Banned Solidity Features

The following Solidity language features are banned from Compose facets and modules.

Compose restricts certain Solidity features to keep facet and library code simpler, more consistent, and easier to reason about.

Because of Compose's architecture, many of these features are either unnecessary or less helpful.

Exceptions

These restrictions do not apply to tests.

These restrictions do not apply to developers using Compose in their own projects.

Banned Solidity Features

Facets may not inherit from any contract or interface.

🚫 Not allowed

contract MyContract is OtherContract {
    ...
}

// or

contract MyContract is IMyInterface {
    ...
}

If you want inheritance, your facet is probably too large. Split it into smaller facets. Compose replaces inheritance with on-chain facet composition.

Facets may not define constructors.

🚫 Not allowed

constructor() {
    owner = msg.sender;
    ...
}

Facets may not define or use modifiers.

🚫 Not allowed

modifier onlyOwner() {
    require(msg.sender == owner, "Caller is not the owner");
    _;
}

Storage variables may not include visibility specifiers.

🚫 Not allowed

uint256 public counter;

Visibility labels are unnecessary because Compose uses ERC-8042 Diamond Storage across all facets.
This rule does not apply to constant or immutable variables, which may be declared internal.

All facet functions must be declared as internal or external.

🚫 Not allowed

function approve(address _spender, uint256 _value) public {
    ...
}

🚫 Not allowed

library MyLib {
    ...
}

🚫 Not allowed

using SomethingMod for uint256;

The ternary operator (condition ? valueIfTrue : valueIfFalse) may not be used.

🚫 Not allowed

string memory status = balance >= cost ? "Paid" : "Insufficient funds";

Always use braces {} around if and else bodies.

🚫 Not allowed

if (x < 10) count++;

✅ Required

if (x < 10) {
    count++;
}

No contract or module may use selfdestruct

🚫 Not allowed

selfdestruct(owner);