Why are some OpTags not available as NodeHandle?

[lmondada]

We had introduced a OpTag::DataflowParent to refer to nodes with sibling graphs that have an input and output node (useful within DFG rewriting).

I wanted to express this requirement by specifying a NodeHandle in my HugrView trait, but noticed that it is not available. HugrView<DfgID> is too specific. It seems NodeHandles are only available for concrete OpTypes. Is there a reason for this?

I've gone with HugrView<Root> where Root: ContainerHandle<ChildrenHandle = DataflowOpID>, which is esentially equivalent to what I need. Is the ContainerHandle trait making OpTag::DataflowParent redundant? Can we get rid of redundancy AND keep such requirements reasonable concise?

EDIT: I think this question applies to all "abstract" OpTags, i.e. OpTags that are supersets of OpTypes.

[aborgna-q]

Ideally we'd use OpTags as root node restrictions, but notice that tags are enum values while NodeHandles are types implementing a trait.

We cannot parametrize types with values, so we're restricted to using handle types with associated tags instead.
(Rust supports const generics like Array<const N: usize>, but it doesn't allow arbitrary enums like OpTag).

We could define more generic handles for each available tag, and NodeHandle::try_cast where necessary. I just worry that the explicit calls may get annoying.