What is it? The link points to a discussion more deep than I’m willing to read.
replies(10):
Short version.
(x =: y) is an expression that:
1. assigns the value y to the variable x
2. has the value y.
So `print((x := 1) + 1)` prints '2', and sets x=1.
A ton of languages [eg: c, js] have '=' work this way. And a ton of style guides for those languages tell you to avoid using it like that, because it's confusing. So this is a bit controversial.