Since I'm making a new bootstrap executable to facilitate FENCE!, I'm patching it to modern behaviors for PARSE3. This includes the behavior of SKIP as being arity-1 and taking how much to skip.
I'm quite pleased with most UPARSE redesigns, including that one. But the substitute for "match any single item" being the <any> tag gave me pause to ask "does this feel right, in light of modern understandings?"
I think I've come to believe that TAG! combinators are all arity-0, which <any> is. But most other tag combinators synthesize things from midair without advancing the input. Not advancing the input might be a good rule for a tag combinator?. Perhaps that's over-restrictive.
Might it be better if it were just one?
>> parse [1020] [x: one]
>> x
== 1020
It might suggest a class of combinators that captured more items (two, three, etc.). Hence they would be a worse choice than e.g. skip 2 if you didn't want to synthesize the values, so hopefully people wouldn't reach for them unless they really wanted blocks. But that would create an irregularity that ONE wouldn't make a block when the others did (if ONE gave back a block, it means the captured thing was itself a block). Or maybe it's a PICK where the series is implicit?
>> parse [1020 304] [x: two]
>> x
== 304
(That would make it seem like it should be FIRST and SECOND instead of ONE and TWO, but I don't particularly like FIRST for this... I don't think.)
I think I like ONE. It's easy to type, and I think it seems more meaningful than <any>. Especially when there is an ANY combinator that does something more related to the non-PARSE ANY.
We could also have an arity-0 NEXT combinator that could return the next position, if you were trying to convey more that you were just skipping ahead and not intending to take any input.