ERC-20

It's time for something that every smart contract developer has done at least once. Let's try to implement Erc20 standard. Of course, we are going to use the Odra Framework.

The ERC-20 standard establishes a uniform specification for fungible tokens. This implies that each token shares attributes that make it indistinguishable from another token of the same type and value.

Framework features

A module we will write in a minute, will help you master a few Odra features:

• Advanced storage using key-value pairs,
• Odra types such as Address,
• Advanced event assertion.

Code

Our module features a considerably more complex storage layout compared to the previous example.

It is designed to store the following data:

1. Immutable metadata - name, symbol, and decimals.
2. Total supply.
3. Balances of individual users.
4. Allowances, essentially indicating who is permitted to spend tokens on behalf of another user.

Module definition

erc20.rs

use odra::prelude::*;
use odra::{Address, casper_types::U256, Mapping, Var};

#[odra::module(events = [Transfer, Approval])]
pub struct Erc20 {
    decimals: Var<u8>,
    symbol: Var<String>,
    name: Var<String>,
    total_supply: Var<U256>,
    balances: Mapping<Address, U256>,
    allowances: Mapping<(Address, Address), U256>
}

• L10 - For the first time, we need to store key-value pairs. In order to do that, we use Mapping. The name is taken after Solidity's native type mapping.
• L11 - Odra does not allows nested Mappings as Solidity does. Instead, you can create a compound key using a tuple of keys.

Metadata

erc20.rs

...
use odra::Event;

...

#[odra::module]
impl Erc20 {
    pub fn init(&mut self, name: String, symbol: String, decimals: u8, initial_supply: U256) {
        let caller = self.env().caller();
        self.name.set(name);
        self.symbol.set(symbol);
        self.decimals.set(decimals);
        self.mint(&caller, &initial_supply);
    }

    pub fn name(&self) -> String {
        self.name.get_or_default()
    }

    pub fn symbol(&self) -> String {
        self.symbol.get_or_default()
    }

    pub fn decimals(&self) -> u8 {
        self.decimals.get_or_default()
    }

    pub fn total_supply(&self) -> U256 {
        self.total_supply.get_or_default()
    }
}

impl Erc20 {
   pub fn mint(&mut self, address: &Address, amount: &U256) {
        self.balances.add(address, *amount);
        self.total_supply.add(*amount);
        
        self.env().emit_event(Transfer {
            from: None,
            to: Some(*address),
            amount: *amount
        });
    }
}

#[derive(Event, PartialEq, Eq, Debug)]
pub struct Transfer {
    pub from: Option<Address>,
    pub to: Option<Address>,
    pub amount: U256
}

• L6 - The first impl block, marked as a module, contains functions defined in the ERC-20 standard.
• L8-L14 - A constructor sets the token metadata and mints the initial supply.
• L33 - The second impl is not an Odra module; in other words, these functions will not be part of the contract's public interface.
• L34-L43 - The mint function is public, so, like in regular Rust code, it will be accessible from the outside. mint() uses the notation self.balances.add(address, *amount);, which is syntactic sugar for:

use odra::UnwrapOrRevert;

let current_balance = self.balances.get(address).unwrap_or_default();
let new_balance = <U256 as OverflowingAdd>::overflowing_add(current_balance, current_balance).unwrap_or_revert(&self.env());
self.balances.set(address, new_balance);

Core

To ensure comprehensive functionality, let's implement the remaining features such as transfer, transfer_from, and approve. Since they do not introduce any new concepts, we will present them without additional remarks.

erc20.rs

...
use odra::OdraError;

#[odra::module]
impl Erc20 {
    ...

    pub fn transfer(&mut self, recipient: &Address, amount: &U256) {
        let caller = self.env().caller();
        self.raw_transfer(&caller, recipient, amount);
    }

    pub fn transfer_from(&mut self, owner: &Address, recipient: &Address, amount: &U256) {
        let spender = self.env().caller();
        self.spend_allowance(owner, &spender, amount);
        self.raw_transfer(owner, recipient, amount);
    }

    pub fn approve(&mut self, spender: &Address, amount: &U256) {
        let owner = self.env().caller();
        self.allowances.set(&(owner, *spender), *amount);
        self.env().emit_event(Approval {
            owner,
            spender: *spender,
            value: *amount
        });
    }

    pub fn balance_of(&self, address: &Address) -> U256 {
        self.balances.get_or_default(&address)
    }

    pub fn allowance(&self, owner: &Address, spender: &Address) -> U256 {
        self.allowances.get_or_default(&(*owner, *spender))
    }
}

impl Erc20 {
    ...

    fn raw_transfer(&mut self, owner: &Address, recipient: &Address, amount: &U256) {
        let owner_balance = self.balances.get_or_default(&owner);
        if *amount > owner_balance {
            self.env().revert(Error::InsufficientBalance)
        }
        self.balances.set(owner, owner_balance - *amount);
        self.balances.add(recipient, *amount);
        self.env().emit_event(Transfer {
            from: Some(*owner),
            to: Some(*recipient),
            amount: *amount
        });
    }

    fn spend_allowance(&mut self, owner: &Address, spender: &Address, amount: &U256) {
        let allowance = self.allowance(owner, spender);
        if allowance < *amount {
            self.env().revert(Error::InsufficientAllowance)
        }
        let new_allowance = allowance - *amount;
        self.allowances
            .set(&(*owner, *spender), new_allowance);
        self.env().emit_event(Approval {
            owner: *owner,
            spender: *spender,
            value: allowance - *amount
        });
    }
}

#[derive(Event, PartialEq, Eq, Debug)]
pub struct Approval {
    pub owner: Address,
    pub spender: Address,
    pub value: U256
}

#[derive(OdraError)]
pub enum Error {
    InsufficientBalance = 1,
    InsufficientAllowance = 2,
}

Now, compare the code we have written, with Open Zeppelin code. Out of 10, how Solidity-ish is our implementation?

Test

erc20.rs

#[cfg(test)]
pub mod tests {
    use super::*;
    use odra::{casper_types::U256, host::{Deployer, HostEnv, HostRef}};

    const NAME: &str = "Plascoin";
    const SYMBOL: &str = "PLS";
    const DECIMALS: u8 = 10;
    const INITIAL_SUPPLY: u32 = 10_000;

    fn setup() -> (HostEnv, Erc20HostRef) {
        let env = odra_test::env();
        (
            env.clone(),
            Erc20HostRef::deploy(
                &env,
                Erc20InitArgs {
                    symbol: SYMBOL.to_string(),
                    name: NAME.to_string(),
                    decimals: DECIMALS,
                    initial_supply: INITIAL_SUPPLY.into()
                }
            )
        )
    }

    #[test]
    fn initialization() {
        // When deploy a contract with the initial supply.
        let (env, erc20) = setup();

        // Then the contract has the metadata set.
        assert_eq!(erc20.symbol(), SYMBOL.to_string());
        assert_eq!(erc20.name(), NAME.to_string());
        assert_eq!(erc20.decimals(), DECIMALS);

        // Then the total supply is updated.
        assert_eq!(erc20.total_supply(), INITIAL_SUPPLY.into());

        // Then a Transfer event was emitted.
        assert!(env.emitted_event(
            erc20.address(),
            &Transfer {
                from: None,
                to: Some(env.get_account(0)),
                amount: INITIAL_SUPPLY.into()
            }
        ));
    }

    #[test]
    fn transfer_works() {
        // Given a new contract.
        let (env, mut erc20) = setup();

        // When transfer tokens to a recipient.
        let sender = env.get_account(0);
        let recipient = env.get_account(1);
        let amount = 1_000.into();
        erc20.transfer(&recipient, &amount);

        // Then the sender balance is deducted.
        assert_eq!(
            erc20.balance_of(&sender),
            U256::from(INITIAL_SUPPLY) - amount
        );

        // Then the recipient balance is updated.
        assert_eq!(erc20.balance_of(&recipient), amount);

        // Then Transfer event was emitted.
        assert!(env.emitted_event(
            erc20.address(),
            &Transfer {
                from: Some(sender),
                to: Some(recipient),
                amount
            }
        ));
    }

    #[test]
    fn transfer_error() {
        // Given a new contract.
        let (env, mut erc20) = setup();

        // When the transfer amount exceeds the sender balance.
        let recipient = env.get_account(1);
        let amount = U256::from(INITIAL_SUPPLY) + U256::one();

        // Then an error occurs.
        assert!(erc20.try_transfer(&recipient, &amount).is_err());
    }

    #[test]
    fn transfer_from_and_approval_work() {
        let (env, mut erc20) = setup();

        let (owner, recipient, spender) =
            (env.get_account(0), env.get_account(1), env.get_account(2));
        let approved_amount = 3_000.into();
        let transfer_amount = 1_000.into();

        assert_eq!(erc20.balance_of(&owner), U256::from(INITIAL_SUPPLY));

        // Owner approves Spender.
        erc20.approve(&spender, &approved_amount);

        // Allowance was recorded.
        assert_eq!(erc20.allowance(&owner, &spender), approved_amount);
        assert!(env.emitted_event(
            erc20.address(),
            &Approval {
                owner,
                spender,
                value: approved_amount
            }
        ));

        // Spender transfers tokens from Owner to Recipient.
        env.set_caller(spender);
        erc20.transfer_from(&owner, &recipient, &transfer_amount);

        // Tokens are transferred and allowance decremented.
        assert_eq!(
            erc20.balance_of(&owner),
            U256::from(INITIAL_SUPPLY) - transfer_amount
        );
        assert_eq!(erc20.balance_of(&recipient), transfer_amount);
        assert!(env.emitted_event(
            erc20.address(),
            &Approval {
                owner,
                spender,
                value: approved_amount - transfer_amount
            }
        ));
        assert!(env.emitted_event(
            erc20.address(),
            &Transfer {
                from: Some(owner),
                to: Some(recipient),
                amount: transfer_amount
            }
        ));
        // assert!(env.emitted(erc20.address(), "Transfer"));
    }

    #[test]
    fn transfer_from_error() {
        // Given a new instance.
        let (env, mut erc20) = setup();

        // When the spender's allowance is zero.
        let (owner, spender, recipient) =
            (env.get_account(0), env.get_account(1), env.get_account(2));
        let amount = 1_000.into();
        env.set_caller(spender);

        // Then transfer fails.
        assert_eq!(
            erc20.try_transfer_from(&owner, &recipient, &amount),
            Err(Error::InsufficientAllowance.into())
        );
    }
}

• L146 - Alternatively, if you don't want to check the entire event, you may assert only its type.

What's next

Having two modules: Ownable and Erc20, let's combine them, and create an ERC-20 on steroids.