Be dangerous and mutate a immutable Enum in haxe.

Lens.hx

class Test {
    static function main(){
        var a = Cons(1, Nil);
        trace(a);
        trace(ListLens.mutate(a, 2));
        trace(a);
        
        /*
        Output from the above traces - this is dangerous ;-)
        
        Test.hx:4: Cons(1,Nil)
        Test.hx:5: Cons(2,Nil)
        Test.hx:6: Cons(2,Nil)
        */
    }
}
enum List<T> {
    Nil;
    Cons(head : T, tail : List<T>);
}
class ListLens {

    public static function mutate<T>(list : List<T>, value : T) : List<T> {
        #if js
            untyped __js__('list[1] = value');
        #elseif flash
            untyped list.params[0] = value;
        #elseif neko
            untyped list.args[0] = value;
        #end
       return list;
    }
}

Adding AVM2 to ListLens

Adding Neko to ListLens