Counter¶

Source: fourier/examples/counter.fou.

The minimum interesting Fourier contract — one slot of storage, an init, a read, a write.

Full source¶

// Minimal counter — demonstrates init + a single mutating method.

contract Counter {
    storage value: uint @ 0;

    fn init() {
        value = 42;
    }

    pub fn get() -> uint {
        return value;
    }

    pub fn inc() {
        value = value + 1;
    }
}

Walkthrough¶

storage value: uint @ 0;¶

One storage slot, pinned to slot 0. Holds a single 256-bit unsigned integer. After init, this slot reads 42; after each inc() call, the value at slot 0 increases by 1.

The pin to slot 0 is arbitrary — any slot from 0 to 2**256 - 2 would work. Slot 2**256 - 1 is reserved for the init guard.

fn init() { value = 42; }¶

The optional initializer. Runs exactly once, atomically inside the deploy transaction. After this runs, the contract reads value == 42 regardless of who calls get() first.

init is private (no pub). It is not assigned a selector. The compiler enforces no params, no return, no early return. See Contracts / init.

pub fn get() -> uint { return value; }¶

Selector 0x01 (first declared pub fn). Reads slot 0 and returns it.

Bytecode (approximate):

_fn_get:
JUMPDEST
POP                ; discard selector
PUSH 0
SLOAD              ; read slot 0
PUSH 0x40
MSTORE
PUSH 32
PUSH 0x40
RETURN

pub fn inc() { value = value + 1; }¶

Selector 0x02. Reads slot 0, adds 1, writes back. No return value.

_fn_inc:
JUMPDEST
POP
PUSH 0
SLOAD              ; current value
PUSH 1
ADD                ; +1
PUSH 0
SSTORE             ; write back
STOP               ; implicit terminator for void fn

Note: ADD wraps modulo 2**256. Calling inc() after the counter hits 2**256 - 1 rolls over to 0 with no error. Use safe_add if you'd prefer to revert.

Calldata for each selector¶

Deployment + invocation transcript¶

Conceptual flow:

1. Compile counter.fou to bytecode.
2. Submit a deploy tx with data.type = "deploy", data.code = <hex>. The deploy address is derived from (deployer, deployer_nonce).
3. The VM stores the code; immediately invokes the new code with empty calldata; the init prologue checks the flag (unset), sets it, runs value = 42, falls through to the empty-calldata short-circuit that halts with STOP.
4. Submit a get() call tx: data.calldata = "01". Receipt return_data = 0000...002a (42 in hex).
5. Submit inc() calls. Each increments the slot.

What to try next¶

• Add pub fn dec() { value = value - 1; }. What selector does it get? (Hint: it's the third pub fn.) Does dec() from value == 0 revert or wrap?
• Add a precondition: pub fn inc_if_owner() { require(caller() == owner); value = value + 1; }. You'll need to add a storage owner: address @ 1; and set it in init.
• Swap value + 1 for safe_add(value, 1) and observe overflow behavior change from wrap to revert.