|x| on a vector is not well-typed. The notation for norms of vectors is ||x||, namely 2 bars on each side.

There still is a naming collision between |X| (cardinality of a set) and |x| (absolute value of a scalar). Sometimes this happens. It generally is still unambiguous though, as mathematicians tend to use different segments of the alphabet for different purposes, and also additionally tend to capitalize sets, and leave scalars lower case.