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Nim Ethers

A port of the ethers.js library to Nim. Allows you to connect to an Ethereum node.

This is very much a work in progress; expect to see many things that are incomplete or wrong. Use at your own risk.

Installation

Use the Nimble package manager to add ethers to an existing project. Add the following to its .nimble file:

requires "ethers >= 0.9.0 & < 0.10.0"

Usage

To connect to an Ethereum node, you require a Provider. Currently, only a JSON-RPC provider is supported:

import ethers
import chronos

let provider = JsonRpcProvider.new("ws://localhost:8545")
let accounts = await provider.listAccounts()

To interact with a smart contract, you need to define the contract functions in Nim. For example, to interact with an ERC20 token, you could define the following:

type Erc20 = ref object of Contract

proc totalSupply(token: Erc20): UInt256 {.contract, view.}
proc balanceOf(token: Erc20, account: Address): UInt256 {.contract, view.}
proc transfer(token: Erc20, recipient: Address, amount: UInt256) {.contract.}
proc allowance(token: Erc20, owner, spender: Address): UInt256 {.contract, view.}
proc approve(token: Erc20, spender: Address, amount: UInt256) {.contract.}
proc transferFrom(token: Erc20, sender, recipient: Address, amount: UInt256) {.contract.}

Notice how some functions are annotated with a {.view.} pragma. This indicates that the function does not modify the blockchain. See also the Solidity documentation on state mutability

Now that you've defined the contract interface, you can create an instance of it using its deployed address:

let address = Address.init("0x.....")
let token = Erc20.new(address, provider)

The functions that you defined earlier can now be called asynchronously:

let supply = await token.totalSupply()
let balance = await token.balanceOf(accounts[0])

These invocations do not yet change the state of the blockchain, even when we invoke those functions that lack a {.view.} pragma. To allow these changes to happen, we require an instance of a Signer first.

For example, to use the 4th account on the Ethereum node to sign transactions, you'd instantiate the signer as follows:

let signer = provider.getSigner(accounts[3])

And then connect the contract and signer:

let writableToken = token.connect(signer)

This allows you to make changes to the state of the blockchain:

await writableToken.transfer(accounts[7], 42.u256)

Which transfers 42 tokens from account 3 to account 7

And lastly, don't forget to close the provider when you're done:

await provider.close()

Events

You can subscribe to events that are emitted by a smart contract. For instance, to get notified about token transfers you define the Transfer event:

type Transfer = object of Event
  sender {.indexed.}: Address
  receiver {.indexed.}: Address
  value: UInt256

Notice that Transfer inherits from Event, and that some event parameters are marked with {.indexed.} to match the definition in Solidity.

Note that valid types of indexed parameters are:

uint8 | uint16 | uint32 | uint64 | UInt256 | UInt128 |
int8 | int16 | int32 | int64 | Int256 | Int128 |
bool | Address | array[ 1..32, byte]

Distinct types of valid types are also supported for indexed fields, eg:

type
  DistinctAlias = distinct array[32, byte]
  MyEvent = object of Event
    a {.indexed.}: DistinctAlias
    b: DistinctAlias # also allowed for non-indexed fields

You can now subscribe to Transfer events by calling subscribe on the contract instance.

proc handleTransfer(transfer: Transfer) =
  echo "received transfer: ", transfer

let subscription = await token.subscribe(Transfer, handleTransfer)

When a Transfer event is emitted, the handleTransfer proc that you just defined will be called.

When you're no longer interested in these events, you can unsubscribe:

await subscription.unsubscribe()

Custom errors

Solidity's custom errors are supported. To use them, you declare their type and indicate in which contract functions they can occur. For instance, this is how you would define the "InsufficientBalance" error to match the definition in this Solidity example:

type
  InsufficientBalance = object of SolidityError
    arguments: tuple[available: UInt256, required: UInt256]

Notice that InsufficientBalance inherits from SoldityError, and that it has an arguments tuple whose fields match the definition in Solidity.

You can use the {.errors.} pragma to declare that this error may occur in a contract function:

proc transfer*(token: Erc20Token, recipient: Address, amount: UInt256)
  {.contract, errors:[InsufficientBalance].}

This allows you to write error handling code for the transfer function like this:

try:
  await token.transfer(recipient, 100.u256)
except InsufficientBalance as error:
  echo "insufficient balance"
  echo "available balance: ", error.arguments.available
  echo "required balance: ", error.arguments.required

Utilities

This library ships with some optional modules that provides convenience utilities for you such as:

ethers/erc20 module provides you with ERC20 token implementation and its events

Contribution

If you want to run the tests, then before running nimble test, you have to have installed NodeJS and started a testing node:

$ cd testnode
$ npm ci
$ npm start

Thanks

This library is inspired by the great work done by the ethers.js (no affiliation) and nim-web3 developers.

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