CapnProto C++ as library for parsing textual schemas

[John Demme]

Hello all-

I've spent a lot of time digging around the CapnProto C++ code base, but I can't figure how to point a function to a textual capnp schema and get out the CodeGenRequest. Parsing to a capnp::Schema is not sufficient as that class only represents the root node with (AFAICT) no way to access the other nodes and other information in the CodeGenRequest. I saw there was infrastructure to do this in the compiler, but those header files aren't distributed with the library code.

~John

[Kenton Varda]

Hi John,

`capnp::SchemaParser` will parse capnp files and give you a `capnp::ParsedSchema`, which is like `capnp::Schema` but also has the `getNested()` method which allows you to traverse the whole tree of child nodes. So it's not just the root node.

That said, it's true that it's not super-convenient to construct a `CodeGeneratorRequest` from this. You would need to traverse the tree and flatten it into a list, which is easy enough, but filling in `requestedFiles` (especially `imports`) might be tricky.

Out of curiosity, why do you need to construct a `CodeGeneratorRequest`?

-Kenton

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[John Demme]

Hi Kenton-

I only see:

  ParsedSchema getNested(kj::StringPtr name) const;
  // Gets the nested node with the given name, or throws an exception if there is no such nested
  // declaration.

If there were a getNested() which returned a list of ParsedSchemas that'd probably be sufficient. Knowing what file housed a particular node would be nice, but not strictly necessary.

I'm writing software which takes in a capnp schema along with a bunch of other stuff. It's basically a message passing compiler which can #include capnp schemas to use for message schemas. Only there's no language associated with it, so a source-to-source translator (codegen) doesn't apply.

~John

[Kenton Varda]

`schema.getProto().getNestedNodes()` returns a list of `capnp::schema::Node::NestedNode`, which has `name` and `id` fields. So that gives you the names of the nested declarations, which you can get plug into `getNested()`.

It's a little ugly, but in general, the C++ schema classes only have methods for things that aren't already easy to get from the underlying capnp struct.

That said, I could be convinced that a `getAllNested()` that returns some sort of iterable would be worthwhile here.

-Kenton

[John Demme]

Yeah, I thought of that but it was too ugly. Also, I had some C# code which worked on a CodeGenRequest lying around from an abandoned version of this. There was another reason as well, though I don't remember it now...

If that method existed, I'd use it ;)

~John

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[John Demme]

https://github.com/capnproto/capnproto/pull/1016

Wasn't that difficult ;)

~John

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