Mutability in jakt is modeled around value semantics, i.e. if one copies an immutable value such as a number or a struct instance the copy can be mutable:

let a = 1
mut b = a
b = 2

which is totally fine. However classes and boxed enums have reference semantics, so chaning the "copy" will also changed the "copied" value, because it was not in fact copies. To fix this two things would have to be changed:

• it should not be allowed to assign an immutable instance to mutable variables
• for function return values of boxed enum or class type it should be possible to declare them immutable.

for the latter again I see three possibilities

• add a const qualifier for return types
• make class/boxed enum return values immutable by default and allow making them mutabke explicitly by adding a mut qualifier
• infer mutablity from whatever is returned

the latter could actually be combined with one of the other two

To really make using immutable instances safe imho it would have to be guarantueed that all variables holding a reference to an instance are immutable. That means either an instance would have to be immutable at construction time and then it should also not be possible to assign it to a mutable variable. or assigningments chaning mutability would have to assert that the reference count is 1.