kitchen/modules/adding-machine

A simple adding machine checks for overflow and emits an event with the result, without using storage. In the module 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));
            Ok(())
        }
    }
}

decl_event!(
    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

decl_event!(
    pub enum Event<T> {
        ...
    }
)

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

decl_event!(
    pub enum Event<T> 
    where
        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),
    }
)