golang: json arrays that also have methods

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Question content

I have a data structure like this:

type (
  parent struct {
    items []*child
  }
  child struct {
    field string `json:"field"`
  }
)

I also hope parent has a way:

func (p *parent) example() { }

But the json requirement is that the parent is just an array:

[
  {
    "field": "data"
  }
]

I want parent to be a simple array, but in order for parent to have methods, it cannot be of array type.

Is there a way to solve these two problems with one data structure?

(To make things more complicated, the actual data structure I have to use has two levels: greatgrandparent contains []grandparent, while grandparent has A parent containing parent. The json structure is defined externally, the array has no key names, and I want methods for each of the four structures.)

Solution

In order for the parent to have methods, it cannot be an array type.

It can, it just has to have a name because only named types (or pointers to named types) can implement methods. The following is valid go code:

type parent []*child

func (p parent) example() { /* ... */ }

Note that the above parent is a slice rather than an array. Arrays in go have static length, you can't increase them and you can't shrink them, slices on the other hand have dynamic length, you can resize them at will. Arrays and slices are closely related, but not the same.

Another way is to have the structure type implement the json.unmarshaler/json.marshaler interface:

type parent struct { Items []*child }

func (p *parent) UnmarshalJSON(data []byte) error {
    return json.Unmarshal(data, &p.Items)
}

func (p parent) MarshalJSON() ([]byte, error) {
    return json.Marshal(p.Items)
}

The above will generate the required json structure.

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