6. Diagnostic Notation

CBOR is a binary interchange format. To facilitate documentation and debugging, and in particular to facilitate communication between entities cooperating in debugging, this section defines a simple human-readable diagnostic notation. All actual interchange always happens in the binary format.

Note that this truly is a diagnostic format; it is not meant to be parsed. Therefore, no formal definition (as in ABNF) is given in this document. (Implementers looking for a text-based format for representing CBOR data items in configuration files may also want to consider YAML [YAML].)

The diagnostic notation is loosely based on JSON as it is defined in RFC 4627, extending it where needed.

The notation borrows the JSON syntax for numbers (integer and floating point), True (>true<), False (>false<), Null (>null<), UTF-8 strings, arrays, and maps (maps are called objects in JSON; the diagnostic notation extends JSON here by allowing any data item in the key position). Undefined is written >undefined< as in JavaScript. The non-finite floating-point numbers Infinity, -Infinity, and NaN are written exactly as in this sentence (this is also a way they can be written in JavaScript, although JSON does not allow them). A tagged item is written as an integer number for the tag followed by the item in parentheses; for instance, an RFC 3339 (ISO 8601) date could be notated as:

0("2013-03-21T20:04:00Z")

or the equivalent relative time as

1(1363896240)

Byte strings are notated in one of the base encodings, without padding, enclosed in single quotes, prefixed by >h< for base16, >b32< for base32, >h32< for base32hex, >b64< for base64 or base64url (the actual encodings do not overlap, so the string remains unambiguous). For example, the byte string 0x12345678 could be written h'12345678', b32'CI2FM6A', or b64'EjRWeA'.

Unassigned simple values are given as "simple()" with the appropriate integer in the parentheses. For example, "simple(42)" indicates major type 7, value 42.

6.1. Encoding Indicators

Sometimes it is useful to indicate in the diagnostic notation which of several alternative representations were actually used; for example, a data item written >1.5< by a diagnostic decoder might have been encoded as a half-, single-, or double-precision float.

The convention for encoding indicators is that anything starting with an underscore and all following characters that are alphanumeric or underscore, is an encoding indicator, and can be ignored by anyone not interested in this information. Encoding indicators are always optional.

A single underscore can be written after the opening brace of a map or the opening bracket of an array to indicate that the data item was represented in indefinite-length format. For example, [_ 1, 2] contains an indicator that an indefinite-length representation was used to represent the data item [1, 2].

An underscore followed by a decimal digit n indicates that the preceding item (or, for arrays and maps, the item starting with the preceding bracket or brace) was encoded with an additional information value of 24+n. For example, 1.5_1 is a half-precision floating-point number, while 1.5_3 is encoded as double precision. This encoding indicator is not shown in Appendix A. (Note that the encoding indicator "_" is thus an abbreviation of the full form "_7", which is not used.)

As a special case, byte and text strings of indefinite length can be notated in the form (_ h'0123', h'4567') and (_ "foo", "bar").