Macro single_value_storage 

macro_rules! single_value_storage {
    ($name:ident, $value_ty:ty, $key:expr, $err:expr) => { ... };
    ($name:ident, $value_ty:ty, $key:expr) => { ... };
}

Creates an Odra module that stores a single value under a given named key.

The module has two methods: set and get. If the value is not set and an error is passed as the fourth argument, get will revert with the provided error.