Custom types in entrypoints

Using custom types in entrypoints requires our type to implement the Serde trait. This is because when calling an entrypoint, the input is sent as an array of felt252 to the entrypoint, and we need to be able to deserialize it into our custom type. Similarly, when returning a custom type from an entrypoint, we need to be able to serialize it into an array of felt252. Thankfully, we can just derive the Serde trait for our custom type.

// Deriving the `Serde` trait allows us to use
// the `Person` type as an entrypoint parameter and as a return value
#[derive(Drop, Serde)]
pub struct Person {
    pub age: u8,
    pub name: felt252,
}
 
#[starknet::contract]
pub mod SerdeCustomType {
    use super::Person;
 
    #[storage]
    struct Storage {}
 
    #[abi(embed_v0)]
    impl SerdeCustomType of super::ISerdeCustomType<ContractState> {
        fn person_input(ref self: ContractState, person: Person) {}
 
        fn person_output(self: @ContractState) -> Person {
            Person { age: 10, name: 'Joe' }
        }
    }
}