Expand after all that stuff?

Question

Often I find myself in the need of expanding some macro, whose contents should be given as an argument to another command. The usual solution is to invoke some \expandafter concoctions to somehow control the expansion, but I wonder if (at least for some common cases) there is an easier way.

As an example, imagine that I want to write something like the following

\somecommand{Some Argument}{\secondarg}

But I want to expand \secondarg before actually attempting to process \somecommand.

For this kind of scenarions, would it be possible to define a command so that

\expandafterallthat{\somecommand{Some Argument}}{\secondarg}

expands to something like

\somecommand{SomeArgument}{Contents of secondarg macro}

Answer 1

This is provided by etextools as \expandnext:

\usepackage{etextools}
\expandnext{\somecommand{Some Argument}}{\secondarg}

But if you only like to expand the second argument once you can use a double argument swapping, a trick I use in some of my packages:

\documentclass{article}
%% Implementation
\def\myswap#1#2{#2{#1}}

\def\expandafterallthat#1#2{%
    \expandafter\myswap\expandafter{#2}{#1}%
}
%%%%%%%%%%%%%%%%

% Test case:
%
\makeatletter
\def\somecommand#1#2{%
    \def\first{#1}%
    \@onelevel@sanitize\first
    \par\texttt{First argument: \first}%
    \def\second{#2}%
    \@onelevel@sanitize\second
    \par\texttt{Second argument: \second}%
}
\makeatother

\def\secondarg{\empty Some stuff\empty}

\begin{document}

\expandafterallthat{\somecommand{\empty Some Argument\empty}}{\secondarg}

\end{document}

This correctly prints: (the \emptys are to see if the argument got expanded further)

first argument: \empty Some Argument\empty
Second argument: \empty Some stuff\empty

Answer 2

Since Joseph did not mention expl3 yet, let me do that: the LaTeX3 bundle includes a module for expansion control, l3expan. In particular, this includes functions to expand arguments of a command, of the form \exp_args:N... where the dots are letters corresponding to how the various arguments should be treated.

Many letters are in use: n for a brace group which should be passed with no modification, o to expand once, f to expand fully from the left, stopping at the first non-expandable token, x to expand fully throughout, like in an \edef. There is also N for a single non-braced token which should be passed as is, c for text that should be made into a control sequence, then treated as an N argument, and V and v for variables (see the l3expan documentation for details).

Here, you want to expand the second argument once, and leave the first unchanged, so the variant you need is \exp_args:Nno: leave the function unchanged (N), the first argument, braced, unchanged (n), and the second expanded once (o).

\newcommand{\foo}[2]{\showtokens{#1... #2}}
\newcommand{\sometext}{Some text.}
\RequirePackage{expl3}
\ExplSyntaxOn
\exp_args:Nno \foo {\error} {\sometext}
\ExplSyntaxOff

Answer 3

Written without a package

\documentclass{article}
\newcommand\expandafterallthat[2]{%
  \expandafter\expandafterallthataux\expandafter{\romannumeral -`0#2 }{#1}%
}
\newcommand\expandafterallthataux[2]{#2{#1}}
\newcommand\somecommand[2]{\detokenize{#1:::#2}}
\newcommand\secondarg{\thirdarg}
\newcommand\thirdarg{\fourtharg}
\newcommand\fourtharg{example}
\begin{document}
\expandafterallthat{\somecommand{Some Argument}}{\secondarg}

\end{document}

will multiply-expand input. (You don't say whether to expand #2 once or several times.)