Can one create a two-part spacing command?

Question

I would like to create a two-part spacing command. Specifically, I'd like two commands: \mySpaceOne and \mySpaceTwo. If \mySpaceOne is followed by a \mySpaceTwo, then it generates a certain amount of space; otherwise, it does nothing and does not consume the next token.

Is this possible? How would one begin to create such a command?

Answer 1

\makeatletter
\newcommand\mySpaceOne{\@ifnextchar\mySpaceTwo{\hspace{1em}}{}}
\makeatother

This will consume all space tokens, however.

Answer 2

Similar answer to egreg but using the TeX primitive \futurelet

\documentclass{article}
\begin{document}
\def\myspacetwo{}
\def\myspaceone{%
 \futurelet\ifoptions
    \choosemacro
}

\def\choiceone{\rule{20pt}{20pt}}
\def\choicetwo{}
\def\choosemacro{%
 \ifx\ifoptions\myspacetwo%
     \let\choice = \choiceone 
 \else
    \let\choice = \choicetwo
 \fi
\choice
}
\myspaceone\myspacetwo 
\myspaceone
\end{document}

A \futurelet takes normally three tokens:

     \futurelet\tokena\tokenb\tokenc

TeX executes a let\tokena=\tokenc generating a copy of \tokenc stored under the name of \tokena.

It then removes \tokena from the main token list. Following TeX expands \tokenb.

This token is for all practical purposes a macro with the following properties:

1. The macro will use \tokena, which is a copy of \tokenc, to find out what \tokenc is, in other words what token is to be expected later.
2. It then causes another macro to be expanded which will ultimately absorb \tokenc.