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Comment by truth_machine | Hacker News Reader

So it looks like a better title would have been "saving the state of the turing machine on the bitcoin blockchain", as the claim of Turing completeness[1] seems disingenious - bitcoin script itself has no looping constructs and is decidedly non turing complete. The user has to call the contract as many times as necessary to ensure that Turning machine transitions between states, and the same user checks that the computation terminated.

1: The claim is "It is straightforward to adapt the Turing machine contract above to implement any other Turing machines, by simply changing the states, the symbols and transition function. Thus, any Turing machine can be simulated on Bitcoin, conclusively proving Bitcoin is Turing-Complete by definition. QED."

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The presented solution does *not* loop inside bitcoin script itself, as you suggest, but outside whereby bitcoin transactions perform the state transfer. The machine runs for as long as someone pays for it (or it goes into accepting state) - which makes sense because if there was a one-time-fee for unbounded or potentially infinite runtime, you could create a program that never terminates. This can be compared to Ethereum where every step in execution costs fees and the caller needs to ensure that a sufficient amount of fees (gas) is paid.

Well, in Etherium, provided that sufficient amount of gas is paid for, I could have a contract that implements several (many?) iterations of the Turing machine - or any other computation.

With the approach proposed in the article I need to have an external Turing-complete "controller" that would keep calling the contract.

At this point, what is the benefit I am getting from having this "contract" at all? I would be better off with a just serializing the state of my machine and putting it into OP_RETURN, getting a much smaller blob to store on the chain. So I will save on the fees, could implement my Turing machine (or anything else, really) in the language of my choice, not constrained by the absence of loops and function calls.

Article essentially uses bitcoin blockchain as a database (I hesitate to use the word "ledger"), and the use of "contract" is just a gimmick, seemingly introduced just to prop the absurd claim that bitcoin somehow becomes turing-complete when external turing-complete controller performs "contract calls".

In Ethereum, the caller specifies the gas amount beforehand to ensure that the execution finishes. In the presented bitcoin-based solution, the caller prepares the transactions beforehand that finish the execution; it then publishes the transactions.

> At this point, what is the benefit I am getting from having this "contract" at all? I would be better off with a just serializing the state of my machine and putting it into OP_RETURN, getting a much smaller blob to store on the chain. > Article essentially uses bitcoin blockchain as a database

This is just plain wrong and not at all what this article is about. In the article, a script is developed that enforces state transfer by the specified transition table, i.e. only a specific set of bitcoin transactions are allowed on the state, namely the ones from the transition table.

> This is just plain wrong and not at all what this article is about. In the article, a script is developed that enforces state transfer by the specified transition table, i.e. only a specific set of bitcoin transactions are allowed on the state, namely the ones from the transition table.

So what is the article about, then? I started this thread disagreeing with the claim that material presented in the article somehow makes bitcoin turing complete and claiming that it, in fact, is not. You seem to be arguing this point with me, but I am not exactly sure what your (counter)arguments are.

Looping is not required to qualify as a Turing machine, and it never was. Bitcoin intentionally doesn't have loops, and higher level language sCrypt offer just that.

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