Yeah I know exactly what you mean in Haskell. Its just that in scala given the java inter-op there are a whole hunch of escape hatches that will always be there. I remember in the coursera exercises when you submitted code that used mutable vars...it would flag your code and deduct points. So essentially even if someone provided a purefn annotation...to actually enforce that, the compiler would have to scan each function and disbar i/o, calls to rng, calls to global vars... tall order.

Which coursera course? For Scala, or functional programming, or something else? Do you have a link to it?

There's only one so far: FP in Scala, run by Odersky himself.