Macro callback 

macro_rules! callback {
    ($callback_name:ident(($($var:ident : $typ:ty),+)) -> $action_ty:ty $body:block) => { ... };
    ($callback_name:ident($var:ident : $typ:ty) -> $action_ty:ty $body:block) => { ... };
}

Creates a callback instance. Must accept a single argument, so () should be used when no arguments are needed and tuples where more than one value need to be passed.

Example

callback!(task_done_callback(result: String) -> Action {
    SomeAction { result }
})

callback!(multiple_arguments_callback((arg1: u64, arg2: u64)) -> Action {
    MultipleArgumentsAction { value: arg1 + arg2 }
})