156 lines
4.3 KiB
Markdown
156 lines
4.3 KiB
Markdown
Nim Ethers
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==========
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A port of the [ethers.js][0] library to Nim. Allows you to connect to an
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Ethereum node.
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This is very much a work in progress; expect to see many things that are
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incomplete or wrong. Use at your own risk.
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Installation
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------------
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Use the [Nimble][2] package manager to add `ethers` to an existing
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project. Add the following to its .nimble file:
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```nim
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requires "ethers >= 0.2.2 & < 0.3.0"
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```
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Usage
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-----
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To connect to an Ethereum node, you require a `Provider`. Currently, only a
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JSON-RPC provider is supported:
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```nim
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import ethers
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import chronos
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let provider = JsonRpcProvider.new("ws://localhost:8545")
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let accounts = await provider.listAccounts()
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```
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To interact with a smart contract, you need to define the contract functions in
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Nim. For example, to interact with an ERC20 token, you could define the
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following:
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```nim
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type Erc20 = ref object of Contract
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proc totalSupply(token: Erc20): UInt256 {.contract, view.}
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proc balanceOf(token: Erc20, account: Address): UInt256 {.contract, view.}
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proc transfer(token: Erc20, recipient: Address, amount: UInt256) {.contract.}
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proc allowance(token: Erc20, owner, spender: Address): UInt256 {.contract, view.}
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proc approve(token: Erc20, spender: Address, amount: UInt256) {.contract.}
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proc transferFrom(token: Erc20, sender, recipient: Address, amount: UInt256) {.contract.}
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```
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Notice how some functions are annotated with a `{.view.}` pragma. This indicates
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that the function does not modify the blockchain. See also the Solidity
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documentation on [state mutability][3]
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Now that you've defined the contract interface, you can create an instance of
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it using its deployed address:
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```nim
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let address = Address.init("0x.....")
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let token = Erc20.new(address, provider)
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```
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The functions that you defined earlier can now be called asynchronously:
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```nim
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let supply = await token.totalSupply()
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let balance = await token.balanceOf(accounts[0])
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```
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These invocations do not yet change the state of the blockchain, even when we
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invoke those functions that lack a `{.view.}` pragma. To allow these changes to
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happen, we require an instance of a `Signer` first.
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For example, to use the 4th account on the Ethereum node to sign transactions,
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you'd instantiate the signer as follows:
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```nim
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let signer = provider.getSigner(accounts[3])
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```
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And then connect the contract and signer:
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```nim
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let writableToken = token.connect(signer)
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```
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This allows you to make changes to the state of the blockchain:
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```nim
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await writableToken.transfer(accounts[7], 42.u256)
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```
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Which transfers 42 tokens from account 3 to account 7
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Events
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------
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You can subscribe to events that are emitted by a smart contract. For instance,
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to get notified about token transfers you define the `Transfer` event:
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```nim
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type Transfer = object of Event
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sender {.indexed.}: Address
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receiver {.indexed.}: Address
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value: UInt256
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```
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Notice that `Transfer` inherits from `Event`, and that some event parameters are
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marked with `{.indexed.}` to match the definition in Solidity.
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Note that valid types of indexed parameters are:
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```nim
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uint8 | uint16 | uint32 | uint64 | UInt256 | UInt128 |
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int8 | int16 | int32 | int64 | Int256 | Int128 |
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bool | Address | array[ 1..32, byte]
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```
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Distinct types of valid types are also supported for indexed fields, eg:
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```nim
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type
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DistinctAlias = distinct array[32, byte]
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MyEvent = object of Event
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a {.indexed.}: DistinctAlias
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b: DistinctAlias # also allowed for non-indexed fields
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```
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You can now subscribe to Transfer events by calling `subscribe` on the contract
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instance.
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```nim
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proc handleTransfer(transfer: Transfer) =
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echo "received transfer: ", transfer
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let subscription = await token.subscribe(Transfer, handleTransfer)
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```
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When a Transfer event is emitted, the `handleTransfer` proc that you just
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defined will be called.
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When you're no longer interested in these events, you can unsubscribe:
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```nim
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await subscription.unsubscribe()
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```
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Subscriptions are currently only supported when using a JSON RPC provider that
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is created with a websockets URL such as `ws://localhost:8545`.
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Thanks
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------
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This library is inspired by the great work done by the [ethers.js][0] (no
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affiliation) and [nim-web3][1] developers.
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[0]: https://docs.ethers.io/
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[1]: https://github.com/status-im/nim-web3
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[2]: https://github.com/nim-lang/nimble
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[3]: https://docs.soliditylang.org/en/v0.8.11/contracts.html#state-mutability
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