Relax restriction on root node.

[cqc-alec]

Update spec as per #94 .

[ss2165]

Thanks!
I think it might be simpler to just say the root can be any container node? Then in the table just comment that the parent column only applies if the container node is not root.

[ss2165]

for another PR but just noting that we also need to make function names not metadata at which point main no longer needs to be the first.

[aborgna-q]

I think it might be simpler to just say the root can be any container node?

Does it even need to be a container? We may just have a single leaf node (E.g. in some replace operation).

[cqc-alec]

I think it might be simpler to just say the root can be any container node? Then in the table just comment that the parent column only applies if the container node is not root.

Yeah I wondered about this, and even whether there was a good reason not to allow any type of node. But maybe some nodes can't really be decontextualized -- like Input and Output (which aren't container nodes), and BasicBlock (which is). Do you think a standalone BasicBlock makes sense?

[aborgna-q]

I don't see any reason to disallow those, they may be used during some operations. As long as the executable HUGRs and loadable HUGRs are well defined, the rest can be just a piece of a bigger hugr. (We need to relax the validation of the root node, e.g. for connected ports).

With something like that, we could maybe define a pure view on a HUGR that just exposes a single node and their children (say, focused on a basic block).

[ss2165]

Happy with no restriction on type of root node. I think we should say somewhere near the top that the type of the root significantly affects the interpretation of the HUGR though.

[cqc-alec]

Happy with no restriction on type of root node. I think we should say somewhere near the top that the type of the root significantly affects the interpretation of the HUGR though.

Allowing BasicBlock nodes to be root would invalidate some statements in the spec such as:

The first output of the DSG contained in a BasicBlock has type Predicate(#t0, #t1,...#tn), where the node has N successors, and the remaining outputs are a row #x. #ti with #x appended matches the inputs of successor i.

We could say "except when the BasicBlock is a root", but I don't see that it has much meaning then.

[ss2165]

Allowing any nodes other than module as the root invalidates edge validities. For instance, you include DFG and CFG but they currently require their internal and external signatures to match (internal signatures of BasicBlock in the CFG case). We just need to add a line about root node having no edges and how that supersedes other requirements.

[ss2165]

👍