Code That Says Exactly What It Means

[Kagamin]

On Thursday, 30 October 2025 at 05:23:29 UTC, Peter C wrote:
> An putting aside any idea of adding scopeprivate to D, let's 
> remind ourselves that abstraction is one of the central 
> principles of software engineering, what an abstract data type 
> actually is, and why it's important for it to have the capacity 
> to establish and maintain its own invariants:
>
> https://learn.adacore.com/courses/ada-in-practice/chapters/abstract_data_types.html
>
> If I put these two types in the same module, where you can 
> reasonably argue they belong, then the concerns in the article 
> above become an immediate reality in D:

Ironically in C# enumerator accesses all collection members 
directly, including `_version`. Here's 
System.Collections.Generic.List:
```
public struct Enumerator : IEnumerator<T>, IEnumerator
{
	private readonly List<T> _list;
	private int _index;
	private readonly int _version;
	private T? _current;

	internal Enumerator(List<T> list)
	{
		_list = list;
		_index = 0;
		_version = list._version;
		_current = default;
	}

	public void Dispose()
	{
	}

	public bool MoveNext()
	{
		List<T> localList = _list;

		if (_version == localList._version && ((uint)_index < 
(uint)localList._size))
		{
			_current = localList._items[_index];
			_index++;
			return true;
		}
		return MoveNextRare();
	}

	private bool MoveNextRare()
	{
		if (_version != _list._version)
		{
			ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
		}

		_index = _list._size + 1;
		_current = default;
		return false;
	}

	public T Current => _current!;

	object? IEnumerator.Current
	{
		get
		{
			if (_index == 0 || _index == _list._size + 1)
			{
				ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumOpCantHappen();
			}
			return Current;
		}
	}

	void IEnumerator.Reset()
	{
		if (_version != _list._version)
		{
			ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
		}

		_index = 0;
		_current = default;
	}
}
```