Using pure to create immutable

[Lutger Blijdestijn]

Jesse Phillips wrote:

> The discussion on Reddit brought to my attention that pure functions can
> return and assign to an immutable.
> 
> 
http://www.reddit.com/r/programming/comments/knn5p/thoughts_on_immutability_in_d/c2lsgek
> 
> I am trying to modify the example request to make use of this, but have
> failed.
> 
> 
http://www.reddit.com/r/programming/comments/knn5p/thoughts_on_immutability_in_d/c2lrfpm
> 
> test.d(4): Error: cannot implicitly convert expression
> (makeFromArray([1,2,3])) of type test.List!(int).List to immutable(List)
> 
> Is this a bug? I can't identify where this issue would lie (works with
> inheritance and templating).
> 
> void main() {
>    immutable a = makeFromArray([1,2,3]);
> }

For this to work, the arguments to makeFromArray have to be immutable:

void main() {
    immutable(int)[] data = [1,2,3];
    immutable a = makeFromArray(data);
}

> private abstract class List(T) {
>    abstract bool isEmpty () const;
>    abstract T head () const;
>    abstract const(List!T) tail () const;
> 
> 
> }
> private final class Cons(T): List!T {
>    immutable T head_;
>    Cons!T tail_; // not immutable here for a reason
> 
>    this(T h, Cons!T t) { head_ = h; tail_ = t; }
> 
>    override bool isEmpty() const { return false; }
>    override T head () const { return head_; }
>    override const(Cons!T) tail () const { return tail_; }
> }
> 
> List!T makeFromArray(T)(T[] array) pure {

makeFromArray must take and return immutable types:

immutable(Cons!T) makeFromArray(T)(immutable(T[]) array) pure

>    if (array.length == 0) { return null; }
> 
>    auto result = new Cons!T(array[0], null);
>    auto end = result;
> 
>    for (int i = 1; i < array.length; ++i) {
>       end.tail_ = new Cons!T(array[i], null);
>       end = end.tail_;
>    }
> 
>    return result;
> }