Inserting command with optional argument as optional argument Problem

Question

If I define the following two commands:

\newcommand{\TestA}[1][]{a^{#1} aaa}
\newcommand{\TestB}[1][]{#1}

and then call it by

$\TestA[\TestB[111]]$

I get the error:

"Argument of \\TestB has an extra  }."

But $\TestB[111]$ and $\TestA[111]$ works...

Is this a bug or am I doing something wrong?

Answer 1

You are confusing latex with the square backets for the optional arguments. When latex sees

 \TestA[\TestB[111]]

it thinks that the optional argument for \TestA is \TestB[111. Now it tries to evaluate this expression and gets confused because \TestB is not conforming to syntax.

To fix this you need to put brackets around the optional argument to \TestA:

\TestA[{\TestB[111]}]