Currently the compile marks all closures in a method mutable even though there may be a mix of mutable and immutable:

class Foo
{
  Void main() 
  {
    a := Actor(ActorGroup()) |Obj m->Obj| { echo(m); return m }    
    3.times |Int i| { a.send(a) }     
  }
}

sys::NotImmutableErr: Receive func not immutable: |sys::Obj->sys::Obj|
  fan.sys.Actor.make$ (Actor.java:44)
  fan.sys.Actor.make (Actor.java:29)
  ...

Promoted to ticket #532 and assigned to brian

Ticket resolved in 1.0.43

Don't mark a closure immutable unless it actually captures local variables, even if other closures in that method do capture local variables.

Sounds great, by the way. Fan's hardcore concurrency emphasis is sweet. It seems to have a really nice organization/balance of functional (i.e., immutability-oriented) and imperative features.