GP mentionned "maybes". In Haskell (and others) you have for example both maybe and either. When you've got an either you can have either (ah!) the left to indi

GP mentionned "maybes". In Haskell (and others) you have for example both maybe and either. When you've got an either you can have either (ah!) the left to indicate an error (and which error) or the right to hold the correct value. And the type system forces you to at deal with both cases (like a maybe forces you to deal with the case where it's "maybe not").