Consider adding a tagless boolean encoding

[popematt]

Tagless encodings help to minimize the space for encoding the arguments to an e-expression. The tagless types in the spec right now focus on encoding values that have an opcode followed by additional data. However, I think boolean values represent an opportunity as well.

Assuming that we can create a macro-shaped parameter using a macro that itself has no parameters, here is an example of how one could create a 2-bit representation of a boolean value.

(macro true_flag () true)

(macro my_configuration (true_flag::$is_foo_enabled?
                         true_flag::$is_bar_enabled?
                         true_flag::$is_baz_enabled?
                         true_flag::$is_quux_enabled?)
       { 
           foo: (if_single $is_foo_enabled true false), 
           bar: (if_single $is_bar_enabled true false),
           baz: (if_single $is_baz_enabled true false), 
           quux: (if_single $is_quux_enabled true false), 
       }
)

And then one could invoke this macro in text and binary, respectively:

(:my_configuration () (:) () (:))

00000000  00 01 00 01
\______/   ^  ^  ^  ^
   |       |  |  |  └ One argument
   |       |  |  └ Zero arguments
   |       |  └ One argument
   |       └ Zero arguments
   Opcode 00 - macro with address 0

The text representation is not very human-friendly because we are abusing the absence/presence of an argument to indicate whether the value should be true or false. In binary, we get 4 arguments represented in only 1 byte (via the argument encoding bitmap).

We should consider adding an official, non-hacky way for users to achieve this. Perhaps we could define a special tagless type called bool_flag that can only be used with ? or ! cardinalities. Then, in text, it can accept true and false, and in binary, it can be encoded using the argument encoding bits:

Bits	Meaning
00	void
01	false
10	true

If #330 is adopted, then we could instead say that bool_flag parameters must always be ! and we can simply use 0 for false and 1 for true, which is a truly minimal encoding for boolean values.