Nested commands with parametrized names

Question

Inside a command I would like to define multiple new commands. Their names are parametrized by arguments, but they should also be able to call each other during definition. Here is an example:

\documentclass{article}  

\newcommand{\defFooAndBar}[2]{%
    \expandafter\newcommand\csname foo#1\endcsname{#2}
    \expandafter\newcommand\csname bar#1\endcsname{let us expand \foo#1 and show #2.}
}

\defFooAndBar{One}{the second argument}
\defFooAndBar{Two}{the second argument}

\begin{document}  

\fooOne
\barOne
\fooTwo
\barTwo

\end{document} 

However, there is an error generated:

! Undefined control sequence. \barOne ->let us expand \foo One and show the argument.

Apparently, Latex puts a blank space after \foo and therefore \foo#1 is not treated as a single entity. Is it possible to indicate the parser that they belong together?

Answer 1

TeX works strictly with tokens. In your code

let us expand \foo#1 and show #2.

\foo makes a single token and #1 is a parameter token that will be replaced by the actual argument at macro call. So when you do

\defFooAndBar{One}{the second argument}

the second command is defined as

let us expand \foo One and show the second argument

(the space is just to show where one token ends).

Solution: form the token with \csname...\endcsname just as you do for building the tokens to be defined:

\newcommand{\defFooAndBar}[2]{%
    \expandafter\newcommand\csname foo#1\endcsname{#2}%
    \expandafter\newcommand\csname bar#1\endcsname{let us expand \csname foo#1\endcsname and show #2.}%
}

No \expandafter is needed here; it is necessary instead for forming the token \fooOne (or similar) before \newcommand enters into action.