Fetch onchain data with function calls

Monitoring Address Token Balance

Another, similar example, is monitoring the token balance of an address. In this example, we'll use the Uniswap V2 USDC/USDT Pool. In Uniswap V2, the exchange rate of the two tokens is determined by their liquidity in the pool. Because both USDC and USDT are supposed to be $1, their liquidity should be almost the same. Monitoring for this will allow us to catch potential depeg events in either stablecoin.

use Call, PercentageDiff from hexagate;

source pool: address = 0x3041CbD36888bECc7bbCBc0045E3B1f144466f5f;
source usdc: address = 0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48;
source usdt: address = 0xdac17f958d2ee523a2206206994597c13d831ec7;

source usdcBalance: integer = Call {
    contract: usdc,
    signature: "function balanceOf(address) returns (uint256)",
    params: tuple(pool)
};

source usdtBalance: integer = Call {
    contract: usdt,
    signature: "function balanceOf(address) returns (uint256)",
    params: tuple(pool)
};

source percentageDiff: integer = PercentageDiff {
    before: usdcBalance,
    after: usdtBalance
};

source threshold: integer = 2; // Up to 2% difference is allowed

rule {
    description: "Uniswap USDC/USDT pool balance differ by more than ${percentageDiff}%",
    condition: percentageDiff <= threshold
};

There are some similarities to the previous example, but also some differences. Let's break them down:

---

use Call, PercentageDiff from hexagate;

source pool: address = 0x3041CbD36888bECc7bbCBc0045E3B1f144466f5f;
source usdc: address = 0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48;
source usdt: address = 0xdac17f958d2ee523a2206206994597c13d831ec7;

This is pretty similar to the first example, but we are importing different sources, and also declaring more addresses in advance. We will use these addresses next.

---

source usdcBalance: integer = Call {
    contract: usdc,
    signature: "function balanceOf(address) returns (uint256)",
    params: tuple(pool)
};

source usdtBalance: integer = Call {
    contract: usdt,
    signature: "function balanceOf(address) returns (uint256)",
    params: tuple(pool)
};

USDC and USDT are both ERC20 tokens (which means they are contracts). These contract expose a function called balanceOf, which we can use to fetch the token balance of a specific address.

We can use the Call datasource in Gate to call functions on specific contracts. It receives 3 parameters:

• The contract address to call

  • We use the USDC and USDT contract addresses, since we need the balance in both tokens of the pool, to compare them

• The EVM function signature

  • We use the standard ERC20 balanceOf function signature

• The EVM function parameters

  • We send the pool address in both calls, since we want to fetch the USDC and USDT balances of that address

  • This parameter is a tuple, since some EVM functions can receive more than 1 parameter

In general, using the Call datasource is a common way to fetch onchain data in Gate.

---

source percentageDiff: integer = PercentageDiff {
    before: usdcBalance,
    after: usdtBalance
};

source threshold: integer = 2; // Up to 2% difference is allowed

rule {
    description: "Uniswap USDC/USDT pool balance differ by more than ${percentageDiff}%",
    condition: percentageDiff <= threshold
};

We use another datasource, PercentageDiff, to calculate the difference between the balances, as a percentage. We then create a rule which ensures every block that the difference between both balances is never above 2%.