null and type safety

[BCS]

Reply to Walter,

> Steven Schveighoffer wrote:
> 
>> Couldn't one design a struct wrapper that implements this behavior?
>> 
> If that cannot be done in D, then D needs some design improvements.
> Essentially, any type should be "wrappable" in a struct which can
> alter the behavior of the wrapped type.
> 
> For example, you should also be able to create a ranged int that can
> only contain values from n to m:
> 
> RangedInt!(N, M) i;
> 
> Preserving this property of structs has driven many design choices in
> D, particularly with regards to how const fits into the type system.
> 

Why not explicitly support this with bodied typedefs? 



typedef int MyInt(int m, int M)
{
    MyInt opAssign(int i) { assert(m <= i && i <= M); this = i; }

    MyInt opAssign(int m_, int M_)(MyInt!(m_,M_) i) // <- that doesn't work 
but...
    {
       static if(m > m_ && M_ > M) assert(m <= i && i <= M);  // only assert 
as needed
       else static if(m > m_) assert(m <= i);
       else static if(M < M_) assert(i <= M);
       this = i;
    }

    // this is only to define the return type, the normal code for int is 
still generated
    MyInt!(m+m_, M+M_) opAdd(int m_, int M_)(MyInt!(m_,M_) i) = this; 
}