Macro expansion inside array

Question

I got stuck with the following piece of TeX:

\def\foo{A}
\def\bar{\foo}
\let\baz\bar
1: \baz
\def\foo{B}
\\
2: \baz
\\

\begin{array}[t]{r}
\def\foo{C}
3: \baz
\\
\def\foo{D}
4: \baz
\\
5: \baz
\end{array}

\begin{enumerate}
\def\foo{E}
\item[6:] \baz

\item[7:]
\def\foo{F}
\baz

\item[8:] \baz
\end{enumerate}

This piece of TeX is rendered as follows:

1: A
2: B

3: C
4: D
5: B

6: E
7: F
8: F

Now, lines 1 and 2 behave as expected, after redefining \foo, the new value is printed. However, when wrapped in an array, line 5 prints B where I would expect D. The macros work as expected when inside enumerate (lines 6-8).

How is it that line 5 prints as B instead of D and how to fix the definition of the macros? I presume there is something weird going on with the expansion inside array. I tried to change array for align but that manifests the same problem.

Answer 1

This is completely expected

\def\foo{A}     % now \foo expands to A
\def\bar{\foo}  % now \bar expands to \foo
\let\baz\bar    % now \baz expands to \foo
1: \baz         % we get \foo, then A
\def\foo{B}     % now \foo expands to B
\\
2: \baz         % we get \foo, then B
\\

\begin{array}[t]{r}
\def\foo{C}     % now \foo expands to C
3: \baz         % we get \foo, then C
\\              % end of cell, \foo reverts to B
\def\foo{D}     % now \foo expands to D
4: \baz         % we get \foo, then D
\\              % end of cell, \foo reverts to B
5: \baz         % we get \foo, then B
\end{array}

\begin{enumerate}
\def\foo{E}     % now \foo expands to E
\item[6:] \baz  % we get \foo, then E

\item[7:]
\def\foo{F}     % now \foo expands to F
\baz            % we get \foo, then F

\item[8:] \baz  % we get \foo, then F
\end{enumerate}

Cells in array form groups. It's not clear what you want to achieve, but if you want that the definition of \foo transcends groups, you need \gdef rather than \def.

In particular, what's the role of \bar? You never use it, except when defining \baz as an alias to it.