What spacing around colon in function notation?

Question

Is there some standard for which spacing is to be preferred in the f: X -> Y notation for a function?

Three possibilities:

$f \colon X \to Y$
\\
$f: X \to Y$
\\
$f \mathpunct{:} X \to Y$

The respective outputs are shown here:

Answer 1

The TeXbook, Chapter 16, p. 134:

If you want the `:' character to be treated as a punctuation mark instead of as a relation, just call it \colon:

$f:A\to B$

$f\colon A\to B$

Hence there shouldn't be :, but \colon.