Are item tags VLQs?

[rakudrama]

The grammar uses / for a tag for extension items:

VendorExtensionItem :
 / VendorExtensionName
 / VendorExtensionName VlqList

InvalidItem :
 InvalidTag
 InvalidTag VlqList
InvalidTag :
 Vlq but not B C D /

All the other defined tags happen to be 1-length VLQs (A, B, C ...)
An InvalidTag is also a VLQ.
This can be taken as an implication that tags are supposed to be VLQs.

/ is not a VLQ, since the digit has a continuation bit set, requiring a following digit. For example /B as a VLQ is 63 (unsigned).
As a vendor extension item it is names[1].
If the tag-space is supposed to be arbitrarily extendable, it would make sense if the tags were always VLQs.

The implication of a non-VLQ tag is that VLQs and tags need to be recognized separately — we can't pre-preemptively decode VLQs since we need context sensitivity to distinguish between / the tag and / the first digit of multi-digit VLQ.

I came across this issue as I was trying to write the character-level scanner as a state machine, but currently you can't do context-insensitive recognition of VLQs and then 'push' them to a recognizer.

[szuend]

Good catch! Tags are supposed to be unsigned VLQs. From the second paragraph in "Scopes structure":

The scopes field is a comma (,) separated list of items. Each item is a list of base64 VLQs. The first base64 VLQ in each item is a tag that specifies which item is described by the rest of the VLQs.

(We should probably add the "unsigned" there.)

This was an over-sight, we wanted to add the highest, single-letter, unsigned VLQ for vendor extensions. I guess this should be f instead of / so we don't have the continuation bit.

[rakudrama]

The choice of / or '31' was possibly an aesthetic choice for two reasons — intended to be highest single letter VLQ, and / looks distinctive.

If a single-letter VLQ is required, then any single letter VLQ will do, and one adjacent to the others would make any kind of dispatch (array lookup, switch) more compact. A would be a fine choice too, or the next-highest unused in the current grammar.