Decoding a group with optional entry

[oliviermartin]

The current CDDL specification uses in "Figure 5: Personal Data: Example for extensibility" this example:

NameComponents = (
	? firstName: tstr,
	? familyName: tstr,
)

How do you know when parsing a CBOR buffer which tstr is which one if the CBOR buffer only contains one tstr?

The group as it does not use key, there is no way to identify which tstr is represented.

[cabo]

This group can be used in an array or a map. In a map, the keys tell you which of the group entries are present. In an array, you have no such information. The stickiness of PEG causes the first text string found to be matched to the firstName entry, and the second one to the familyName entry. In a way, the array works as if you had written

NameComponents = (
    ? (firstName: tstr,
    ? familyName: tstr)
)

which may be a better way to write it as it exposes the semantics more visibly.