Make ? indicate a single-letter variable in math mode

var.tex

\documentclass{article}
\usepackage[utf8]{inputenc}

\DeclareUnicodeCharacter{3BB}{\lambda} % λ

\newcommand{\var}[1]{\mathtt{#1}}

\let\qm=?
\catcode`?=13
\def?{%
  \ifmmode%
  \expandafter\var%
  \else%
  \qm%
  \fi%
}

\begin{document}
In text mode, the question mark still indicates a question:

\begin{quotation}
  How does this work?
\end{quotation}

In math mode, however, it indicates a single-letter variable:
\[(a,b) \quad\hat=\quad λ?f.~?f~a~b\]
\end{document}