[v3] RFC: Changing the API for Get()

[lestrrat]

Because all of jwx objects where allowed to carry both pre-defined and arbitrary fields that appear on the JSON objects, we needed two sets of accessors.

The first group consisted of accessors whose target fields were pre-defined, and their type known (and agreed on by the users) before hand. These field could have nicely named and typed accessors like the following:

token.NotBefore() // time.Time
token.Subject() // string

This doesn't work for all the extra fields that the user may specify. Therefore we created the Get/Set APIs

token.Get(`x-my-very-special-claim`)

The problem with this API is that the Get API could only return interface{}, because we don't know the type of value beforehand. This means that to use this value we still needed one more type conversion:

v, _ := token.Get(`x-my-very-special-claim`)
if s, ok := v.(string); ok {
  // finally got a usable value
}

For v3, we'd like to change this signature to

Get(string, interface{}) error

This allows you to pass a pointer to a typed variable, if you as the user knows what the value is supposed to be:

// I know, as the person using this particular flavor of token,  that this is a float64
var val float64
if err := token.Get(`x-my-very-special-claim`, &val); err != nil {
   ...
}
// Use val

This work much like how json.Unmarshal works, and should be relatable to most Go users, and shorten the number of lines that they have to write by a handful of lines.

[fehrnah]

Maybe we could use a generic version of the get method ?

Something like

func Get[T any] (fieldName string) (T, error) {
    // ...
}

and the use would look like

val, err := token.Get[float64](`x-my-very-special-claim`)
if err != nil {
    //...
}

[lestrrat]

At least as of today, no can do even if we wanted to.

[lestrrat]

This has been implmented in #990. Closing