[asonge]

I think this is exactly it.

If you had a multi-paradigm language, you'd need not only just some way to break the rules easily and have that bubble up, you'd also want this rule-breaking behavior to be exposed up the chain from any libraries. You'd also have to enforce the benefits of immutability and such through social pressure in order for users to see large parts of the benefits of side-effect isolation.

A lot of the choices FP languages make that are less about side-effects (like strict typing systems) are also often an all-or-none proposition. Optional typing systems have very limited benefits if libraries aren't well-typed. The static analyzers can only do so much.

[dozzie]

Oh, they can do surprisingly much. See how Erlang works with its optional typespecs and dialyzer.