Many languages (C++ templates, C#, Java, among others) write things like List<T> to mean a "list where every item is of type T". This feature is usually called

Many languages (C++ templates, C#, Java, among others) write things like List<T> to mean a "list where every item is of type T". This feature is usually called something like "generics" or "parametric polymorphism". It's not haskell notation but I hoped that it would give an easier flavour. > Are promises and futures always monads? Only if you can construct the necessary operations (or if they are provided), and they relate to each other correctly. For example, if you have a value m of type M<A>, (join (pure m)) should be identical to m. The language you're using also needs to be able to usefully do something with this fact, or it's just trivia. In Haskell, we can define a type class called Monad and write operations that work on any monad, without knowing or caring which one is in use (that's the caller's job to decide). This would be hard-to-impossible to do in a less expressive language like C.