The convention dates back to at least ML in the 70s, possibly even longer time back. So it is not some new thing, it's recent rise in popularity is more a refle

The convention dates back to at least ML in the 70s, possibly even longer time back. So it is not some new thing, it's recent rise in popularity is more a reflection in MLs increasing influence in language design space. If I'd had to guess, the syntax might be inspired by math notation such as x ∈ ℝ