Decoding optional and default values with decco ppx in rescript

DecodingWithDecco.res

module RequiredField = {
  @decco
  type t = {value: string}
}

module RequiredFieldWithDefault = {
  @decco
  type t = {
    @decco.default("fallback")
    value: string,
  }
}

module OmittableOrNullableField = {
  @decco
  type t = {value: option<string>}
}

module OmittableOrNullableFieldWithDefault = {
  @decco
  type t = {
    @decco.default(Some("fallback"))
    value: option<string>,
  }
}

let fieldOmitted = %raw(`{}`)
let fieldPresentButNull = %raw(`{"value": null}`)
let fieldPresentButUndefined = %raw(`{"value": undefined}`)
let fieldPresent = %raw(`{"value": "present"}`)

Js.log((
  "RequiredField",
  RequiredField.t_decode(fieldOmitted), // Error: value not a string
  RequiredField.t_decode(fieldPresentButNull), // Error: value not a string
  RequiredField.t_decode(fieldPresentButUndefined), // Error: value not a string (undefined is not a valid JSON value)
  RequiredField.t_decode(fieldPresent), // Ok {value: "present"}
))
Js.log((
  "RequiredFieldWithDefault",
  RequiredFieldWithDefault.t_decode(fieldOmitted), // Ok {value: "fallback"}
  RequiredFieldWithDefault.t_decode(fieldPresentButNull), // Error: value not a string (Was hoping for Ok {value: "fallback"} here)
  RequiredFieldWithDefault.t_decode(fieldPresentButUndefined), // Error: value not a string
  RequiredFieldWithDefault.t_decode(fieldPresent), // Ok {value: "present"}
))
Js.log((
  "OmittableOrNullableField",
  OmittableOrNullableField.t_decode(fieldOmitted), // Ok {value: None}
  OmittableOrNullableField.t_decode(fieldPresentButNull), // Ok {value: None}
  OmittableOrNullableField.t_decode(fieldPresentButUndefined), // Error: value not a string
  OmittableOrNullableField.t_decode(fieldPresent), // Ok {value: Some("present")}
))
Js.log((
  "OmittableOrNullableFieldWithDefault",
  OmittableOrNullableFieldWithDefault.t_decode(fieldOmitted), // Ok {value: None} 
  OmittableOrNullableFieldWithDefault.t_decode(fieldPresentButNull), // Ok {value: None}
  // (Note that falling back to None makes this no better than the OmittableOrNullableField case)
  OmittableOrNullableFieldWithDefault.t_decode(fieldPresentButUndefined), // Error: value not a string
  OmittableOrNullableFieldWithDefault.t_decode(fieldPresent), // Ok {value: Some("present")}
))