3. Shared Account

Full working code: The complete source code for this example is available in example-contracts/shared-account/

This example demonstrates a simple multi-tenant shared account introducing static contract imports, complex storage structs, authorization logic, ID generation, and cross-contract calls.

WIT Interface

Exports functions for multi-tenant account management with token deposits and withdrawals:

package root:component;

world root {
  include kontor:built-in/built-in;
  use kontor:built-in/context.{view-context, proc-context, signer};
  use kontor:built-in/error.{error};
  use kontor:built-in/foreign.{contract-address};
  use kontor:built-in/numbers.{integer, decimal};

  export init: func(ctx: borrow<proc-context>);

  export open: func(ctx: borrow<proc-context>, token: contract-address, n: integer, other-tenants: list<string>) -> result<string, error>;

  export deposit: func(ctx: borrow<proc-context>, token: contract-address, account-id: string, n: integer) -> result<_, error>;

  export withdraw: func(ctx: borrow<proc-context>, token: contract-address, account-id: string, n: integer) -> result<_, error>;

  export balance: func(ctx: borrow<view-context>, account-id: string) -> option<integer>;

  export token-balance: func(ctx: borrow<view-context>, token: contract-address, holder: string) -> option<integer>;

  export tenants: func(ctx: borrow<view-context>, account-id: string) -> option<list<string>>;
}

Rust Implementation

The interface! macro generates an interface for cross-contract calls to the token contract with a dynamic address (passed as a parameter). Use import! when the dependency contract has been previously published and the contract address is known at compile time.
The StorageRoot macro, used for the root storage type, and the Storage macro, used for nested storage types in the Account struct, enable persistent storage.
other_tenants uses Map<String, bool> because the storage layer does not currently support list types, and Map provides a limited interface with the keys method for iteration. Even with a List type using a Map here could make sense. Instead of bool an enum or struct that defines the their “role” in the account could be written.
authorized Verifies procedure permissions
open: Verifies token balance, generates an ID using crypto::generate_id(), sets the account, and transfers tokens to ctx.contract_signer()
deposit/withdraw: Authorize the caller, verify balances, update storage, and call the token contract following CEI pattern
balance/tenants: Query the storage for account details

use stdlib::*;

contract!(name = "shared-account");

interface!(name = "token", path = "../token/contract/wit");

#[derive(Clone, Default, Storage)]
struct Account {
    pub other_tenants: Map<String, bool>,
    pub balance: Integer,
    pub owner: String,
}

#[derive(Clone, Default, StorageRoot)]
struct SharedAccountStorage {
    pub accounts: Map<String, Account>,
}

fn authorized(signer: &Signer, account: &AccountModel) -> bool {
    account.owner() == signer.to_string()
        || account
            .other_tenants()
            .get(signer.to_string())
            .is_some_and(|b| b)
}

fn insufficient_balance_error() -> Error {
    Error::Message("insufficient balance".to_string())
}

fn unauthorized_error() -> Error {
    Error::Message("unauthorized".to_string())
}

fn unknown_error() -> Error {
    Error::Message("unknown account".to_string())
}

impl Guest for SharedAccount {
    fn init(ctx: &ProcContext) {
        SharedAccountStorage::default().init(ctx);
    }

    fn open(
        ctx: &ProcContext,
        token: ContractAddress,
        n: Integer,
        other_tenants: Vec<String>,
    ) -> Result<String, Error> {
        let signer = ctx.signer();
        let balance =
            token::balance(&token, &signer.to_string()).ok_or(insufficient_balance_error())?;
        if balance < n {
            return Err(insufficient_balance_error());
        }
        let account_id = ctx.generate_id();
        ctx.model().accounts().set(
            account_id.clone(),
            Account {
                balance: n,
                owner: ctx.signer().to_string(),
                other_tenants: Map::new(
                    &other_tenants
                        .into_iter()
                        .map(|t| (t, true))
                        .collect::<Vec<_>>(),
                ),
            },
        );
        token::transfer(&token, signer, &ctx.contract_signer().to_string(), n)?;
        Ok(account_id)
    }

    fn deposit(
        ctx: &ProcContext,
        token: ContractAddress,
        account_id: String,
        n: Integer,
    ) -> Result<(), Error> {
        let signer = ctx.signer();
        let balance =
            token::balance(&token, &signer.to_string()).ok_or(insufficient_balance_error())?;
        if balance < n {
            return Err(insufficient_balance_error());
        }
        let account = ctx
            .model()
            .accounts()
            .get(account_id)
            .ok_or(unknown_error())?;
        if !authorized(&signer, &account) {
            return Err(unauthorized_error());
        }
        account.update_balance(|b| b + n);
        token::transfer(&token, signer, &ctx.contract_signer().to_string(), n)
    }

    fn withdraw(
        ctx: &ProcContext,
        token: ContractAddress,
        account_id: String,
        n: Integer,
    ) -> Result<(), Error> {
        let signer = ctx.signer();
        let account = ctx
            .model()
            .accounts()
            .get(account_id)
            .ok_or(unknown_error())?;
        if !authorized(&signer, &account) {
            return Err(unauthorized_error());
        }
        let balance = account.balance();
        if balance < n {
            return Err(insufficient_balance_error());
        }
        account.set_balance(balance - n);
        token::transfer(&token, ctx.contract_signer(), &signer.to_string(), n)
    }

    fn balance(ctx: &ViewContext, account_id: String) -> Option<Integer> {
        ctx.model().accounts().get(account_id).map(|a| a.balance())
    }

    fn token_balance(
        _ctx: &ViewContext,
        token: ContractAddress,
        holder: String,
    ) -> Option<Integer> {
        token::balance(&token, &holder)
    }

    fn tenants(ctx: &ViewContext, account_id: String) -> Option<Vec<String>> {
        ctx.model().accounts().get(account_id).map(|a| {
            [a.owner()]
                .into_iter()
                .chain(a.other_tenants().keys())
                .collect()
        })
    }
}

Key points:

Uses interface! not import! for dynamic token contract address
ctx.model() for storage access
Map operations: accounts().get(id) returns Option<AccountModel>
Nested struct updates: account.update_balance(|b| b + n)
Error helpers return Error::Message(...to_string())
authorized() function takes &AccountModel not &AccountWrapper
ctx.generate_id() creates unique account IDs
Calls token::balance() and token::transfer() with contract addresses

Testing

The test demonstrates creating accounts, depositing, withdrawing, and testing authorization:

#[cfg(test)]
mod tests {
    use testlib::*;

    interface!(name = "token", path = "../token/contract/wit");
    interface!(name = "shared-account");

    #[testlib::test]
    async fn test_shared_account_contract() -> Result<()> {
        let alice = runtime.identity().await?;
        let bob = runtime.identity().await?;
        let claire = runtime.identity().await?;
        let dara = runtime.identity().await?;

        let token = runtime.publish(&alice, "token").await?;
        let shared_account = runtime.publish(&alice, "shared-account").await?;

        token::mint(runtime, &token, &alice, 100.into()).await??;

        let account_id = shared_account::open(
            runtime,
            &shared_account,
            &alice,
            token.clone(),
            50.into(),
            vec![&bob, &dara],
        )
        .await??;

        let result = shared_account::balance(runtime, &shared_account, &account_id).await?;
        assert_eq!(result, Some(50.into()));

        shared_account::deposit(runtime, &shared_account, &alice, token.clone(), &account_id, 25.into())
            .await??;

        let result = shared_account::balance(runtime, &shared_account, &account_id).await?;
        assert_eq!(result, Some(75.into()));

        shared_account::withdraw(runtime, &shared_account, &bob, token.clone(), &account_id, 25.into())
            .await??;

        let result = shared_account::balance(runtime, &shared_account, &account_id).await?;
        assert_eq!(result, Some(50.into()));

        shared_account::withdraw(runtime, &shared_account, &alice, token.clone(), &account_id, 50.into())
            .await??;

        let result = shared_account::balance(runtime, &shared_account, &account_id).await?;
        assert_eq!(result, Some(0.into()));

        let result = shared_account::withdraw(runtime, &shared_account, &bob, token.clone(), &account_id, 1.into())
            .await?;
        assert_eq!(
            result,
            Err(Error::Message("insufficient balance".to_string()))
        );

        let result = shared_account::withdraw(runtime, &shared_account, &claire, token.clone(), &account_id, 1.into())
            .await?;
        assert_eq!(result, Err(Error::Message("unauthorized".to_string())));

        let result =
            shared_account::token_balance(runtime, &shared_account, token.clone(), &alice).await?;
        assert_eq!(result, Some(75.into()));

        let result = token::balance(runtime, &token, &bob).await?;
        assert_eq!(result, Some(25.into()));

        let result = shared_account::tenants(runtime, &shared_account, &account_id)
            .await?
            .unwrap();
        assert_eq!(result.iter().len(), 3);
        assert!(result.contains(&alice.to_string()));
        assert!(result.contains(&dara.to_string()));
        assert!(result.contains(&bob.to_string()));

        Ok(())
    }
}

Basics

Reference

Examples

WIT Interface

Rust Implementation

Testing

Basics

Reference

Examples

​WIT Interface

​Rust Implementation

​Testing

WIT Interface

Rust Implementation

Testing