具有未知属性的接口

Question

I'm trying to use the Unmarshal method from the frontmatter package to parse front matter from a markdown file.

The type signature for the function is as follows

func Unmarshal(data []byte, v interface{}) (err error)

I've got the byte data and I understand that I need to pass an interface/struct with the appropriate fields as the second argument—however I don't know what the fields are going to be in the files that I parse and it's important that I don't lose data.

Internally this package uses yaml.v2 which provides a more comprehensive example for defining the interface before unmarshaling.

type T struct {
    A string
    B struct {
        RenamedC int   `yaml:"c"`
        D        []int `yaml:",flow"`
    }
}

Then create an instance of the struct t and pass a pointer to t through to Unmarshal.

t := T{}

err := yaml.Unmarshal([]byte(data), &t)

As I understand, this is only going to work if the YAML looks like this:

a: Easy!
b:
  c: 2
  d: [3, 4]

The second example looks closer to what I need. Rather than creating a struct, it seems to use a map of interface{} -> interface{}.

m := make(map[interface{}]interface{})

err = yaml.Unmarshal([]byte(data), &m)

I'm relatively new to Go and to me this looks like a generic map, which would be ideal for reading in unknown values.

I've adapted the example for my own project and ended up with the following code.

m := make(map[interface{}]interface{})
err := frontmatter.Unmarshal(data, &m)

But this results in a runtime error

panic: reflect: NumField of non-struct type

Full stacktrace here.

---

Am I heading in the right direction? If so, what's going wrong?

Answer 1

Turns out that the raw yaml.Unmarshal method is robust enough to only pick up the front matter, even when presented with the entire file.

The solution I ended up using looks like this.

// read file made up of front matter and content
data, err := ioutil.ReadFile(file)

if err != nil {
  log.Fatal(err)
}

meta := make(map[interface{}]interface{})
yaml.Unmarshal([]byte(data), &meta)

This means dropping the frontmatter package and using the yaml package directly.