条件（动态）结构标签

Question

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?

Answer 1

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.