In Ada2012 there are "subtypes". Subtypes can have tighter constraints (such as type invariants) than their base types.

I have a struct X in D. Is it possible to define a type equivalent to X except that having tighter invariants?

As I understand if I derive Y from X, then it is no more X; that is I cannot use X (even provided it matches Y invariants) where I need Y. So in D it is impossible, right?

I think in D it's impossible, but want to be sure and so ask.

On Tuesday, 1 January 2019 at 14:05:43 UTC, Victor Porton wrote:
> In Ada2012 there are "subtypes". Subtypes can have tighter constraints (such as type invariants) than their base types.
>
> I have a struct X in D. Is it possible to define a type equivalent to X except that having tighter invariants?
>
> As I understand if I derive Y from X, then it is no more X;
1. As Ada is strongly typed, this is the same there, no?
2. You cannot derive structs in D. Therefore, I assume, that you meant that X and Y are classes.

> that is I cannot use X (even provided it matches Y invariants) where I need Y. So in D it is impossible, right?

It is possible, as invariants are inherited implicitly, see
https://dlang.org/spec/contracts.html#Invariants
paragraph 9.

´´´
import std.experimental.all;

void main()
{
	auto x = new X();
	auto y = new Y();
	x.d = 0.0;
	y.d = 1.0;
	x.fun;
	y.fun;
}

class X
{
	double d;

	invariant
	{
		assert(!d.isNaN);
	}
}

class Y : X
{
	invariant
	{
		assert(d > 0);
	}
}

void fun(T)(T t)
{
	assert(t);
}
´´´

On Tue, 01 Jan 2019 14:05:43 +0000, Victor Porton wrote:
> In Ada2012 there are "subtypes". Subtypes can have tighter constraints (such as type invariants) than their base types.
> 
> I have a struct X in D. Is it possible to define a type equivalent to X except that having tighter invariants?

In D, structs don't participate in inheritance. Classes do, but invariants aren't inherited; they're specific to the class in which the function is defined. (Which is probably a bug and I filed https://issues.dlang.org/ show_bug.cgi?id=19537.)

I see three ways to make this work today:

1. Use alias this to wrap the struct. Add invariants that deal with the
wrapped struct's fields.
2. Use compile-time reflection to copy the fields over, then manually copy
the invariants over and add more.
3. Use a class. Define a virtual function like 'doInvariant()`. Call it
from the base class's invariant{} block.

> As I understand if I derive Y from X, then it is no more X; that is I cannot use X (even provided it matches Y invariants) where I need Y. So in D it is impossible, right?

With classes, a derived class instance can be used anywhere you expect a base class instance. The reverse is not true.