{{/* This is somewhat parallel to unmarshal_helper.go.tmpl, but, as usual, in
     reverse.  Note the helper accepts a pointer-to-interface, for
     consistency with unmarshaling and with the API we expect of custom
     marshalers. */}}

func __marshal{{.GoName}}(v *{{.GoName}}) ([]byte, error) {
    {{/* Determine the GraphQL typename, which the unmarshaler will need should
         it be called on our output. */}}
    var typename string
    switch v := (*v).(type) {
    {{range .Implementations -}}
    case *{{.GoName}}:
        typename = "{{.GraphQLName}}"

        {{/* Now actually do the marshal, with the concrete type. (Go only
             marshals embeds the way we want if they're structs.)  Except that
             won't work right if the implementation-type has its own
             MarshalJSON method (maybe it in turn has an interface-typed
             field), so we call the helper __premarshalJSON directly (see
             marshal.go.tmpl). */}}
        {{if .NeedsMarshaling -}}
        premarshaled, err := v.__premarshalJSON()
        if err != nil {
            return nil, err
        }
        result := struct {
            TypeName string `json:"__typename"`
            *__premarshal{{.GoName}}
        }{typename, premarshaled}
        {{else -}}
        result := struct {
            TypeName string `json:"__typename"`
            *{{.GoName}}
        }{typename, v}
        {{end -}}
        return json.Marshal(result)
    {{end -}}
    case nil:
        return []byte("null"), nil
    default:
        return nil, {{ref "fmt.Errorf"}}(
            `unexpected concrete type for {{.GoName}}: "%T"`, v)
    }
}