How to typeset function restrictions

Question

I wonder how to typeset the restriction of a function to some subdomain, a la $f|_A$ or $f\restriction_A$ , but with the vertical bar being as big as possible.

Answer 1

If you want the vertical bar to be at least \big, the following doesn't have the spacing issues of Ulrich's solution:

\newcommand\restr[2]{{% we make the whole thing an ordinary symbol
  \left.\kern-\nulldelimiterspace % automatically resize the bar with \right
  #1 % the function
  \vphantom{\big|} % pretend it's a little taller at normal size
  \right|_{#2} % this is the delimiter
  }}

If you don't need the extra height for normal size symbols, comment the \vphantom line.

Use it as \restr{f}{A}.

Answer 2

Does $\left.f\right|_A$ provide what you are looking for?

Answer 3

How about \newcommand\restr[2]{\ensuremath{\left.#1\right|_{#2}}}, which can be used like this: $\restr{f}{A}$?

(Warning, haven't tested...)