Polkadot Smart Contracts

About Me

• Kian Paimani aka. @kianenigma
• Engineering Lead @ Parity Technologies
• Polkadot Core Fellow
• blog.kianenigma.com (<-- slides)

What is a Smart Contract

Blockchain models that we can think of:

• Database 🤮
• State Machine ♾️
• Computer 💻

Note:

First is utter BS
Second is accurate, but a bit to math-y
Third one is accurate, and one that most people relate to, we will use this one

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What is a Smart Contract

• Computer has code and memory.
• Users can interact with the code (trigger a transfer)
• In a blockchain, we call these the stf/runtime, and state respectively.

Aside: The main novelty of a computer is that it will execute code correctly, and therefore you can TRUST the memory. 🪄

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What is a Smart Contract

• First generation blockchains had a fixed code.
• What is the code of the Bitcoin computer?
  • Transfer of BTC

Note:

First blockchains were computers with a (almost) fixed code. Bitcoin's code is ONLY how to do transfer of BTC, and therefore the State of the Bitcoin computer was only user balances. Nothing more.

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What is a Smart Contract

• Smart Contracts are the first step into making blockchain computers programmable.

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What is a Smart Contract

• Ethereum is a programmable blockchain
• What is the code of the Ethereum computer?
  • Transfer of ETH
  • Upload a contract
  • Call into an existing contract, which might alter state

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What is a Smart Contract

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What is a Smart Contract

• Given: blockchains are computers with code and memory
• Smart contracts are essentially a way to extend the code and memory

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What is a Smart Contract

• Execution of these contracts is as secure as the execution of the main blockchain's code. 🪄
• If you can trust Ethereum and its ability to correctly transfer ETH, you should also trust that it can correctly execute whatever is the code of any smart contract.

Ethereum Contracts

A Smart Contract has:

• code
• address (at which it can be invoked)
• state/memory
• balance: money owned by the contract

Note:

Bringing it all together

Case Study

Simple Escrow Contract

• Buyer deposits funds
• Both buyer and seller approve
  • Transfer funds to seller
• Timeout
  • Return funds to buyer

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Case Study

Web3 Roulette

• Deposit Phase
  • Anyone can deposit money into the contract
• Roulette Phase
  • One participants get nothing back, their money split among everyone else

How to Write a Contract

• Ethereum Contracts are written in a language Solidity.
• Part of the Ethereum code is to execute Solidity code.
• This happens in a virtual machine known as EVM

Note:

EVM: Ethereum Virtual Machine

How to Execute a Contract

If Ethereum wants to execute a totally untrusted code, what challenges does it have?

• Deterministic cost (gas)
• Deterministic output (EVM)

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How to Execute a Contract

• Ability to meter ⏰ the cost of the execution of a contract is an important requirement
• This is why when interacting with any contract, you specify gas_limit

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Extending Memory

What about the fact that a contract can add more data to its own memory, which in turn bloats the overall blockchain memory?

• State cost needs to be taken into account
  • Part of gas
  • Deposit
  • State Rent

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Other Properties of Contracts

• Atomic
• Immutable

Smart Contracts in Polkadot

Polkadot

• Computer who's code allows other Blockchains to be executed and validated.
• Polkadot always assumed one of these external blockchains (Parachains) would deliver smart contract capability.

Past

• Polkadot's code is WASM bytecode (not EVM or similar).
• An experiment to compile Rust to WASM as a smart Contract
  • pallet-contracts and Ink!.
• Execute EVM compatible Smart Contracts as well
  • pallet-evm

Note:

Both of these initiated in Parity, but are pursued as community projects now.

Now

• A new, much better virtual machine based on RiskV: PolkaVM
• Fork pallet-contracts to execute any Smart Contract that compiles to PolkaVM.
• Compiler to compile Solidity to PolkaVM bytecode

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Now

Outcome: Polkadot will have fast Solidity-compatible Smart Contracts in H2 2025

Outro

• Smart Contracts are the most basic form of building an application on top of a secure blockchain computer 🪄.
• Battle tested approach that has been used for almost a decade now

Questions