> Which languages would you say are really good for defining your own types? Would they be a good fit for the example I provided where a function needs to accept integers larger than zero?
I'm not an Ada expert, but it has excellent support for range-restricted integer types.[0]
Ada's 'discriminated types' are also fun. They let you create members which only exist when they're applicable. [1]
> Do they also allow you to define your own operations on those types
Looks like Ada supports operator overloading, yes. [2]
Ada is also great at separating the various aspects of OOP into separate language concepts, instead of going "everything is done with classes!!" as many popular languages do, leading to a lot of confusion in this thread.