Immutability

All types in Q# are value types. Q# doesn't have a concept of a reference or pointer. Instead, it allows you to reassign a new value to a previously declared variable via an assignment expression. For example, there's no distinction in behavior between reassignments for variables of type Int or variables of type Int[]. Consider the following sequence of statements:

    mutable arr1 = new Int[3];
    let arr2 = arr1;
    arr1 w/= 0 <- 3;

The first statement instantiates a new array of integers [0,0,0] and assigns it to arr1. The next statement assigns that value to a variable with name arr2. The last statement creates a new array instance based on arr1 with the same values except for the value at index 0 which is set to 3. The newly created array is then assigned to the variable arr1. The last line makes use of the abbreviated syntax for evaluate-and-reassign statements, and could equivalently have been written as arr1 = arr1 w/ 0 <- 1;.
After running the three statements, arr1 will contain the value [3,0,0] while arr2 remains unchanged and contains the value [0,0,0].

Q# clearly thus distinguishes the mutability of a handle and the behavior of a type. Mutability within Q# is a concept that applies to a symbol rather than a type or value; it applies to the handle that allows you to access a value rather than to the value itself. It is not represented in the type system, implicitly or explicitly.

Of course, this is merely a description of the formally defined behavior; under the hood, the actual implementation uses a reference counting scheme to avoid copying memory as much as possible. The modification is specifically done in place as long as there's only one currently valid handle that accesses a certain value.