with non-positive numbers \romannumeral will not deliver any token:
%\romannumneral-expansion in progress
\name\name\name0 \expandafter\expandafter\expandafter{f o o }\expandafter{b a r }{c r a z y }
Processing the first \name yields:
%\romannumneral-expansion in progress
\name\name0 \expandafter\expandafter\expandafter\f␣o␣o␣\expandafter{b a r }{c r a z y } .
Processing the second \name yields:
%\romannumneral-expansion in progress
\name0 \expandafter\expandafter\expandafter\f␣o␣o␣\expandafter\b␣a␣r␣{c r a z y } .
Processing the third \name yields:
%\romannumneral-expansion in progress
0 \expandafter\expandafter\expandafter\f␣o␣o␣\expandafter\b␣a␣r␣\c␣r␣a␣z␣y␣ .
Now \romannumeral finds the number 0 . Therefore \romannumeral-expansion gets aborted and \romannumeral won't deliver any token:
\expandafter\expandafter\expandafter\f␣o␣o␣\expandafter\b␣a␣r␣\c␣r␣a␣z␣y␣ .
Be aware that \name internally applies \csname while applying \csname as a side effect yields assigning the control sequence in question the meaning of the \relax-primitive in case the control sequence in question was undefined before applying \csname. That assignment will be restricted to the current scope even if the \globaldefs-parameter had a positive value at the time of applying \csname.
%%\errorcontextlines=1000
\documentclass[a4paper]{article}
\usepackage{textcomp}%
\makeatletter
\newcommand\name{}%
\long\def\name#1#{\romannumeral0\UD@innername{#1}}%
\newcommand\UD@innername[2]{%
\expandafter\UD@exchange\expandafter{\csname#2\endcsname}{ #1}%
}%
\newcommand\UD@exchange[2]{#2#1}%
\makeatother
%\newcommand\foo[2]{%
\name\newcommand{foo}[2]{%
\noindent
Control sequence whose name does not contain any space.\\
Argument 1: \textit{\textlangle#1\textrangle}\\
Argument 2: \textit{\textlangle#2\textrangle}\\
}%
\name\newcommand{foo }[2]{%
\noindent
Control sequence whose name has a trailing space.\\
Argument 1: \textit{\textlangle#1\textrangle}\\
Argument 2: \textit{\textlangle#2\textrangle}\\
}%
\name\newcommand{ f o o }[2]{%
\noindent
Control sequence whose name is interspersed with spaces.\\
Argument 1: \textit{\textlangle#1\textrangle}\\
Argument 2: \textit{\textlangle#2\textrangle}\\
}%
\begin{document}
\name{foo}{Arg 1}{Arg 2}
\name{foo }{Arg 1}{Arg 2}
\name{ f o o }{Arg 1}{Arg 2}
Nesting \texttt{\string\name}:
\name\expandafter\newcommand\expandafter*\expandafter{C o N f u SiO n}\expandafter{%
\romannumeral\name\name\name0 %
\expandafter\expandafter\expandafter{F O O}\expandafter{B A R}{C R A Z Y}%
}%
\texttt{\name\string{C o N f u SiO n} is \name\meaning{C o N f u SiO n}}%
\\
Playing around with grouping:
%Be aware that \texttt itself opens up a new scope for typesetting its argument.
%\globaldefs=1\relax
\texttt{%
\begingroup\name\string{w e i r d } is \name\endgroup\meaning{w e i r d }%
}%
\texttt{%
\name\string{w e i r d } is \name\meaning{w e i r d }%
}%
\end{document}
