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From: "James O'Beirne" <james.obeirne@gmail.com>
Date: Thu, 26 Oct 2023 12:04:52 -0400
Message-ID: <CAPfvXfK667yenYeZi_4iUWskdVaJgS37PC3X0dqYNaehPErDcQ@mail.gmail.com>
To: Andrew Poelstra <apoelstra@wpsoftware.net>, 
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: multipart/alternative; boundary="0000000000002096c30608a0bc44"
Subject: Re: [bitcoin-dev] Proposed BIP for OP_CAT
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I have to admit - I'm somewhat baffled at the enthusiasm for a "just CAT"
softfork, since I can't see that it would achieve much. It's indicative to
me that there isn't a compelling example to date that (i) actually has
working code and (ii) only relies upon CAT. I'm not averse to CAT, just
confused that there's a lot of enthusiasm for a CAT-only fork.

To do actually-interesting covenants, afacit you'd need "introspection"
opcodes and/or CHECKSIGFROMSTACK - and even then, for almost all
applications I'm familiar with, that kind of CAT-based approach would be
much more circuitous than the alternatives that have been discussed for
years on this list.

> Vaults

I don't think this is actually a use-case that CAT materially helps with.
Andrew's posts, while well written and certainly foundational, do not
sketch a design for vaults that someone would actually use. I don't see how
CAT alone (without many auxiliary introspection opcodes) facilitates vaults
that clear the usability hurdles I describe in this paper:
https://jameso.be/vaults.pdf. For example, batched withdrawals and partial
unvaultings don't seem possible.

Even with introspection opcodes, Burak's (
https://brqgoo.medium.com/emulating-op-vault-with-elements-opcodes-bdc7d8b0fe71)
prototype wasn't able to handle revaulting - an important feature for
usability (https://twitter.com/jamesob/status/1636546085186412544).

> Tree signatures

To what extent does Taproot obviate this use?

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<div dir=3D"ltr"><div>I have to admit - I&#39;m somewhat baffled at the ent=
husiasm for a &quot;just CAT&quot; softfork, since I can&#39;t see that it =
would achieve much. It&#39;s indicative to me that there isn&#39;t a compel=
ling example to date that (i) actually has working code and (ii) only relie=
s upon CAT. I&#39;m not averse to CAT, just confused that there&#39;s a lot=
 of enthusiasm for a CAT-only fork.<br></div><div><br></div><div>To do actu=
ally-interesting covenants, afacit you&#39;d need &quot;introspection&quot;=
 opcodes and/or CHECKSIGFROMSTACK - and even then, for almost all applicati=
ons I&#39;m familiar with, that kind of CAT-based approach would be much mo=
re circuitous than the alternatives that have been discussed for years on t=
his list.</div><div><br></div><div>&gt; Vaults</div><div><br></div><div>I d=
on&#39;t think this is actually a use-case that CAT materially helps with. =
Andrew&#39;s posts, while well written and certainly foundational, do not s=
ketch a design for vaults that someone would actually use. I don&#39;t see =
how CAT alone (without many auxiliary introspection opcodes) facilitates va=
ults that clear the usability hurdles I describe in this paper: <a href=3D"=
https://jameso.be/vaults.pdf">https://jameso.be/vaults.pdf</a>. For example=
, batched withdrawals and partial unvaultings don&#39;t seem possible.=C2=
=A0</div><div><br></div><div>Even with introspection opcodes, Burak&#39;s (=
<a href=3D"https://brqgoo.medium.com/emulating-op-vault-with-elements-opcod=
es-bdc7d8b0fe71">https://brqgoo.medium.com/emulating-op-vault-with-elements=
-opcodes-bdc7d8b0fe71</a>) prototype wasn&#39;t able to handle revaulting -=
 an important feature for usability (<a href=3D"https://twitter.com/jamesob=
/status/1636546085186412544">https://twitter.com/jamesob/status/16365460851=
86412544</a>).</div><div><br></div><div>&gt; Tree signatures</div><div><br>=
</div><div>To what extent does Taproot obviate this use?<br></div></div>

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