How to perform arithmetic within siunitx?

Question

I often put comments in my documents to remind me where a number comes from:

\SI{39.68}{\milli\metre} deep % 0.31 * 128

I'd like LaTeX to calculate the product by itself. I wrote a function to do that with fp, and it works (although I'd like a shorter one). But it doesn't work within \SI:

\newcommand{\mult}[2]{%
    \FPmul{\tempSum}{#1}{#2}%
    \FPround\roundedSum\tempSum2%
    \roundedSum%
}

\mult{0.31}{128} mm deep % works

\SI{\mult{0.31}{128}}{\milli\metre} deep % does not work

How to solve this?

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! siunitx error: "invalid-number"
! 
! Invalid numerical input '\edef {\FP@mul \tempSum 0.31..\relax 128..\relax
! }\edef {\FP@round \roundedSum ..\relax }{2}'.
! 
! See the siunitx documentation for further information.
! 
! For immediate help type H <return>.
!...............................................

Answer 1

siunitx parses numerical inputs with some options. However the input of the number isn't expanded before manipulating. To use tokens inside the first mandatory argument you can use the option parse-numbers=false.

If you define an expandable version of \mult you can use it as a valid input of \SI. In the example below this is done by \DeclareExpandableDocumentCommand provided by xparse.

\documentclass{article}
\usepackage{siunitx}
\usepackage{xparse} 
\ExplSyntaxOn
\DeclareExpandableDocumentCommand \mult {  m m  }
 {
  \fp_eval:n { round( #1 * #2  ,2 ) }
 }
\ExplSyntaxOff
\begin{document}
\mult{0.31}{128}

\SI[parse-numbers=false]{\mult{0.31}{128}}{\milli\metre} deep

\SI{\mult{0.31}{128}}{\milli\metre} 
\end{document} 

---

Using the module l3fp you can define arithmetic functions very easy. In the example above I used your definition with two mandatory arguments. However you can also use:

\NewDocumentCommand \Func { O{2} m   }
 {
  \fp_eval:n { round( #2   , #1 ) }
 }

Now \Func has two arguments the first one is optional and specifies the rounding. The second is passed to fp_eval:

\Func{0.38*4}