Expandable string substitution

Question

I am trying to define a macro

\formatstr{\mystring}{(string empty)}{the string is: #1}

which expands to (string empty) when \mystring is empty (as judged, e.g. by etoolbox's \csdefempty) and expands to the string is: \mystring otherwise.

I could write, for example,

\newcommand*\formatstr[3]{%
  \def\fmt##1{#3}%
  \ifdefempty#1{#2}{\fmt{#1}}%
}

which works as I want, except for the fact that \formatstr itself is not expandable because of the \def assignment.

Is there any other way in which I could substitute the #1 in the third argument of \formatstr with \mystring in an expandable way?

Edit: Let me try to more clearly explain what I'm trying to do

Using etoolbox I can type things such as

\ifdefempty{\mystring}{(empty string)}{the string \mystring\ is funny}

But this gets tedious if I have to write

\ifdefempty{\mystringwithaverylongname}{(empty string)}{the string \mystringwithaverylongname\ is long}

So what I would like to be able to write is

\formatstr{\mystringwithaverylongname}{(empty string)}{some string #1 is nice}

Or perhaps

\formatstr{\mystringwithaverylongname}{(empty string)}{i've heard that :str is interesting}

What I want to know if is it possible to substitute the #1 (or :str) in the third argument with the contents coming from the first?

Answer 1

You need a wrapper around \ifdefempty functioning as an argument processor. For instance:

\def\myifdefempty#1#2#3:str#4|{%
  \ifdefempty{#1}{#2}{#3#1#4}%
}

Full minimal example:

\documentclass{article}

\usepackage{etoolbox}

\usepackage{hyperref}

\newcommand*{\formatstr}[3]{%
  \myifdefempty{#1}{#2}#3|%
}

\def\myifdefempty#1#2#3:str#4|{%
  \ifdefempty{#1}{#2}{#3#1#4}%
}

\begin{document}

\tableofcontents

\def\mystring{}

\section{\formatstr{\mystring}{an empty string}{the string :str is funny}}

\def\mystring{`a'}

\section{\formatstr{\mystring}{an empty string}{the string :str is funny}}

\end{document}

Answer 2

The usual idiom is something like

\makeatletter

\begingroup
\lccode`\!=0
\lowercase{\endgroup
\newcommand*\formatstr[3]{%
  \if!#1!%
   \expandafter\@firstofthree
   \else
   \expandafter\@secondoftwo
    \fi{#2}{#3}{#1}}}

\long\def\@firstofthree#1#2#3{#1}


\makeatother

\def\foo{}


\typeout{A: \formatstr\foo{string is emty}{the string is: \fmt}}


\def\fmt#1{[[#1]]}
\def\foo{hello}


\typeout{B: \formatstr\foo{string is emty}{the string is: \fmt}}

\stop

which produces the output (in \typeout to show it is expandable

A: string is emty
B: the string is: [[hello]]

---

Or this version which does not require a \fmt helper macro.

Here, as noted in comments, the syntax is

  \fmtstring\token{empty}{non-empth-pre}{non-empty-post}

with empty being used in the empty case and the expansion of

non-empth-pre\token non-empty-post

being used in the non-empty case.

A: string is empty
B: the string is: [hello]

from the slightly simpler code:

\makeatletter

\begingroup
\lccode`\!=0
\lowercase{\endgroup
\newcommand*\formatstr[4]{%
  \if!#1!%
   \expandafter\@firstoftwo
   \else
   \expandafter\@secondoftwo
    \fi{#2}{#3#1#4}}}



\makeatother

\def\foo{}
\typeout{A: \formatstr\foo{string is empty}{the string is: [}{]}}



\def\foo{hello}
\typeout{B: \formatstr\foo{string is empty}{the string is: [}{]}}

\stop

Answer 3

I guess this is what Joseph Wright is saying, which sounds logical to me, but I don't think I understand the OP well. \ifdefempty is expandable in a rather expensive way.

\documentclass{article}
\usepackage{etoolbox}
\newcommand*\formatstr[3]{\ifdefempty{#1}{#2}{#3#1}}
\newcommand*\bformatstr[3]{\ifblank{#1}{#2}{#3{#1}}}
\begin{document}
\def\fmt#1{[[#1]]}
\def\foo{}
\def\vgap{\par\bigskip}

\textbf{Case A1}: \formatstr\foo{string is empty}{the string is: \fmt}
\vgap
\textbf{Case A2}: \bformatstr{}{string is empty}{the string is: \fmt}
\vgap
\def\foo{hello}
\textbf{Case B1}: \formatstr\foo{string is empty}{the string is: \fmt}
\vgap
\textbf{Case B2}: \bformatstr{hello}{string is empty}{the string is: \fmt}
\end{document}