[brightball]

The difference between Elixir/Erlang and a statically typed functional language is that it's built for distribution.

In a distributed system where machines register on a cluster and messages are passed constantly across isolated heaps, processes and entire machines transparently all of the assumed protections of a static type system break down. You can't enforce static types across a cluster anymore than you can across a JSON requests to somebody else's API without a lot of extra overhead.

Static types are essentially contracts and in order to enforce a contract like that on a cluster of different machines that would mean you'd have to exchange contracts everytime a new machine joined the cluster...for all modules and functions...with every machine on the cluster. And then you have to determine how to handle contract violations.

Clustered message passing operates more like request routing in that regard. Send to this machine, to this module, to a function named this, with this arity, that matches this pattern.

The Elixir gradual typing approach balances this reality extremely well in my opinion. You get type checking if you want it and can specify it in more detail...but it won't promise something that it can't guarantee across the cluster, across deploys and across changes.