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digitalmars.D.learn - Subtypes with tighter constraints

reply Victor Porton <porton narod.ru> writes:
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.
Jan 01 2019
next sibling parent Alex <sascha.orlov gmail.com> writes:
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);
}
´´´
Jan 01 2019
prev sibling parent Neia Neutuladh <neia ikeran.org> writes:
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.
Jan 01 2019