Function function 

pub async fn function<S, A, E>(
    name: &E::FnName,
    args: A,
    env: &mut E,
) -> Option<Result<BoxFuture<'static, ExitStatus>, S::Error>>
where
    E: FunctionEnvironment<Fn = S> + FunctionFrameEnvironment + SetArgumentsEnvironment + ?Sized,
    E::Args: From<A>,
    S: Clone + Spawn<E>,

Creates a future adapter that will attempt to execute a function (if it has been defined) with a given set of arguments.