Adding Machine


A simple adding machine checks for overflow and emits an event with the result, without using storage. In the pallet's file,

pub trait Trait: system::Trait {
    type Event: From<Event> + Into<<Self as system::Trait>::Event>;

decl_module! {
    pub struct Module<T: Trait> for enum Call where origin: T::Origin {
        fn deposit_event() = default;

        fn add(origin, val1: u32, val2: u32) -> Result {
            let _ = ensure_signed(origin)?;
            // checks for overflow
            let result = match val1.checked_add(val2) {
                Some(r) => r,
                None => return Err("Addition overflowed"),
            Self::deposit_event(Event::Added(val1, val2, result));

    pub enum Event {
        Added(u32, u32, u32),

NOTE: The event described above only wraps u32 values. If we want/need the Event type to contain multiple types from our runtime, then the decl_event would use the following syntax

    pub enum Event<T> {

In some cases, the where clause can be used to specify type aliasing for more readable code

    pub enum Event<T>
        Balance = BalanceOf<T>,
        <T as system::Trait>::AccountId,
        <T as system::Trait>::BlockNumber,
        <T as system::Trait>::Hash,
        FakeEvent1(AccountId, Hash, BlockNumber),
        FakeEvent2(AccountId, Balance, BlockNumber),