Question: How do I apply a transformer to a child field of a struct depending on the value of another field in the struct? about go-cmp

[dsnet]

Glad I could help!

You should consider adding the example code as an example in the godoc.

Yea, the examples could use some love. It's on my list of things to do someday.

[dsnet]

I think (according to my understanding, it will attempt to apply the transfer from the Mystruct struct and will not attempt to apply it to the fields within Mystruct.

You are correct that it will apply on Mystruct, which you can transform into something else entirely. The cmpopts.AcyclicTransformer would only be needed if the output is (or recursively contains) the Mystruct type.

Alternatively, you could use cmp.FilterPath, which since 0.3.0 is strictly more powerful than cmp.FilterValues. Every cmp.PathStep in the cmp.Path has a Values method, which can be used to interrogate information about the parent values.

Thus, you could do something like this:

cmp.FilterPath(func(p cmp.Path) bool {
    // Filter specifically for accesses to the Mystruct.Data field.
    if p.Index(-2).Type() != reflect.TypeOf(Mystruct{}) {
    	return false
    }
    if sf, ok := p.Index(-1).(cmp.StructField); !ok || sf.Name != "Data" {
    	return false
    }

    // Filter specifically for some specific data type.
    x, y :=  p.Index(-2).Values()
    return x.(Mystruct).DataType == myType && y.(Mystruct).DataType == myType
}, cmp.Transformer("unmarshalMyType", func(b []byte) T {
   return unmarshalMyType(b)
}))

[dsnet]

Here's a more advanced example with multiple transformed types:

cmp.FilterPath(func(p cmp.Path) bool {
    // Filter specifically for accesses to the Mystruct.Data field.
    if p.Index(-2).Type() != reflect.TypeOf(Mystruct{}) {
    	return false
    }
    if sf, ok := p.Index(-1).(cmp.StructField); !ok || sf.Name != "Data" {
    	return false
    }
}, cmp.Options{
	// The sub-options below can safely assume that the filter above is applied.

	// This filter only applies on "fooType".
	cmp.FilterPath(func(p cmp.Path) bool {
	    x, y :=  p.Index(-2).Values()
	    return x.(Mystruct).DataType == fooType && y.(Mystruct).DataType == fooType
	}, cmp.Transformer("unmarshalFoo", func(b []byte) Foo {
		return unmarshalFoo(b)
	}),

	// This filter only applies on "barType".
	cmp.FilterPath(func(p cmp.Path) bool {
	    x, y :=  p.Index(-2).Values()
	    return x.(Mystruct).DataType == barType && y.(Mystruct).DataType == barType
	}, cmp.Transformer("unmarshalBar", func(b []byte) Bar {
		return unmarshalBar(b)
	}),
})

// In case you're wondering what happens when x.DataType != y.DataType,
// then none of the options apply and cmp will compare Mystruct as a normal
// struct, which would report a difference (which is probably what you want).

[F21]

Thanks @dsnet ! That was very useful. You should consider adding the example code as an example in the godoc.