I believe that while monads are great, crystal obviates the need for option because String? doesn't mean "nullable string", it means (String | Nil) or "the type which is the union of String and Nil". This is a much more powerful and generic concept than nillable types, as you can call any method in this object which is defined both on String and Nil. One such method (which happens to be defined in every type) is the method try. This is equivalent to a map in monad optional. I'm sure you can see that this generalises to make type unions possible to represent the optional type (with no overhead) and all methods which you could implement on it. Furthermore, its actually more powerful as this is implemented at the type system level, meaning flow typing works, which makes code much cleaner as you can use normal if statements with boolean conjugates to "unwrap" these types (really just removing types from the union) which is cleaner.