[Storage] Augment return type from `Response<ListOfSentMessage, ...>` -> `Response<SentMessage, ...>`

[vincenttran-msft]

Currently, the send_message API is defined as follows:

 pub async fn send_message(
        &self,
        queue_message: RequestContent<QueueMessage, XmlFormat>,
        options: Option<QueueClientSendMessageOptions<'_>>,
    ) -> Result<Response<SentMessage, XmlFormat>> {
        let response = self.client.send_message(queue_message, options).await?;

        Self::extract_first_message(response, |list: &ListOfSentMessage| {
            list.items.clone().unwrap_or_default()
        })
        .await
    }

This is because the underlying generated function is defined as:

pub async fn send_message(
        &self,
        queue_message: RequestContent<QueueMessage, XmlFormat>,
        options: Option<QueueClientSendMessageInternalOptions<'_>>,
    ) -> Result<Response<ListOfSentMessage, XmlFormat>> {

So the feature request is whether there is anyway we can define something, similar to alternateType, but we would want to be able to define in this case:

1. Turn send_message return type from ListOfSentMessage to SentMessage
2. We can define the conversion function in handwritten code to go from ListOfSentMessage -> SentMessage

We just want to be able to define this sort of behavior without having to maintain the handwritten layer just for this functionality.

For completeness, this is the current implementation of extract_first_message() which is what we currently use to do the repacking to the desired return type:

async fn extract_first_sent_message(
        response: Response<ListOfSentMessage, XmlFormat>,
    ) -> Result<Response<SentMessage, XmlFormat>> {
        let status = response.status();
        let headers = response.headers().clone();
        let list = response.into_model()?;
        let first = list
            .items
            .unwrap_or_default()
            .into_iter()
            .next()
            .ok_or_else(|| {
                azure_core::Error::with_message(
                    azure_core::error::ErrorKind::DataConversion,
                    "No messages found in the response.",
                )
            })?;
        let xml_body = xml::to_xml(&first)?;
        Ok(RawResponse::from_bytes(status, headers, xml_body).into())
    }

[heaths]

Have you tried using @alternativeType and then implement TryFrom appropriately?

[jhendrixMSFT]

Note that @@alternateType is a fairly big hammer. i.e. when you say @@alternateType(ListOfSentMessage, SentMessage) it means "replace all references to ListOfSentMessage with SentMessage". For this particular case, that doesn't appear to be problematic.

@vincenttran-msft I put together a mock-up of what this could look like in my fork of the emitter repo, see the commits here (the last two).

[vincenttran-msft]

Hi Joel, I just skimmed through the relevant typespec changes and the emitted code, and that all looks great and would perfectly match our use-case. I did see your follow-up TODOs, which I agree should be resolved but for our context will not be blocking (as once the generated method signature is correct, we will remove the necessity of pub(crate) visibility). Thanks!

[jhendrixMSFT]

Regarding the TODO, agreed it's not blocking. I only call it out because the TryFrom impl makes use of that type, and since it evaporated I had to copy it. Would be ideal if we can make the type pub(crate) so we can use it.

[jhendrixMSFT]

Opened Azure/typespec-azure#3974 to track the issue mentioned in the TODO.

[jhendrixMSFT]

Closing as it appears we have a solution. Please reopen if there are problems.