On Wed, Sep 27, 2023 at 01:25:40PM +0200, Victor Toso wrote:
> This patch handles QAPI union types and generates the equivalent data
> structures and methods in Go to handle it.
>
> The QAPI union type has two types of fields: The @base and the
> @Variants members. The @base fields can be considered common members
> for the union while only one field maximum is set for the @Variants.
>
> In the QAPI specification, it defines a @discriminator field, which is
> an Enum type. The purpose of the @discriminator is to identify which
> @variant type is being used.
>
> Not that @discriminator's enum might have more values than the union's
> data struct. This is fine. The union does not need to handle all cases
> of the enum, but it should accept them without error. For this
> specific case, we keep the @discriminator field in every union type.
I still tend think the @discriminator field should not be
present in the union structs. It feels like we're just trying
to directly copy the C code in Go and so smells wrong from a
Go POV.
For most of the unions the @discriminator field will be entirely
redundant, becasue the commonm case is that a @variant field
exists for every possible @discriminator value.
To take one example
type SocketAddress struct {
Type SocketAddressType `json:"type"`
// Variants fields
Inet *InetSocketAddress `json:"-"`
Unix *UnixSocketAddress `json:"-"`
Vsock *VsockSocketAddress `json:"-"`
Fd *String `json:"-"`
}
If one was just writing Go code without the pre-existing knowledge
of the QAPI C code, 'Type' is not something a Go programmer would
be inclined add IMHO.
And yet you are right that we need a way to represent a @discriminator
value that has no corresponding @variant, since QAPI allows for that
scenario. To deal with that I would suggest we just use an empty
interface type. eg
type SocketAddress struct {
Type SocketAddressType `json:"type"`
// Variants fields
Inet *InetSocketAddress `json:"-"`
Unix *UnixSocketAddress `json:"-"`
Vsock *VsockSocketAddress `json:"-"`
Fd *String `json:"-"`
Fish *interface{} `json:"-"`
Food *interface() `json:"-"`
}
the pointer value for 'Fish' and 'Food' fields here merely needs to
be non-NULL, it doesn't matter what the actual thing assigned is.
With regards,
Daniel
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