composable-move-functions

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Overview

Sui transactions can chain multiple function calls in a single Programmable Transaction Block (PTB). Functions that transfer objects internally instead of returning them break this composability. This skill covers how to design functions that work well in PTBs.

All patterns sourced from https://move-book.com/guides/code-quality-checklist

No public entry

Functions should be either public (composable, can be called from other modules and PTBs) or entry (transaction endpoint only). Never use public entry together.

// WRONG — public entry is redundant and limits composability
public entry fun do_something() { }

// CORRECT — public for composable functions that return values
public fun mint(ctx: &mut TxContext): NFT { }

// CORRECT — entry for intentionally non-composable endpoints
entry fun mint_and_keep(ctx: &mut TxContext) { }

When to use entry: Only for convenience endpoints that are intentionally non-composable — functions that wrap a composable public function and handle transfers to sender.

Return Objects, Don't Transfer Internally

Public functions should return values to the caller rather than transferring them to ctx.sender(). This makes them composable in PTBs — the caller decides what to do with the result.

// WRONG — couples minting with transfer, can't compose
public fun mint_and_transfer(ctx: &mut TxContext) {
    let nft = NFT { id: object::new(ctx) };
    transfer::transfer(nft, ctx.sender());
}

// CORRECT — returns the object, caller decides
public fun mint(ctx: &mut TxContext): NFT {
    NFT { id: object::new(ctx) }
}

// If you need a convenience entry point, add a separate entry wrapper:
entry fun mint_and_keep(ctx: &mut TxContext) {
    let nft = mint(ctx);
    transfer::transfer(nft, ctx.sender());
}

CLI implication for returned values

Functions that return non-drop values cannot be invoked via sui client call — the CLI has no way to consume the returned value, causing an UnusedValueWithoutDrop error. Use sui client ptb instead, where you can chain --assign and --transfer-objects to handle the return value:

sui client ptb \
  --move-call @pkg::module::create_thing --assign thing \
  --transfer-objects "[thing]" @sender

If the function is called frequently from the CLI, consider providing a companion entry wrapper that transfers internally (as shown above).

This applies broadly:

• add_liquidity should return LP coins and remainder coins, not transfer them
• remove_liquidity should return both coins, not transfer them
• swap should return the output coin, not transfer it
• borrow should return the borrowed asset, not transfer it

Parameter Ordering

Function parameters follow a strict order:

1. Objects first — the primary object being acted on
2. Capabilities second — authorization tokens like AdminCap
3. Primitive values — amounts, flags, addresses
4. Clock — always at the end (before ctx), exception to objects-first rule
5. ctx: &mut TxContext last — ALWAYS the final parameter, after all primitives and all other arguments

// WRONG — cap before object, primitives mixed in
public fun authorize_action(
    cap: &AdminCap,
    value: u8,
    app: &mut App,
    ctx: &mut TxContext,
) { }

// CORRECT — object first, cap second, primitives third, ctx last
public fun authorize_action(
    app: &mut App,
    cap: &AdminCap,
    value: u8,
    ctx: &mut TxContext,
) { }

Clock Exception

Clock goes near the end, just before ctx, even though it's an object:

public fun timed_action(
    app: &mut App,
    cap: &AppCap,
    value: u8,
    clock: &Clock,
    ctx: &mut TxContext,
) { }

Quick Reference