Function update 

pub fn update<T, F>(key: &str, default: T, f: F) -> Result<T>
where
    T: DeserializeOwned + Serialize + Clone,
    F: FnOnce(T) -> T,

Update a value in state using a function.

If the key doesn’t exist, uses the default value.

Example

state::update("counter", 0, |n| n + 1)?;