Selectors

Selectors are 4-byte identifiers that tell the EVM which message variant to invoke. They enable ABI compatibility with the broader Ethereum ecosystem.

What Are Selectors?

When a contract is called, the first 4 bytes of the calldata identify the function. This is the selector:

Calldata: 0xa9059cbb0000000000000000...
          ^^^^^^^^ selector (4 bytes)
                  ^^^^^^^^^^^^^^^^... arguments

Fe uses the same selector mechanism as Solidity, ensuring contracts can interoperate.

The Selector Attribute

Specify selectors using the #[selector] attribute:

msg TokenMsg {
    #[selector = 0xa9059cbb]
    Transfer { to: u256, amount: u256 } -> bool,

    #[selector = 0x70a08231]
    BalanceOf { account: u256 } -> u256,
}

Selector Format

Selectors are specified as hexadecimal values:

msg Example {
    #[selector = 0xa9059cbb]  // Hex format (preferred)
    Transfer { to: u256, amount: u256 } -> bool,

    // Decimal format also valid: #[selector = 2835717307]
}

The value must fit in 4 bytes (u32).

Required Selectors

Every message variant must have a selector:

msg Example {
    // Error: missing selector
    NoSelector { value: u256 } -> bool,

    // Correct: has selector
    #[selector = 0x12345678]
    WithSelector { value: u256 } -> bool,
}

Selector Uniqueness

Selectors must be unique across all recv blocks in a contract:

msg MsgA {
    #[selector = 0x12345678]
    Operation { } -> bool,
}

msg MsgB {
    #[selector = 0x12345678]  // Same selector!
    AnotherOp { } -> bool,
}

contract Example {
    recv MsgA {
        Operation { } -> bool { true }
    }

    recv MsgB {
        AnotherOp { } -> bool { true }
        // Error: duplicate selector 0x12345678
    }
}

The compiler detects and reports selector conflicts.

Standard Selectors

For ERC standards, use the established selectors. These are computed as the first 4 bytes of keccak256(signature):

ERC20 Selectors

Function	Signature	Selector
transfer	`transfer(address,uint256)`	`0xa9059cbb`
approve	`approve(address,uint256)`	`0x095ea7b3`
transferFrom	`transferFrom(address,address,uint256)`	`0x23b872dd`
balanceOf	`balanceOf(address)`	`0x70a08231`
allowance	`allowance(address,address)`	`0xdd62ed3e`
totalSupply	`totalSupply()`	`0x18160ddd`

ERC721 Selectors

Function	Signature	Selector
ownerOf	`ownerOf(uint256)`	`0x6352211e`
safeTransferFrom	`safeTransferFrom(address,address,uint256)`	`0x42842e0e`
approve	`approve(address,uint256)`	`0x095ea7b3`
getApproved	`getApproved(uint256)`	`0x081812fc`

Computing Selectors

Selectors are the first 4 bytes of keccak256(function_signature):

keccak256("transfer(address,uint256)")
= 0xa9059cbb2ab09eb219583f4a59a5d0623ade346d962bcd4e46b11da047c9049b
       ^^^^^^^^
       Selector: 0xa9059cbb

Tools like cast sig from Foundry can compute these:

cast sig "transfer(address,uint256)"
# Output: 0xa9059cbb

Fe will provide a sol_sig const function for computing selectors at compile time:

msg TokenMsg {
    #[selector = sol_sig("balanceOf(address)")]
    BalanceOf { account: u256 } -> u256,
}

This will allow you to define selectors using the Solidity signature string directly, with no runtime cost. The computation happens at compile time.

Custom Selectors

For non-standard interfaces, you can use any unique 4-byte value:

msg CustomProtocol {
    #[selector = 0x00000001]
    Initialize { config: u256 },

    #[selector = 0x00000002]
    Process { data: u256 } -> bool,

    #[selector = 0x00000003]
    Query { id: u256 } -> u256,
}

Just ensure they don’t conflict with selectors in other messages the contract handles.

Why Explicit Selectors?

Fe requires explicit selectors rather than auto-generating them because:

ABI compatibility: Matching standard selectors ensures interoperability
Naming freedom: Fe uses snake_case conventions, but ERC standards use camelCase function names. Explicit selectors let you write idiomatic Fe while remaining fully compatible with Ethereum standards
Clarity: The selector is visible in the source code
Stability: Renaming a variant doesn’t accidentally change its selector
Verification: Easy to verify against interface specifications

For example, you can name your variant BalanceOf or balance_of. It doesn’t matter because the selector 0x70a08231 (derived from balanceOf(address)) is what the EVM uses for routing:

msg Erc20 {
    // Fe-style naming, but ABI-compatible with ERC20's balanceOf(address)
    #[selector = 0x70a08231]
    BalanceOf { account: u256 } -> u256,
}

Summary

Concept	Description
Selector	4-byte function identifier
`#[selector = 0x...]`	Attribute to specify selector
Uniqueness	Selectors must be unique per contract
Standard selectors	Use established values for ERC compatibility