When you do `f(x=5)`, x doesn't get set to 5 anywhere you can see (it happens inside the body of f, which you are treating as a black box). So logically, in the context of the code that's calling f, it's not an assignment operation.

Admittedly this kind of goes out the window with a lot of tidyverse non-standard eval stuff. E.g. if you do `df %>% mutate(x = 5)`, then x does get set to 5 in the data frame, so that is logically an assignment of x in a visible scope using '='. Going by my logic, functions like mutate should ideally use a different symbol for operations that alter the input like this, but we run up against syntactic limitations of the language.

I think the argument is that, to the uninitiated, x = x + 2 looks more like an invalid equality than "replace the value stored in x with its current value plus 2".

Personally, it seems to me that = for assignment is so ubiquitous that this argument has become stale.

I still use the arrow because it's the prevailing recommendation in R style guides.