I'm new to golang generics and have the following setup.

1. I've gathered loads of different kinds of reports.
2. Each report has enclosing fields
3. So I wrapped it in a ReportContainerImpl

I've used a type argument of [T Reportable] where the Reportable is defined as follows

type Reportable interface {
    ExportDataPointReport | ImportDataPointReport | MissingDataPointReport | SensorThresoldReport
}

Each of the type in the type constraint is structs that is to be embedded in the container.

type ReportContainerImpl[T Reportable] struct {
    LocationID string `json:"lid"`
    Provider string `json:"pn"`
    ReportType ReportType `json:"m"`
    Body T `json:"body"`
}

I use a discriminator ReportType to determine the concrete type when Unmarshal.

type ReportType string

const (
    ReportTypeExportDataPointReport ReportType = "ExportDataPointReport"
    ReportTypeImportDataPointReport ReportType = "ImportDataPointReport"
    ReportTypeMissingDataPointReport ReportType = "MissingDataPointReport"
    ReportTypeSensorThresoldReport ReportType = "SensorThresoldReport"
)

Since go does not support type assertion for struct (only interfaces) it is not possible to cast the type when Unmarshal. Also go does not support pointer to the "raw" generic type. Hence, I've created a interface that the ReportContainerImpl implements.

type ReportContainer interface {
    GetLocationID() string
    GetProvider() string
    GetReportType() ReportType
    GetBody() interface{}
}

The problem I then get is that I cannot do type constrains on the return type in any form or shape and am back at "freetext semantics" on the GetBody() function to allow for type assertion when Unmarshal is done.

    container, err := UnmarshalReportContainer(data)

    if rep, ok := container.GetBody().(ExportDataPointReport); ok {
      // Use the ReportContainerImpl[ExportDataPointReport] here...
    }

Maybe I'm getting this wrong? - but however I do this, I always end up with somewhere needs a interface{} or to know the exact type before Unmarshal

• Do you have a better suggestion how to solve this in a type (safer) way?

Cheers, Mario :)

For completeness I add the UnmarshalReportContainer here

func UnmarshalReportContainer(data []byte) (ReportContainer, error) {

    type Temp struct {
        LocationID string `json:"lid"`
        Provider string `json:"pn"`
        ReportType ReportType `json:"m"`
        Body *json.RawMessage `json:"body"`
    }

    var temp Temp
    err := json.Unmarshal(data, &temp)
    if err != nil {
        return nil, err
    }

    switch temp.ReportType {
    case ReportTypeExportDataPointReport:
        var report ExportDataPointReport
        err := json.Unmarshal(*temp.Body, &report)
        return &ReportContainerImpl[ExportDataPointReport]{
            LocationID: temp.LocationID,
            Provider:   temp.Provider,
            ReportType: temp.ReportType,
            Body:       report,
        }, err

      // ...
    }
}