COLLECT Semantics: KEEP vs. KEEP SPREAD

Development Semantics
1

When reimplementing COLLECT for UPARSE on top of the "pending" model, I mentioned a potential avenue for optimization by not immediately splicing blocks, but keeping them in a list and then merging the whole list together if-and-when the COLLECT got to the success point:

The idea of saving splicing until the end might sound good, but... it doesn't work if we presume mutations.

Just imagine a delaying implementation of ordinary COLLECT:

>> delayed-splice-collect [
      block: [a b c]
      keep spread block
      clear block
      insert block [d e f]
      keep spread block
  ]
== [d e f d e f]

That's not what we expect from today's COLLECT.

Though if you're not splicing, you expect mutations to affect what you've already collected. It's true in all Redbols today, since you're keeping the block by identity:

red>> collect [
         block: [a b c]
         keep block
         clear block
         insert block [d e f]
         keep block
      ]
== [[d e f] [d e f]]

Persistent Vector Looks Awfully Tempting :face_with_monocle:

Looking at things like this is always a throwback to saying "mutable state by default is garbage". But it depends on what example you're looking at. Sometimes it's good, sometimes it's bad.

I guess the thing is to just explore what the good parts are. Maybe someone (or some AI) will blend in some Clojure bits at some point in a derivative...but that person (or AI) is not going to be me!

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There are several good reasons for SPLICE! to be immutable, and this is one of them.

Outside of the benefits for COLLECT in PARSE, it makes the immutability of NONE seem less random... you just take for granted that all splices are immutable.

This would mean that if you SPREAD something, it becomes shallow-locked. If that's not what you want, it's your responsibility to shallow copy it.

It's not particularly oppressive for the existing uses, most of which are just spread compose [...] or spread reduce [...] anyway. And SPLICE isn't meant to be passed to the first argument of things like APPEND anyway.

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