[Cbor] nan'' application-extension literal

[Carsten Bormann <[e-mail]>]

Now that we have a more detailed understanding of NaNs in IEEE 754 and thus CBOR, I went ahead and designed an EDN syntax for representing diverse NaN values in diagnostic input and output.  
It seemed to me that a nan’’ application—extension literal will be the best way to integrate such a syntax without a need for a change in the main EDN syntax (which will rarely actually need to express NaN values outside the default for 0.0/0.0).

The syntax is: nan’payload-rep’, where payload-rep is the EDN representation of a floating point number with an absolute value between 1.0 and 2.0 (both sides exclusive).
The payload-rep number is obtained by flipping the topmost bit of the exponent of the NaN.
The payload is usually expressed as a hexadecimal floating point literal; 0x1.0p0 (exclusive) to 0x1.8p0 (exclusive) is a signaling NaN, and 0x1.8p0 (inclusive) to 0x2.0p0 (exclusive) is a quiet NaN.
(Decimal floating point literals are accepted, but they usually lead to periodic fractions that don’t make a lot of sense for NaNs, except for specifically chosen values such as 1.5, 1.25, 1.875, ....
The printer as implemented below always emits hex floats.)

Implemented in cbor-diag 0.9.1 (using the cbor-pure library coming with that) — `gem update cbor-diag` to get that.

In Ruby code, the nan'' syntax is available in the cbor-diag parser after requiring cbor-diag-app/nan, and in the printer by adding the :nan option (otherwise the printer maps everything to NaN, the backwards-compatible behavior).
E.g.,

>> puts (0.0/0.0).cbor_diagnostic(nan: true)
nan'0x1.8p0’

For the tools, the printer option for nnnn2diag.rb is -n (NaN), the parser option for diag2nnnn.rb is (as usual) -anan.  
(Combine to -nanan for playing around.)

Comments welcome.

Next: (1) write it up; (2) implement the necessary flags in cbor.me <http://cbor.me/> as well.

Grüße, Carsten