deriveCompose: Unmarshal

[awalterschulze]

Unmarshal takes two arguments of which one is a return type. So it should still be possible to compose it without the user making a special function.

func getUsername(u *User) (string, error) {
   if u.Username == "" {
       return "", fmt.Errorf("no username")
   }
   return u.Username, nil
}

username, err := deriveCompose(json.Unmarshal, getUsername)(buf)

func deriveCompose(f func([]byte, interface{}) error, g func(*User) (string, error)) func([]byte) (string, error) {
    return func(buf []byte) (string, error) {
         u := new(User)
         if err := f(buf, u); err != nil {
             return "", err
         }
         return g(u)
    }
}