move-patterns

name	move-patterns
description	Move design patterns — events, error handling, one-time witness (OTW), capability pattern, and pure functions/composability.

1. Events

Emit events for all state-changing operations that clients need to observe:

use sui::event;

public struct LiquidityAdded has copy, drop {
    pool_id: ID,
    amount_x: u64,
    amount_y: u64,
    lp_minted: u64,
}

// Inside function:
event::emit(LiquidityAdded {
    pool_id: object::id(pool),
    amount_x,
    amount_y,
    lp_minted,
});

---

2. Error Handling

Error constants use EPascalCase and u64 values:

const EInsufficientLiquidity: u64 = 0;
const EZeroAmount: u64 = 1;

assert!(amount > 0, EZeroAmount);

Clever errors

Annotating a constant with #[error] allows it to carry a human-readable message. The value can be any valid constant type — vector<u8> is most common for string messages:

#[error]
const EInsufficientLiquidity: vector<u8> = b"Insufficient liquidity in pool";

assert!(reserves > 0, EInsufficientLiquidity);
abort EInsufficientLiquidity

At runtime, the Sui CLI and GraphQL server automatically decode these into a readable message:

Error from '0x2::amm::swap' (line 42), abort 'EInsufficientLiquidity': "Insufficient liquidity in pool"

Gotcha: clever error abort codes encode the source line number, so their u64 value can change if the file is reformatted or lines shift. Don't hardcode clever error abort codes in tests or off-chain tooling — match by constant name instead.

**assert! without an abort code** is also valid and auto-derives a clever abort code from the source line:

// ✅ Valid — line number is embedded automatically
assert!(amount > 0);

This is fine for internal invariants where the line number alone is enough context.

---

3. One-Time Witness (OTW) Pattern

Use the OTW pattern for modules that need a unique, uncopyable proof-of-publication (e.g., coin types, publisher objects):

public struct MY_MODULE has drop {}

fun init(otw: MY_MODULE, ctx: &mut TxContext) {
    // The OTW name must exactly match the module name in ALL_CAPS
    let publisher = package::claim(otw, ctx);
    transfer::public_transfer(publisher, ctx.sender());
}

---

4. Capability Pattern

Use capability objects to gate privileged functions instead of checking ctx.sender(). This is more composable and testable — the capability can be held by a contract, not just a wallet:

// ✅ Capability-gated
public struct AdminCap has key, store { id: UID }

public fun set_fee(_: &AdminCap, pool: &mut Pool, new_fee: u64) {
    pool.fee_bps = new_fee;
}

// ❌ Sender check — not composable with other contracts
public fun set_fee(pool: &mut Pool, ctx: &TxContext) {
    assert!(ctx.sender() == pool.admin, ENotAdmin);
}

Note the parameter order: the object (pool) comes before the primitive (new_fee), and _: &AdminCap follows the objects-then-capabilities ordering from the syntax skill's §3 Visibility.

---

5. Pure Functions and Composability

Keep core logic functions pure — they take objects by reference/value and return values. Do not call transfer::transfer to deliver results to the caller:

// ✅ Pure — composable with other protocols
public fun swap<X, Y>(
    pool: &mut Pool<X, Y>,
    coin_in: Coin<X>,
    ctx: &mut TxContext,
): Coin<Y> {
    // ... swap logic
}

// ❌ Transfer inside core logic breaks composability
public fun swap<X, Y>(pool: &mut Pool<X, Y>, coin_in: Coin<X>, ctx: &mut TxContext) {
    let coin_out = /* ... */;
    transfer::public_transfer(coin_out, ctx.sender()); // ❌
}

Internal transfers are acceptable when they serve the function's own mechanics (e.g., burning tokens to @0x0, sharing a newly created object). The key rule is: don't transfer the caller's results — return them instead so the caller can compose.

Return excess coins even if their value is zero — let the caller decide what to do with them.