Rename NotADag

[cqc-alec]

Validation can fail for reasons other than not being a directed acyclic graph, so the name is misleading. Perhaps InvalidGraph?

[aborgna-q]

That's a bug in the Dag checker with multiports 😅. I have a fix in the works.

[cqc-alec]

If it's a bug in the DAG checker then it's also a documentation bug:

    #[error("The children of an operation {optype:?} must form a dag with single source and sink. Loops are not allowed, nor are dangling nodes not in the path between the input and output. In node {node:?}.")]
    NotADag { node: Node, optype: OpType },

(The docs already imply that there are more conditions than not being a DAG.)

[aborgna-q]

The "single source and sink" requirement is an special kind of DAG. I think we are still in the NotADag category, and the user should be able to see the docs or the error message with the specific info.

[cqc-alec]

Just seems odd to get an error called NotADag for something that is in fact a DAG, just not the right kind of DAG.

[aborgna-q]

Fair enough.

I'd go with something more specific rather than more generic tough, InvalidGraph doesn't communicate much about the failing check.
NotASingleSinkSourceDag seems like a mouthful :P

[cqc-alec]

Hmm...

https://cs.stackexchange.com/questions/67849/what-do-you-call-a-dag-with-a-single-root-source

How about NotABoundedDAG? "Bounded" because if we interpret it as a poset then it has a maximal and a minimal element?