Define a new command with parameters inside newcommand

Question

I have read the question and answers about:

• Defining a newcommand, with variable name, inside another newcommand
• How to define a macro to create a new macro with a name passed as its argument?
• What exactly do \csname and \endcsname do?

What I want to do is similar, but one step further. I would like to define a new command with a variable name, but the new command should have parameters (exactly one in my case).

I tried to use the hash symbol # twice, like this:

\newcommand{\deffun}[1]{\expandafter\newcommand[1]\csname #1\endcsname[1]{#1(##1)}}

But this does not seem to work and I am too confused or ignorant to produce a mwe with nesting of macro definitions with parameters. For an answer, pointing me to the relevant place of the documentation would be enough. It looks like nesting of macro definitions is not a commonly used nor explained feature.

PD: I was almost right, it is like this:

\newcommand{\deffun}[1]{\expandafter\newcommand\csname #1\endcsname[1]{#1(##1)}}

Answer 1

There are a number of ways to achieve this:

\documentclass{article}

\def\deffunA#1{\expandafter\def\csname #1\endcsname##1{#1(##1)}}

\newcommand{\deffunB}[1]{\expandafter\newcommand\csname #1\endcsname[1]{#1(##1)}}

\usepackage{etoolbox}
\newcommand{\deffunC}[1]{\csdef{#1}##1{#1(##1)}}

\begin{document}

\deffunA{abc}
$\abc{123}$

\deffunB{ghi}
$\ghi{123}$

\deffunC{jkl}
$\jkl{123}$

\end{document}

\deffunA uses a pure TeX way of defining a macro, while \deffunB is a LaTeX-related way of doing it. \deffunC uses etoolbox which provides a neat interface for defining macros. See section 3.1.1 Macro Definitions of the documentation. In fact, \csdef uses the LaTeX-defined \@namedef... yet another way of achieving what you're after.

Answer 2

The error is in the wrong \newcommand[1].

Here's a way using expl3:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\deffun}{m}
 {
  \exp_args:Nc \newcommand{#1}[1]{#1(##1)}
 }
\ExplSyntaxOff

\begin{document}

\deffun{foo}

\texttt{\meaning\foo}

\end{document}

The output is

\long macro:#1->foo(#1)

The trick is that \exp_args:Nc jumps over the next token (N), takes the next braced group as an argument and makes a control sequence name out of it (c), and restarts from the jumped over token.

A possible “more proper” way, taking into account that the macro to be defined has just one parameter, would be

\NewDocumentCommand{\deffun}{m}
 {
  \cs_new:cpn {#1} ##1 { #1 ( ##1 ) }
 }

that essentially uses the same idea; note that \cs_new:... refuses to define an already existing command, but the definition is global (contrary to \newcommand).

Several variations are possible.