The blockchain is a novel approach to support distributed systems enabling a common, consistent view of a shared state among distributed nodes. There, smart contracts are computer programs that allow users to deploy arbitrary computations, in charge of automatically regulate state transitions and enforce properties. In this paper we speculate on how the blockchain and smart contracts could take advantage of a logic programming approach, and, complementarily, on how logic programming can benefit from the blockchain infrastructure. Accordingly, we discuss some possible research directions and open questions for future research.

1. From the Blockchain to Logic Programming and back
Research perspectives
Giovanni Ciatto1 Roberta Calegari1 Stefano Mariani2
Enrico Denti1 Andrea Omicini1
1
{roberta.calegari, giovanni.ciatto, enrico.denti,
andrea.omicini}@unibo.it
2
stefano.mariani@unimore.it
1Dipartimento di Informatica, Scienza e Ingegneria—Universit`a di Bologna
2Dipartimento di Scienze e Metodi dell’Ingegneria, Universit`a di Modena e Reggio Emilia
Talk @ 19th Workshop From Objects to Agents
Palermo, June 29, 2018
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 1 / 30

2. Outline
1 Motivations & Context
2 Blockchain and smart contracts
3 Logic-based smart contracts
4 The blockchain for distributed Logic Programming
5 Conclusions and future works
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 2 / 30

3. Motivations & Context
Next in Line. . .
1 Motivations & Context
2 Blockchain and smart contracts
3 Logic-based smart contracts
4 The blockchain for distributed Logic Programming
5 Conclusions and future works
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 3 / 30

4. Motivations & Context
Context
Blockchain Technology (BCT)
Novel way to achieve replication of business-logic & data on the net
consistency, untamperability, fault-tolerance, trust over decentralisation
eg cryptocurrencies (e.g. Bitcoin) or identity/access/asset management
Smart-contracts-enabled BCT (ScBCT)
An interpreter as the replicated application, executing smart contracts
ie trusted computational entities automatically handling our assets
Ethereum as the most prominent example
Logic programming (LP)
Programming as goal-oriented reasoning, on top of declarative language
supports meta-programming and provides understandable outputs (proofs)
employed in cognitive agents, e.g. AgentSpeak [Rao96]
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 4 / 30

5. Motivations & Context
Motivations
Can LP be useful within the scope of ScBCTs?
LP may positively affect smart contracts’
declarativeness, observability and mutability . . .
interaction semantics, through message-passing . . .
. . . paving the way towards agent-oriented smart contracts
Can BCTs be useful within the scope of LP?
BCTs as a shared blackboard for distributed cooperative reasoning
Smart contracts as logic services, `a la LPaaS [CDMO18]
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 5 / 30

6. Blockchain and smart contracts
Next in Line. . .
1 Motivations & Context
2 Blockchain and smart contracts
3 Logic-based smart contracts
4 The blockchain for distributed Logic Programming
5 Conclusions and future works
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 6 / 30

7. Blockchain and smart contracts Overview
Blockchain & Smart contracts Overview
ScBCT consist of a P2P network jointly pursuing State Machine
Replication (SMR) [CBPS10, Sch90]
maintaining a consistent view/update over/of a shared state . . .
. . . among mutually-untrusted nodes, a.k.a. miners
replicating an interpreter, i.e. a virtual machine, for smart contracts
by means of the
PoW
Proof-of-Work [Bac02, Nak08] consensus mechanism
providing eventual consistency of the whole system state
endowing cryptocurrencies with their economic value
logging all transactions affecting the shared state on the block-chain
issued by end users
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 7 / 30

8. Blockchain and smart contracts State, transactions & blocks
Blockchain main concepts I
TXs
hash( )
Index i-1
Timestamp
TXs
hash( )
Index i
Timestamp
TXs
hash( )
Index i+1
Timestamp
Statei-1 Statei Statei+1
States, transactions and blocks: the global p.o.v.
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 8 / 30

9. Blockchain and smart contracts State, transactions & blocks
Blockchain main concepts II
Transactions
Blocks
Blockchain
Miners
Clients
Miners, clients and replication: the network p.o.v.
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 9 / 30

10. Blockchain and smart contracts Smart contracts
Smart contracts [Sza96]
Definition
Stateful, reactive, user-defined, immutable, and deterministic processes
executing some arbitrary computation on the blockchain, i.e., while being
consistently replicated over the blockchain network
Stateful — they encapsulate their own state, like OOP’s objects
Reactive — they can only be triggered by issuing some invocation TX
User-defined — users can deploy their smart contracts implementing an
arbitrary logic by issuing a deployment TX
Immutable — their source/byte-code cannot be altered after deployment
Arbitrary — they are expressed with a Turing-complete language
Replicated — the blockchain is essentially a replicated interpreter, employing
a consensus protocol to keep the many replicas consistent
! Disclaimer: focus on the Ethereum platform
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 10 / 30

11. Blockchain and smart contracts Smart contracts
Smart contracts – Features and problems
State encapsulation + sync. interaction + transactional semantics
Object-oriented & imperative metaphor, e.g. Solidity
Side effects are reverted on exceptions
× problems arising from re-entrancy [LCO+16, LL17]
× imperative syntax is understandable to experts only
Immutability + observability of code =⇒ Trust in financial TXs
× may require disassembler or source code
Economic dimension of computations: miners need compensation
quasi-Turing completeness [Woo14]
× per-instruction economic-cost may hinder SW engineering
× Immutability =⇒ No bug fixes
× Pure reactiveness =⇒ No scheduled/periodic computations
× poor practical expressiveness
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 11 / 30

12. Blockchain and smart contracts Smart contracts
Smart contracts on the blockchain I
TXs
hash( )
Index i-1
Timestamp
TXs
hash( )
Index i
Timestamp
TXs
hash( )
Index i+1
Timestamp
Statei-1 Statei Statei+1
Smart contracts being deployed and evolving among states
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 12 / 30

13. Blockchain and smart contracts Smart contracts
Smart contracts on the blockchain II
Transactions
Blocks
Blockchain
Miners
Clients
Smart contracts
Smart contracts being replicated among peers
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 13 / 30

14. Logic-based smart contracts
Next in Line. . .
1 Motivations & Context
2 Blockchain and smart contracts
3 Logic-based smart contracts
4 The blockchain for distributed Logic Programming
5 Conclusions and future works
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 14 / 30

15. Logic-based smart contracts
Logic-based smart contracts
Smart contracts are re-interpreted as logic processes characterised by:
Logic theories as specification scripts
=⇒ Declarativeness & understandability
Strong division between static & dynamic KBs
Static KB defined upon deploy and immutable
Dynamic KB keeping track of the mutable state
=⇒ Controlled mutability by adding/removing rules
Special functors define smart contracts interaction semantics API
eg receive(Message) :- WhatToDo.
eg send(Message, Receiver).
send(M, R) triggers goal receive(Message) on R
TX is aborted if the goal cannot be proven
! Miners role, consensus mechanism, and transactional semantics remain unchanged
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 15 / 30

16. Logic-based smart contracts
Features and open questions
Interpreted Language & mutability =⇒ observability
State and behaviour inspectable without disassembler
Controlled mutability: a smart contract modifying it-self
(Inspectable) behaviour may be added/fixed after deploy
? Which semantics for inter-smart-contracts communication?
! Sync VS async semantics of the send/2 primitive
Async semantics =⇒ encapsulation of control flow
? Delayed messages for post-poned computations
Self-sent messages for periodic computations
Only makes sense for the async semantics
? Economic-related aspects of (logic-based) computation
Who’s paying for message-triggered computations?
Which value for unification? back-tracking? asserts? retracts? . . .
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 16 / 30

17. Logic-based smart contracts
Sync VS Async I
Invocation TX
Method Call
Return from Call
End of TX
hash( )
Index i
Timestamp
TX
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 17 / 30

18. Logic-based smart contracts
Sync VS Async II
Invocation TX
Message TX
Message TX
Message TX
hash( )
Index i
Timestamp
hash( )
Index i+2
Timestamp
hash( )
Index i+3
Timestamp
hash( )
Index i+1
Timestamp
TX TX TX TX
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 18 / 30

19. The blockchain for distributed Logic Programming
Next in Line. . .
1 Motivations & Context
2 Blockchain and smart contracts
3 Logic-based smart contracts
4 The blockchain for distributed Logic Programming
5 Conclusions and future works
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 19 / 30

20. The blockchain for distributed Logic Programming The blockchain as a Logic Theory
The blockchain as a Logic Theory
BCTs essentially provide consistent read/write access to distributed data
What if data were logic terms?
The whole blockchain represents (the evolution of) a logic theory
Clients may issue goals to the blockchain network
or asserting/retracting shared knowledge
Miners may cooperatively participate to the SLD resolution process
Navigating the proof-three in parallel
ie publishing partial results on the blockchain
? Economical incentives for a fair balancing of computational efforts
Preventing already proven goals to be proven again
? How to freeze the knowledge while reasoning?
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 20 / 30

21. The blockchain for distributed Logic Programming Smart contacts enabled BCTs as multiple theories
Smart contacts enabled BCTs as multiple theories
Smart contracts essentially control consistent read/write access their
internal state
What if each smart contract was responsible for a monotonic theory?
Non-monotone edits spawns a new smart contract
in charge of handling the new theory version
the old smart contract and its theory continue to exist
Multiple, possibly para-consistent logic theories
all originating from a common ancestor
each theory expresses what is true in a particular space-time point
each one is handled by a particular smart contract
Clients may query each smart contract’s theory
about a particular view of the world
smart contracts employ SLD resolution
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 21 / 30

22. Conclusions and future works
Next in Line. . .
1 Motivations & Context
2 Blockchain and smart contracts
3 Logic-based smart contracts
4 The blockchain for distributed Logic Programming
5 Conclusions and future works
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 22 / 30

23. Conclusions and future works
Conclusions
The blockchain as a means for replicating programs over the network
ie Smart contracts
Mixing LP and the blockchain is possible in two ways
1 Logic programming and its semantics adopted by smart contracts
Logic-based smart contracts
Also focusing on their interaction semantics
2 The blockchain as a fault-tolerant middleware for logic programs
The blockchain as a blackboard for distributed cooperative reasoning
Smart contracts as multiple para-consistent logic theories
Several questions, mostly arising from the economic dimension of
computation
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 23 / 30

24. Conclusions and future works
Future Works
Developing our own logic-friendly blockchain implementation
? maybe based on Tendermint1
Refining our logic smart contract abstraction towards MAS
Re-designing the Ethereum operational semantics [Woo14] in terms of
asynchronous interactions
Injecting tuProlog2 into some Java-friendly BCT
eg Hyperledger Fabric [ABB+18] or Corda3
1
https://tendermint.com
2
http://apice.unibo.it/xwiki/bin/view/Tuprolog/WebHome
3
https://www.corda.net/
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 24 / 30

25. From the Blockchain to Logic Programming and back
Research perspectives
Giovanni Ciatto1 Roberta Calegari1 Stefano Mariani2
Enrico Denti1 Andrea Omicini1
1
{roberta.calegari, giovanni.ciatto, enrico.denti,
andrea.omicini}@unibo.it
2
stefano.mariani@unimore.it
1Dipartimento di Informatica, Scienza e Ingegneria—Universit`a di Bologna
2Dipartimento di Scienze e Metodi dell’Ingegneria, Universit`a di Modena e Reggio Emilia
Talk @ 19th Workshop From Objects to Agents
Palermo, June 29, 2018
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 25 / 30

26. Appendix
Appendix – The problem with re-entrancy I
contract MyWallet {
function protectSavings ( MoneyHandler handler , uint money) public {
handler.deposit.value(money)(1 /* year */);
}
}
contract MoneyHandler {
struct ClientInfo { uint duration; uint since; uint amount }
mapping(address => ClientInfo) clients;
function deposit(uint duration) public {
ClientInfo storage c = clients[msg.sender ];
c.duration = duration;
c.since = now;
c.amount = msg.value;
}
function withdraw () public {
ClientInfo storage c = clients[msg.sender ];
uint amount = c.amount;
amount += (now - c.since) % (c.duration * 1 years) * c.amount /100;
msg.sender.send(amount);
}
}
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 26 / 30

27. Appendix
Appendix – The problem with re-entrancy II
contract MyWallet {
function protectSavings ( MoneyHandler handler , uint money) public {
handler.deposit.value(money)(1 /* year */);
}
}
contract Mallory {
uint nOfTheft = 3;
/* No deposit function */
function () { // <-- fallback function
if (nOfTheft > 0) {
MyWallet fool = (MyWallet) msg.sender;
fool. protectSavings (this , 1); // <-- re -entrancy!
}
}
}
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 27 / 30

28. References
References I
[ABB+18] Elli Androulaki, Artem Barger, Vita Bortnikov, Christian Cachin, Konstantinos
Christidis, Angelo De Caro, David Enyeart, Christopher Ferris, Gennady Laventman,
Yacov Manevich, Srinivasan Muralidharan, Chet Murthy, Binh Nguyen, Manish
Sethi, Gari Singh, Keith Smith, Alessandro Sorniotti, Chrysoula Stathakopoulou,
Marko Vukoli´c, Sharon Weed Cocco, and Jason Yellick.
Hyperledger Fabric: A Distributed Operating System for Permissioned Blockchains.
Proceedings of the Thirteenth EuroSys Conference on - EuroSys ’18, pages 1–15,
jan 2018.
[Bac02] Adam Back.
Hashcash - A Denial of Service Counter-Measure.
Http://Www.Hashcash.Org/Papers/Hashcash.Pdf, (August):1–10, 2002.
[CBPS10] Bernadette Charron-Bost, Fernando Pedone, and Andr´e Schiper, editors.
Replication: Theory and Practice.
Springer-Verlag, Berlin, Heidelberg, 2010.
[CDMO18] Roberta Calegari, Enrico Denti, Stefano Mariani, and Andrea Omicini.
Logic programming as a service.
arXiv preprint arXiv:1806.02577, 2018.
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 28 / 30

29. References
References II
[LCO+16] Loi Luu, Duc-Hiep Chu, Hrishi Olickel, Prateek Saxena, and Aquinas Hobor.
Making Smart Contracts Smarter.
In Proceedings of the 2016 ACM SIGSAC Conference on Computer and
Communications Security - CCS’16, pages 254–269, New York, New York, USA,
2016. ACM Press.
[LL17] Iuon-chang Lin and Tzu-chun Liao.
A Survey of Blockchain Security Issues and Challenges.
International Journal of Network Security, 1919(55):653–659, 2017.
[Nak08] Satoshi Nakamoto.
Bitcoin: A Peer-to-Peer Electronic Cash System.
Www.Bitcoin.Org, page 9, 2008.
[Rao96] Anand S. Rao.
Agentspeak(l): Bdi agents speak out in a logical computable language.
In Walter Van de Velde and John W. Perram, editors, Agents Breaking Away, pages
42–55, Berlin, Heidelberg, 1996. Springer Berlin Heidelberg.
[Sch90] Fred B. Schneider.
Implementing Fault-tolerant Services Using the State Machine Approach: A
Tutorial.
ACM Comput. Surv., 22(4):299–319, 1990.
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 29 / 30

30. References
References III
[Sza96] Nick Szabo.
Smart Contracts: Building Blocks for Digital Markets.
Alamut.Com, (c):16, 1996.
[Woo14] Gavin Wood.
Ethereum: a secure decentralised generalised transaction ledger.
Ethereum Project Yellow Paper, pages 1–32, 2014.
Ciatto (Univ. of Bologna) BC & LP: Research Perspectives WOA 2018, June 29 30 / 30