Are there placeholders in LaTeX?

Question

Are there placeholders in LaTeX of the form:

A horse is \placeholder able to fly.

\placeholdertext{not}

which should produce an output like:

A horse is not able to fly.

So I look for an analogon of \footnotemark and \footnotetext that works with arbitrary content.

I would like to use that for example in combination with marginpar, when I want to explain some simple term in a paragraph with a complex tikz picture on the side:

Some long paragraph in which I mention some term\marginpar{\placeholder} that I want to explain on the side.

\placeholdertext{
    \begin{tikzpicture}
    % Picture for a better understanding of the used term.
    \end{tikzpicture}
}

Another paragraph with some term\marginpar{\placeholder} that I want to explain.

\placeholdertext{
    Some informative text...
}

Answer 1

You can use the todonotes package to insert not in the margin:

\documentclass{article}
\usepackage{todonotes}

\begin{document}
A horse is \todo{not} able to fly.
\end{document}

Answer 2

From the discussion in the comments, it appears that what is wanted is the ability to define a placeholder after it's used.

First, a quick note: The original poster talked about using \footnotemark in a paragraph and then \footnotetext at the end of the paragraph for the sake of aesthetics in their source code. This is a bad idea. For one thing, there is no guarantee that the \footnotetext and \footnotemark will end up on the same page. For another, the plan breaks down if there is more than one footnote in a paragraph (in that case, the footnote counter will need to be manually adjusted so that the footnote texts will have the correct number).

That said, because of the way LaTeX interprets its input, we can't ordinarily define something after it's used.¹ The way around this is to take advantage of the mechanism that LaTeX uses to handle other forward definitions, the .aux file.

So we'll want to be able to do something along the lines of:

A horse is \PH{horse} able to fly.

\SetPlaceholder{horse}{not}

Since I'm doing this off the top of my head,² I'm going to omit niceties like warnings about undefined placeholders or redefined placeholders. I'll leave that as an exercise for the reader.

Defining \PH is going to be relatively simple. It will just look for a command named PH@name for some name given and use that for output. I'm going to use some internal LaTeX commands³ here so we'll need to start with

\makeatletter

and then define \PH as:

\NewDocumentCommand{\PH}{ m }
   {\@nameuse{#1}}

A placeholder will be defined by:

\NewDocumentCommand{\DefinePlaceHolder}{ m m }
   {\@namedef{#1}{#2}}

You might be thinking, wait a minute, weren't we going to define \SetPlaceHolder? What's this \DefinePlaceHolder?

This is the command that will get written to the .aux file to create the placeholder. So we define \SetPlaceHolder to be:

\NewDocumentCommand{\SetPlaceHolder}{ m m }
   {\@bsphack % ❶
    \protected@write\@auxout{}% ❷
     {\string\DefinePlaceHolder % ❸
        {#1}{#2}}%
    \@esphack % ❶
   }

A couple notes here: The sequence \@bsphack…\@esphack means that \SetPlaceHolder won't cause extra spaces to be produced in the output if it appears in the middle of a paragraph.⁴ We use \protected@write to write the output to the aux file ❷. This will expand any macros that appear that aren't protected with \protect although we can get the same effect with what we're doing by using \string ❸ here.

There's still a lot of improvements that can be made to this code. I would recommend doing texdoc source2e and jumping down to File F which documents how the cross reference code works. There are some challenges to be had if you want the possibility of an empty placeholder since otherwise foo \PH{bar} bam would output two spaces between foo and bam if the placeholder is empty but again, I'll leave all of that as an exercise for the reader.

---

1. This is largely because TeX is essentially a one-pass compilation system. Us older folks who've programmed in 1970s–80s programming languages will be familiar with having to do forward declarations if you want to use a variable or function before it's declared.
2. Which means that all of this code is untested and likely has typos and/or bugs.
3. I was contemplating using Expl3, but a lot of what we want requires other internal commands so I might as well use \@nameuse over \use:c.
4. This is used by many LaTeX commands that don't produce output and the code involved is rather interesting. Among other things it will make sure that you still get end-of-sentence spacing if you write, e.g., foo.\label{bar} And more… The one gotcha is that if you have a command that uses the sphack mechanism at the end of a paragraph it can produce a space there that would not otherwise be output which under certain rare circumstances can cause a blank line to appear at the end of a paragraph.

Answer 3

Of course there are 'placeholders' in (La)TeX -- 'every' macro is a place holder that will be expanded to it's meaning (or is a primitive that does something different)

The xspace package is just for the correct spacing after the macro, but some will frown on this.

The question is now: Is it worth to use much of those place holders? Rather not.

In my opinion, it is better use a glossary, e.g. glossaries package.

\documentclass[a4paper]{scrartcl}

\usepackage{xspace}
\newcommand{\placeholder}{not\xspace}
\begin{document}

A horse is \placeholder able to fly.

\end{document}

Answer 4

There's also the skeldoc package to insert some blank placeholder space. For example the following code:

\documentclass{article}
\usepackage{blindtext}
\usepackage{skeldoc}
\begin{document}
\section{My section}
\skelline
\Blindtext[1]
\skelpar[4]
\Blindtext[1]
\section{\skelline}
\Blindtext[1]
\end{document}

would produce the following document:

More info and examples in the docs: https://github.com/mlhetland/skeldoc.sty