Fe for Solidity Developers

This guide helps Solidity developers understand Fe by highlighting key differences and showing equivalent patterns.

Key Conceptual Differences

Explicit Effects vs Implicit State

Solidity: Functions can access any state variable implicitly.

contract Token {
    mapping(address => uint256) balances;

    function transfer(address to, uint256 amount) public {
        balances[msg.sender] -= amount;  // Implicit access
        balances[to] += amount;
    }
}

Fe: Functions must declare what state they access via effects.

fn transfer(from: Address, to: Address, amount: u256)
    uses mut store: TokenStore  // Explicit declaration
{
    store.balances[from] -= amount
    store.balances[to] += amount
}

Contracts vs Messages

Solidity: Functions are defined directly in contracts.

contract Token {
    function transfer(address to, uint256 amount) public returns (bool) {
        // ...
    }
}

Fe: External interface is defined separately via messages.

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

contract Token {
    recv TokenMsg {
        Transfer { to, amount } -> bool {
            // ...
        }
    }
}

Storage Access

Solidity: Direct access to state variables.

balances[msg.sender] = 100;

Fe: Access through effect-bound storage.

// In recv block
with (TokenStorage = store) {
    TokenStorage.balances[caller()] = 100
}

Syntax Comparison

Variable Declaration

Solidity	Fe
uint256 x = 10;	let x: u256 = 10
uint256 x;	let x: u256 = 0
address owner;	let owner: Address
bool active = true;	let active: bool = true

Types

Solidity	Fe
uint256	u256
int256	i256
uint8	u8
address	Address
bool	bool
string	String<N>
bytes32	u256
mapping(K => V)	Map<K, V>

Functions

Solidity:

function add(uint256 a, uint256 b) public pure returns (uint256) {
    return a + b;
}

Fe:

fn add(a: u256, b: u256) -> u256 {
    a + b
}

Visibility

Solidity	Fe
public	pub
private	(default)
internal	(default within module)
external	via msg and recv

Control Flow

Solidity:

if (x > 0) {
    return true;
} else {
    return false;
}

for (uint i = 0; i < 10; i++) {
    // ...
}

Fe:

if x > 0 {
    return true
} else {
    return false
}

let mut i: u256 = 0
while i < 10 {
    // ...
    i = i + 1
}

Events

Solidity:

event Transfer(address indexed from, address indexed to, uint256 value);

function transfer(...) {
    emit Transfer(from, to, amount);
}

Fe:

struct Transfer {
    #[indexed]
    from: Address,
    #[indexed]
    to: Address,
    value: u256,
}

    log.emit(Transfer { from, to, value: amount })

Error Handling

Solidity:

require(balance >= amount, "Insufficient balance");
revert("Error message");

Fe:

assert(balance >= amount, "Insufficient balance")
revert

Constructors

Solidity:

constructor(uint256 initialSupply) {
    totalSupply = initialSupply;
}

Fe:

contract Token {
    init(initial_supply: u256) uses mut store {
        store.total_supply = initial_supply
    }
}

Pattern Equivalents

ERC20 Transfer

Solidity:

function transfer(address to, uint256 amount) public returns (bool) {
    require(balances[msg.sender] >= amount, "Insufficient balance");
    balances[msg.sender] -= amount;
    balances[to] += amount;
    emit Transfer(msg.sender, to, amount);
    return true;
}

Fe:

Transfer { to, amount } -> bool uses (ctx, mut store, mut log) {
    let from = ctx.caller()
    assert(store.balances[from] >= amount, "Insufficient balance")
    store.balances[from] -= amount
    store.balances[to] += amount
    log.emit(TransferEvent { from, to, value: amount })
    true
}

Access Control

Solidity:

modifier onlyOwner() {
    require(msg.sender == owner, "Not owner");
    _;
}

function mint(address to, uint256 amount) public onlyOwner {
    // ...
}

Fe:

fn require_owner(owner: Address) uses ctx: Ctx {
    assert(ctx.caller() == owner, "Not owner")
}

Mint { to, amount } -> bool uses (ctx, mut store, auth) {
    auth.require(role: MINTER)  // Or: require_owner(store.owner)
    // ...
}

Mappings

Solidity:

mapping(address => mapping(address => uint256)) allowances;

allowances[owner][spender] = amount;
uint256 allowed = allowances[owner][spender];

Fe:

struct Storage {
    allowances: Map<(Address, Address), u256>,
}

store.allowances[(owner, spender)] = amount
let allowed = store.allowances[(owner, spender)]

What’s Different in Fe

No Inheritance

Fe doesn’t have contract inheritance. Use composition instead:

// Instead of: contract Token is Ownable, Pausable
contract Token {
    auth: AccessControl,    // Composition
    pause_state: Pausable,  // Composition
}

No Modifiers

Fe doesn’t have function modifiers. Use helper functions:

fn require_not_paused(paused: bool) {
    assert(!paused, "Contract is paused")
}

// In handler:
require_not_paused(store.paused)

Explicit Selectors

Fe requires explicit ABI selectors:

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

No Overloading

Fe doesn’t support function overloading. Use different names:

// Instead of transfer(address) and transfer(address, uint256)
fn transfer_to(to: Address) { ... }
fn transfer_amount(to: Address, amount: u256) { ... }

Type Inference

Fe infers types where possible:

let x = 10        // u256 inferred
let y: u8 = 10    // Explicit when needed

Migration Tips

1. Start with messages - Define your external interface first with explicit selectors

2. Group storage - Create storage structs instead of individual state variables

3. Add effect annotations - Every function touching state needs uses clauses

4. Replace modifiers - Convert to helper functions with assertions

5. Use composition - Replace inheritance with struct fields

6. Test selectors - Verify your selectors match the Solidity ABI

Quick Reference

Solidity	Fe
msg.sender	ctx.caller()
block.timestamp	ctx.block_timestamp()
block.number	ctx.block_number()
require(...)	assert(...)
emit Event(...)	log.emit(Event { ... })
mapping(K => V)	Map<K, V>
constructor	init
function	fn
public	pub
returns	->