5
In bitcoin, you can create a script like this:
OP_1 OP_IF OP_RETURN OP_ENDIF
But could you place another if statement inside the first?
OP_1 OP_IF OP_1 OP_IF OP_RETURN OP_ENDIF OP_ENDIF
I grant that this is a bit of a silly example (given that the whole thing could be replaced with OP_RETURN) but is that a valid script?
4
Firstly, I'd like to note that you could change your script to OP_1 OP_1 OP_BOOLAND OP_IF OP_RETURN OP_ENDIF and it would be the same.
I'm reading the bitcoin source code, and I can't see a check for nested OP_IF's. These are the IF related checks I've found:
So it would let you nest IF's. In fact, it appears that allowing people to nest IF's is intended. Take this variable, which is used to figure out whether a branch of an IF is executing:
vector<bool> vfExec;
If they didn't want nested IF's, they could have made that a single bool variable.
Amusing fact that I figured out while researching this: The script OP_1 OP_IF OP_ELSE OP_ELSE OP_ENDIF is entirely valid.
1Indeed, you can have multiple OP_ELSE's. Each one inverts the meaning, so <a IF b ELSE c ELSE d ENDIF e> is equivalent to <a IF b d ELSE c ENDIF e>. – Pieter Wuille – 2014-01-26T11:00:45.420