Plain Ol' Go Structs

[zombiezen]

Similar to the support planned for C++: https://groups.google.com/d/topic/capnproto/Reg0wInHBdY/discussion

Design sketch:

package pogs // import "zombiezen.com/go/capnproto2/pogs"

// GeneratedStruct is the interface implemented by a struct generated by capnpc-go.
type GeneratedStruct interface {
  GeneratedStruct() (id uint64, schemaData []byte, s capnp.Struct)
}

// Unmarshal copies from a Cap'n Proto struct to a plain Go struct.
func Unmarshal(val interface{}, s GeneratedStruct) error {
  // ...
}

// Marshal copies from a plain Go struct into a Cap'n Proto struct.
func Marshal(s GeneratedStruct, val interface{}) error {
  // ...
}

Usage:

// Assuming you have a capnp schema foo.capnp:
// struct Foo {
//   bar @0 :UInt64;
// }

type Foo struct {
    Bar uint64
}

func main() {
    msg, err := capnp.Unmarshal(myByteSource)
    if err != nil {
        panic(err)
    }
    foo, err := fooschema.ReadRootFoo(msg)
    if err != nil {
        panic(err)
    }
    // New API:
    var fooStruct Foo
    if err := pogs.Unmarshal(&fooStruct, foo); err != nil {
        panic(err)
    }
    fmt.Println(fooStruct.Bar)

    // And new API for writing back out:
    if err := pogs.Marshal(foo, &fooStruct); err != nil {
        panic(err)
    }
        fmt.Print(msg.Marshal())
}

capnpc-go could be optionally extended to generate the structs automatically, but much like the json package, users could also define narrow Go structs if they only need certain fields.

cc @lvdlvd