Foundry Book

FAQ

This is a collection of common questions and answers. If you do not find your question listed here, hop in the Telegram support channel and let us help you!

Help! I can't see my logs

Forge does not display logs by default. If you want to see logs from Hardhat's console.log or from DSTest-style log_* events, you need to run forge test with verbosity 2 (-vv).

If you want to see other events your contracts emit, you need to run with traces enabled. To do that, set the verbosity to 3 (-vvv) to see traces for failing tests, or 4 (-vvvv) to see traces for all tests.

My tests are failing and I don't know why

To gain better insight into why your tests are failing, try using traces. To enable traces, you need to increase the verbosity on forge test to at least 3 (-vvv) but you can go as high as 5 (-vvvvv) for even more traces.

You can learn more about traces in our Understanding Traces chapter.

How do I use console.log?

To use Hardhat's console.log you must add it to your project by copying the file over from here.

Alternatively, you can use Forge Std which comes bundled with console.log. To use console.log from Forge Std, you have to import it:

import "forge-std/console.sol";

How do I run specific tests?

If you want to run only a few tests, you can use --match-test to filter test functions, --match-contract to filter test contracts, and --match-path to filter test files on forge test.

How do I use a specific Solidity compiler?

Forge will try to auto-detect what Solidity compiler works for your project.

To use a specific Solidity compiler, you can set solc in your config file, or pass --use solc:<version> to a Forge command that supports it (e.g. forge build or forge test). Paths to a solc binary are also accepted. To use a specific local solc binary, you can set solc = "<path to solc>" in your config file, or pass --use "<path to solc>". The solc version/path can also be set via the env variable FOUNDRY_SOLC=<version/path>, but the cli arg --use has priority.

For example, if you have a project that supports all 0.7.x Solidity versions, but you want to compile with solc 0.7.0, you could use forge build --use solc:0.7.0.

How do I fork from a live network?

To fork from a live network, pass --fork-url <URL> to forge test. You can also fork from a specific block using --fork-block-number <BLOCK>, which adds determinism to your test, and allows Forge to cache the chain data for that block.

For example, to fork from Ethereum mainnet at block 10,000,000 you could use: forge test --fork-url $MAINNET_RPC_URL --fork-block-number 10000000.

How do I add my own assertions?

You can add your own assertions by creating your own base test contract and having that inherit from the test framework of your choice.

For example, if you use DSTest, you could create a base test contract like this:

contract TestBase is DSTest {
    function myCustomAssertion(uint a, uint b) {
      if (a != b) {
          emit log_string("a and b did not match");
          fail();
      }
    }
}

You would then inherit from TestBase in your test contracts.

contract MyContractTest is TestBase {
    function testSomething() {
        // ...
    }
}

Similarly, if you use Forge Std, you can create a base test contract that inherits from Test.

For a good example of a base test contract that has helper methods and custom assertions, see Solmate's DSTestPlus.

How do I use Forge offline?

Forge will sometimes check for newer Solidity versions that fit your project. To use Forge offline, use the --offline flag.