[itishappy]

> Does this mean a web-endpoint has to be immutable? If you send it the same parameters multiple times, is it required to respond with the same response every time? If not, does that not mean it is in fact mutable?

This is a concept called purity, and it's only loosely related to immutability. Immutability makes purity easier to implement and reason about, but does not guarantee it. Erlang/Elixer are not pure. For example, `DateTime.now("Etc/UTC")` will return different things at different times.

As a counter-example, Haskell functions are pure, so the `getCurrentTime` function cannot return a value directly as it would be different every time. Instead it uses the return type `IO UTCTime` which act like instructions on how to calculate the time, rather than the time itself.

https://en.wikipedia.org/wiki/Pure_function