This basically exists in Haskell/Idris as the Maybe type. It is used slightly differently than what you're saying but in essence it lets you say: - The thing d

This basically exists in Haskell/Idris as the Maybe type. It is used slightly differently than what you're saying but in essence it lets you say: - The thing doesn't exist, return Nothing (unit in a way) - The inputs produced Just the thing you wanted You have to cover both "didn't work" and "here you go" but its highly useful to not have to worry about NULL versus NOT NULL. https://stackoverflow.com/questions/29456824/what-is-the-maybe-type-and-how-does-it-work https://stackoverflow.com/questions/29456824/what-is-the-may... Your function that decides if something exists or not, could be the thing determining the percentage spiel. Aka a function returning Maybe Bool could return: a Bool with true/false, or Nothing. Nothing being "its not a bool, your inputs are insane/nonsensical", but if it returns a Just Bool, you just unbox the Bool and voila. Just thought you might want to know this is a real thing and is super useful.