Go: JSON and broken APIs

If you’ve ever used Go to decode the JSON response returned by a PHP API, you’ll probably have ran into this error:

json: cannot unmarshal array into Go struct field Obj.field of type map[string]string

The problem here being that PHP, rather than returning the empty object you expected ({}), returns an empty array ([]). Not completely unexpected: in PHP there’s no difference between maps/objects and arrays.

Sometimes you can fix the server:

return (object)$mything;

This ensures that an empty $mything becomes {}.

But that’s not always possible, you might have to work around it on the client. With Go, it’s not all that hard.

First, define a custom type for your object:

type MyObj struct {
    Field map[string]string `json:"field"`


type MyField map[string]string

type MyObj struct {
    Field MyField `json:"field"`

Then implement the Unmarshaler interface:

func (t *MyField) UnmarshalJSON(in []byte) error {  
    if bytes.Equal(in, []byte("[]")) {
        return nil

    m := (*map[string]string)(t)
    return json.Unmarshal(in, m)

And that’s it! JSON deserialization will now gracefully ignore empty arrays returned by PHP.

Some things of note:

  • The method is defined on a pointer receiver (*MyField). This is needed to correctly update the underlying map.
  • I’m casting the t object to map[string]string. This avoids infinite recursion when we later call json.Unmarshal().

October 22, 2019 20:19 #go #php