Why allow recursion? Is there a use-case for this?
Yes. Although there might be a way of accomplishing the same thing without recursion:
Use case:
Imagine we want, in the future, to support "this" OR "that" transactions. Where "this" and "that" are themselves possibly complex multisignature or escrow or whatever transactions.
Most straightforward way might be a new standard transaction that looks like:
Code:
DUP HASH160 <hash160> EQUAL
IF
OP_EVAL "evaluate the this"
ELSE
DUP HASH160 <hash160> EQUALVERIFY
OP_EVAL "... or evaluate the that"
ENDIF
IF
OP_EVAL "evaluate the this"
ELSE
DUP HASH160 <hash160> EQUALVERIFY
OP_EVAL "... or evaluate the that"
ENDIF
So you'd redeem it by putting one script or the other on the stack (along with the signatures needed by the script).
So.... maybe you want to recurse so that the IF/ELSE script is itself part of a standard, single-hash OP_EVAL, so you can use a newfangled bitcoin address to send to it. That would look like:
Code:
scriptSig: <signatures> <this_or_that_script> <IF/ELSE script>
scriptPubKey: DUP HASH160 <hash of IF/ELSE script> EQUALVERIFY OP_EVAL
scriptPubKey: DUP HASH160 <hash of IF/ELSE script> EQUALVERIFY OP_EVAL
I am NOT proposing an IF/ELSE "this or that" standard script type; I think there is plenty of enough work to do to actually make secure wallets and in-the-chain escrow work. But supporting limited recursion for non-standard or future transactions seems easy and safe...
(terminology footnote: calling scriptSig+scriptPubKey "transactions" isn't accurate, the transaction is the bigger thing, but I'm not sure what else to call them; I hope y'all follow what I'm saying)