# Joining two coins of custom currency **URL:** \ **Category:** Move\ **Created:** [July 30, 2025, 10:40am UTC](https://forums.sui.io/t/joining-two-coins-of-custom-currency/48952 "2025-07-30T10:40:40Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![plyoteo](https://avatars.discourse-cdn.com/v4/letter/p/838e76/32.png) [@plyoteo](https://forums.sui.io/u/plyoteo)\ **Post date:** [July 30, 2025, 10:40am UTC](https://forums.sui.io/t/joining-two-coins-of-custom-currency/48952/1 "2025-07-30T10:40:40Z")
Hello SUI Move comrades. Please lend me a helping hand if you will. I am creating a currency of mine, minting it, splitting it and burning it as per code below: ``` module try_nft::dnr; const COIN_URL: vector = b"xyz"; use sui::coin::{Self, Coin, TreasuryCap}; use sui::url; public struct DNR has drop {} fun init(witness: DNR, ctx: &mut TxContext) { let (treasury_cap, metadata) = coin::create_currency( witness, 9, b"My SUI Coin", b"Denero", b"Coin worth crickets", option::some(url::new_unsafe_from_bytes(COIN_URL)), ctx, ); transfer::public_freeze_object(metadata); transfer::public_transfer(treasury_cap, tx_context::sender(ctx)); } public entry fun mint_coin( treasury_cap: &mut TreasuryCap, amount: u64, recipient: address, ctx: &mut TxContext) { coin::mint_and_transfer(treasury_cap, amount, recipient, ctx); } public entry fun burn_coin( treasury_cap: &mut TreasuryCap, coin: Coin) { coin::burn(treasury_cap, coin); } public entry fun split_coin( self: &mut Coin, amount: u64, recipient: address, ctx: & mut TxContext) { let new_split_coin = coin::split(self, amount, ctx); transfer::public_transfer(new_split_coin, recipient); } ``` It compiles just fine and works as a peach when tested on SuiScan. However, when I try to join the two splitted coins, with the snippet below, ``` public entry fun join_coin( self: &mut Coin, c: Coin, recipient: address) { let new_join_coin = coin::join(self, c); transfer::public_transfer(new_join_coin, recipient); } ``` I get a compilation error as below, error[E04005]: expected a single type ┌─ ./sources/try\_nft.move:61:13 │ 61 │ let new\_join\_coin = coin::join(self, c); │ ^^^^^^^^^^^^^ Invalid type for local │ ┌─ /home/plyoteo/.move/https\_\_\_github\_com\_MystenLabs\_sui\_git\_2c930c25f8d3/crates/sui-framework/packages/sui-framework/sources/coin.move:165:18 │ 165 │ public entry fun join(self: &mut Coin, c: Coin) { │ ---- Expected a single type, but found expression list type: ‘()’ Any ideas? I think that I am using the join fun exactly as outlined in sui:🪙:join documentation. I thank you in advance ---
**Author:** ![DevDanny](https://avatars.discourse-cdn.com/v4/letter/d/ecae2f/32.png) [@DevDanny](https://forums.sui.io/u/DevDanny)\ **Post date:** [August 1, 2025, 4:42pm UTC](https://forums.sui.io/t/joining-two-coins-of-custom-currency/48952/2 "2025-08-01T16:42:51Z")
# Understanding the Coin Join and Transfer in SUI Move ## The Problem The original implementation had two key issues: 1. Misunderstanding of `coin::join`’s return value (it returns `()`) 2. Attempting to transfer a mutable reference (`&mut Coin`) which violates Sui’s ownership model ## Why the Original Code Failed ```move public entry fun join_coin( self: &mut Coin, c: Coin, recipient: address ) { let new_join_coin = coin::join(self, c); // Problem 1: No return value transfer::public_transfer(new_join_coin, recipient); // Problem 2: Can't transfer reference } ``` Key problems: 1. `coin::join` modifies `self` in-place and doesn’t return a new coin 2. `public_transfer` requires an owned object with `key + store` abilities, not a reference ## Corrected Solutions ### Option 1: Simple Join (Recommended) ```move public entry fun join_coins( coin1: &mut Coin, // First coin to merge into coin2: Coin // Second coin to be consumed ) { coin::join(coin1, coin2); } ``` ### Option 2: Join and Transfer ```move public entry fun join_and_transfer( treasury_cap: &mut TreasuryCap, // Needed for potential re-minting coin1: Coin, // First coin (will be merged) coin2: Coin, // Second coin (will be consumed) recipient: address, ctx: &mut TxContext ) { // Merge coins let mut merged_coin = coin1; coin::join(&mut merged_coin, coin2); // Transfer the merged coin transfer::public_transfer(merged_coin, recipient); } ``` ## Key Corrections and Insights 1. **Ownership Matters** : 2. **Proper Entry Function Design** : ## Important Notes 1. The `TreasuryCap` parameter in Option 2 is optional but recommended if you might need to mint/burn later 2. Always consider: 3. Remember that after transfer: ---
**Author:** ![plyoteo](https://avatars.discourse-cdn.com/v4/letter/p/838e76/32.png) [@plyoteo](https://forums.sui.io/u/plyoteo)\ **Post date:** [August 1, 2025, 5:59pm UTC](https://forums.sui.io/t/joining-two-coins-of-custom-currency/48952/3 "2025-08-01T17:59:13Z")
DevDanny, Thank you so much, both for the two solutions but mostly for the deep insights into the inner workings of move. I am a 63 yr old programming hobbyist coming from a Python/NodeJS background, so statically typed, compiled languages give me a bit of the chills. After working out the bare essential of Rust I took a head-first plunge into Move. Your response is probably a good reason I wont give up trying. Thank you again, Theodoros Στις Παρ 1 Αυγ 2025, 19:52 ο χρήστης DevDanny via Sui Developer Forum \<[notifications@sui.discoursemail.com](mailto:notifications@sui.discoursemail.com)\> έγραψε: