条件（动态）结构标签

I'm trying to parse some xml documents in Go. I need to define a few structs for this purpose, and my struct tags depend on a certain condition.

Imagine the following code (even though I know it won't work)

if someCondition {
    type MyType struct {
        // some common fields
        Date    []string `xml:"value"`
    }
} else {
    type MyType struct {
        // some common fields
        Date    []string `xml:"anotherValue"`
    }
}

var t MyType
// do the unmarshalling ...

The problem is that these two structs have lots of fields in common. The only difference is in one of the fields and I want to prevent duplication. How can I solve this problem?

The simplest is probably to handle all possible fields and do some post-processing.

For example:

type MyType struct {
    DateField1    []string `xml:"value"`
    DateField2    []string `xml:"anotherValue"`
}

// After parsing, you have two options:

// Option 1: re-assign one field onto another:
if !someCondition {
    parsed.DateField1 = parsed.DateField2
    parsed.DateField2 = nil
}

// Option 2: use the above as an intermediate struct, the final being:
type MyFinalType struct {
    Date    []string `xml:"value"`
}

if someCondition {
    final.Date = parsed.DateField1
} else {
    final.Date = parsed.DateField2
}

Note: if the messages are sufficiently different, you probably want completely different types for parsing. The post-processing can generate the final struct from either.

You use different types to unmarshal. Basically, you write the unmarshaling code twice and either run the first version or the second. There is no dynamic solution to this.

As already indicated, you must duplicate the field. The question is where the duplication should exist.

If it's just a single field of many, one option is to use embedding, and field shadowing:

type MyType struct {
    Date    []string `xml:"value"`
    // many other fields
}

Then when Date uses the other field name:

type MyOtherType struct {
    MyType // Embed the original type for all other fields
    Date     []string `xml:"anotherValue"`
}

Then after unmarshaling of MyOtherType, it's easy to move the Date value into the original struct:

type data MyOtherType
err := json.Unmarshal(..., &data)
data.MyType.Date = data.Date
return data.MyType // will of MyType, and fully populated

Note that this only works for unmarshaling. If you need to also marshal this data, a similar trick can be used, but the mechanics around it must be essentially reversed.