[Lesson 32] [Fund subscription] Question about importing abstract contract containing constant variables

[tcrin]

Hello everyone,

I'm encountering an issue when trying to access the LOCAL_CHAIN_ID constant from an abstract contract in HelperConfig.s.sol. Even though LOCAL_CHAIN_ID is declared as a public constant in CodeConstants, I am unable to access this variable when importing the contract into Interactions.s.sol.

When I build the project, I get the following error:

~/foundry-f23/foundry-smart-contract-lottery$ forge build
[⠊] Compiling...
[⠰] Compiling 45 files with Solc 0.8.19
[⠔] Solc 0.8.19 finished in 253.68ms
Error: 
Compiler run failed:
Error (7576): Undeclared identifier.
  --> script/Interactions.s.sol:54:30:
   |
54 |         if (block.chainid == LOCAL_CHAIN_ID) {
   |                              ^^^^^^^^^^^^^^

What I have tried:

• Checked the visibility of the LOCAL_CHAIN_ID variable (it is public constant).
• Imported the file correctly.
• Hardcoded the chain ID value, which works, but I would prefer a more flexible solution.

❓ ❓Can anyone help me understand why the LOCAL_CHAIN_ID variable is not accessible in Interactions.s.sol?
I would greatly appreciate any insights or suggestions!

---

Relevant Code:

HelperConfig.s.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

import {Script} from "forge-std/Script.sol";
import {VRFCoordinatorV2_5Mock} from "@chainlink/contracts/src/v0.8/vrf/mocks/VRFCoordinatorV2_5Mock.sol";
import {LinkToken} from "../test/mocks/LinkToken.sol";

abstract contract CodeConstants {
    uint256 public constant SEPOLIA_ETH_CHAIN_ID = 1115511;
    uint256 public constant LOCAL_CHAIN_ID = 31337;
}

contract HelperConfig is CodeConstants, Script {
    // Rest of my code
}

Interactions.s.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

import {Script} from "forge-std/Script.sol";
import {HelperConfig, CodeConstants} from "./HelperConfig.s.sol";
import {console} from "forge-std/console.sol";
import {VRFCoordinatorV2_5Mock} from "@chainlink/contracts/src/v0.8/vrf/mocks/VRFCoordinatorV2_5Mock.sol";
import {LinkToken} from "../test/mocks/LinkToken.sol";

contract FundSubscription is Script {
    uint256 public constant FUND_AMOUNT = 3 ether; // 3 LINK

    function fundSubscription(address vrfCoordinator, uint256 subscriptionId, address linkTokenAddress) public {
        console.log("Funding subscription: ", subscriptionId);
        console.log("On chainId: ", block.chainid);

        if (block.chainid == LOCAL_CHAIN_ID) { // This line causes build fail
            // My code here
        }
    }

    function run() external {
        fundSubscriptionUsingConfig();
    }
}

Thanks in advance!

[NikolayPIvanov]

From what I know so far, this has to do with how constants are stored in Solidity - at compile-time they become part of the bytecode - reference to Solidity docs

This means that in your example, LOCAL_CHAIN_ID which is part of CodeConstants contract is nowhere in the bytecode of the FundSubscription contract. I think that if you inherit CodeConstants your code will compile

contract FundSubscription is Script, CodeConstants {
  ...
}

If you wish to have something that you do not need to inherit, check out libraries.

[tcrin]

Thank you very much! I’ve inherited CodeConstants and the build was successful. Your answer was extremely helpful and has given me a better understanding of how constants are stored in Solidity.

[EngrPips]

Awesome.

[heheboii11]

Hey @tcrin, you can see that in the error you have provided it states that LOCAL_CHAIN_ID as undeclared. Please pull theLOCAL_CHAIN_ID from the helperconfig as below

uint256 local_chain_id = HelperConfig.LOCAL_CHAIN_ID();```
and also you need to deploy the helper config in ```interactions.s.sol```, so the code might look like this 
```solidity 
contract FundSubscription is Script {
    uint256 public constant FUND_AMOUNT = 3 ether; // 3 LINK
    HelperConfig helper;
    function fundSubscription(address vrfCoordinator, uint256 subscriptionId, address linkTokenAddress) public {
        console.log("Funding subscription: ", subscriptionId);
        console.log("On chainId: ", block.chainid);
        helper  = new HelperConfig();
        uint256 local_chain_id = HelperConfig.LOCAL_CHAIN_ID();   
        if (block.chainid == local_chain_id) { // This line causes build fail // If you want you can do like this or just pass saving the variable in here in the interaction.s.sol, my advice is just store the variable in here
            // My code here
        }
    }

    function run() external {
        fundSubscriptionUsingConfig();
    }
}

[heheboii11]

Hey, thank you for bringing a new thing that I can learn, has no idea that we can inherit constant!!.

[tcrin]

Hey @heheboii11,
inherit constant - it seems you are misunderstanding.
The reason I can inherit CodeConstants is because it is an abstract contract

[EngrPips]

I think you can inherit non-abstract contracts too, Or am I not remembering correctly?

[tcrin]

That's correct, you can inherit contracts or abstract contracts. However, in the case @heheboii11 mentioned, inherit constants is not possible.

[EngrPips]

That's correct, you can inherit contracts or abstract contracts. However, in the case @heheboii11 mentioned, inherit constants is not possible.