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This guide does not cover smart contract deployment. To learn more about deploying a smart contract with Quais and Hardhat, visit the Hardhat Deployment Tutorial.

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Application Binary Interfaces

All operations that occur on the Quai ledger must be encoded as binary data. To call and interpret data from smart contracts on-chain, we need a simple way to convert between common data types (e.g. strings and numbers) and their binary representaion that the smart contract can understand. Quais uses ABI fragments to encode and interpret smart contract data, allowing developers to easily pass and read data from smart contracts. There are several common formats that Quais accepts to describe a smart contract’s ABI. The Solidity compiler typically provides a JSON ABI for every compiled contract that can be passed to Quais. 
// JSON ABI
const abi = [
  {
      "constant": true,
      "inputs": [],
      "name": "symbol",
      "outputs": [
          {
              "name": "",
              "type": "string"
          }
      ],
      "payable": false,
      "stateMutability": "view",
      "type": "function"
  },
  ...
]

// use the JSON ABI to create a contract
const contract = new quais.Contract(address, abi, provider)
You may also provide the more readable Solidity signature for any relevant methods, errors, or calls to the contract. 
// simplified Solidity signature ABI
const abi = [
	'function decimals() view returns (uint8)',
	'function symbol() view returns (string)',
	'function balanceOf(address addr) view returns (uint)',
]

// use the simplified ABI to create a contract
const contract = new quais.Contract(address, abi, provider)

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Read-only Interactions

Contracts often have read-only methods (view and pure) that can be called to query the state of the smart contract without initiating a transaction. You can call these methods using only a Provider. 
// contract ABI (only includes the methods we care about)
const abi = [
	'function decimals() view returns (uint8)',
	'function symbol() view returns (string)',
	'function balanceOf(address addr) view returns (uint)',
]

// create contract with only a Provider
const contract = new quais.Contract(address, abi, provider)

// get the token symbol
symbol = await contract.symbol() // "ABC"

// get the token decimals
decimals = await contract.decimals() // 18n

// get the balance of an address
balance = await contract.balanceOf('0x1234567890123456789012345678901234567890') // 10000000000000000000n

// format the balance
formatUnits(balance, decimals) // "10.0"

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State-changing Interactions

To call a state-changing method, you’ll need to pass a Signer or Wallet to the contract. 
// contract ABI (only includes the methods we care about)
const abi = ['function transferTo(address to, uint amount)']

// create wallet and pass it to the contract
const wallet = new quais.Wallet(privateKey, provider)
const contract = new quais.Contract(address, abi, wallet)

// transfer some tokens
const tx = await contract.transferTo('0x1234567890123456789012345678901234567890', parseUnits('1.0', 18))

await tx.wait()

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Events

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Listening for Events

You can listen for emitted events from a smart contract using event filters. 
// contract ABI (only includes the events we care about)
const abi = ['event Transfer(address indexed from, address indexed to, uint amount)']

// create contract with only a Provider
const contract = new quais.Contract(address, abi, provider)

// use an event listener to listen for any transfer event
contract.on('Transfer', (from, to, amount, event) => {
	const tokenAmount = formatUnits(amount, 18)
	console.log('Transfer from', from, 'to', to, 'amount', tokenAmount)

	// stop listening for events
	event.removeListener()
})

// use a filter to listen for transfer events from a specific address
const filter = contract.filters.Transfer('0x1234567890123456789012345678901234567890')
contract.on(filter, (from, to, amount, event) => {
	const tokenAmount = formatUnits(amount, 18)
	console.log('Transfer from', from, 'to', to, 'amount', tokenAmount)
})

​
Query Historic Events

To query historic events, you can use the queryFilter method. 
// contract ABI
const abi = ['event Transfer(address indexed from, address indexed to, uint amount)']

// create contract with only a Provider
const contract = new quais.Contract(address, abi, provider)

// query the last 50 blocks for any transfer events
const filter = contract.filters.Transfer
const events = await contract.queryFilter(filter, -50) // returns an array of events

// query all transfer events from a specific address
const filter = contract.filters.Transfer('0x1234567890123456789012345678901234567890')
const events = await contract.queryFilter(filter) // returns an array of events
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