Counterintuitively Julia recommends the use of immutable data types for performance reasons, because immutability enables more flexible compiler optimisations
An immutable variable can be savely shared across functions or even threads without copying.
It can be created on the stack, heap or in a register, whatever the compiler deems most efficient.
In the case, where you want to change a field of an immutable variable (the use case of lenses), immutable types may still be more efficient, because the variable was stack allocated and copying it is cheap or the compiler can correctly infer, that the original object is not in use anymore and thus reuses the data of the old variable for the new one.
Coming from the C++ world, I think immutability by default is pretty need, because it enables many of the optimisations you would get from C++'s move semantics (or Rust's borrow checker) without the hassle.
An immutable variable can be savely shared across functions or even threads without copying. It can be created on the stack, heap or in a register, whatever the compiler deems most efficient.
In the case, where you want to change a field of an immutable variable (the use case of lenses), immutable types may still be more efficient, because the variable was stack allocated and copying it is cheap or the compiler can correctly infer, that the original object is not in use anymore and thus reuses the data of the old variable for the new one.
Coming from the C++ world, I think immutability by default is pretty need, because it enables many of the optimisations you would get from C++'s move semantics (or Rust's borrow checker) without the hassle.