GLib.return{,_val}_if_fail and GLib.warn{,_val}_if_fail should be more Vala friendly
Currently, the only way for such functions to produce useful output is when they are used implicitly with the requires
and ensures
keywords. However, when working with anonymous functions (created through a lambda expression), it's not possible to use these keywords, so it becomes essential to use these functions separately.
Also, sometimes you need to do extra checks before you can check if the received arguments are valid, so again, you need to make use of these functions outside of Vala's syntax support.