This has nothing to do with math or number crunching on the BEAM. This has nothing to do with mutation. This has nothing to do with performance.

This kind of process and function-local static single-assignment code is all over the place in Erlang codebases. It's incredibly common. The other popular method is tail recursion.

I searched for literally 30 seconds and found these:

  - https://github.com/ninenines/cowboy/blob/master/src/cowboy_router.erl#L139-L144
  - https://github.com/ninenines/cowboy/blob/master/src/cowboy.erl#L201-L205
  - https://github.com/ninenines/cowboy/blob/master/src/cowboy_http2.erl#L487-L506