Nested \newcommand's in Latex

Abstract also the formatting command

\newcommand{\newdescriptor}[3]{\newcommand{#1}{#2{#3}}}

Then

\newdescriptor{\sP}{\mathbf}{P}

will do

\newcommand{\sP}{\mathbf{P}}

Nowhere I see any improvement over the direct definition unless you do a further abstraction step:

\newcommand{\whatever}[1]{\mathbf{#1}}
\newcommand{\constant}[1]{\mathsf{#1}}

\newcommand{\newdescriptor}[3]{\newcommand{#1}{#2{#3}}}

\newdescriptor{\sP}{\whatever}{P}
\newdescriptor{\cs}{\constant}{S}

Add \ensuremath and \xspace if you really want. I will never be convinced that typing

The constant \cS is nice

is better than typing

The constant \(\cS\) is nice

To be honest, I find the second way the only good one (on par with $\cS$).

In the replacement text of a definition #2 is replaced by the second argument, #1 is replaced by the first argument, and ## is replaced by # so

\documentclass{article}
\usepackage{xspace}
\newcommand{\makeconstructor}[2]{%
\newcommand{#1}[2]{\newcommand{##1}{\ensuremath{#2{##2}}\xspace}}}

\makeconstructor\makespace\mathcal
\makeconstructor\makewidget\mathsf

\makespace\SP{P}
\makespace\SQ{Q}
\makewidget\WP{P}
\makewidget\WQ{Q}


\begin{document}

$\SP<\SQ<\WP<\WQ$

\end{document}