[kyleburton]

If you're interested in learning about them, those two links are going to do a better job of explaining what both Typed Clojure and Schema are that I am across a few comments.

Wrt the typing of Maps and Arrays, what Clojure supports goes way beyond what Java's type system supports: you can specify that the first element of an array has to be of type X, the second is of type Y, the third can be either NULL or Z. For maps, you can specify that a key must have a particular type and must be in the map, with a specific value while other keys may be present (required or not) and you can compose any of the constraints mentioned on the values the keys may take.

This is different from Generics in that Generics constrain to homogeneity for an entire collection.

You could create formal objects in your Java code to express similar constraints, but you're not achieving the same result: a Java object actually has the property (even if it's null) while a map (or an array) with an optional member will only have it if it's present. With Java you'd need many classes to model all the specific combinations. Java's current type system is significantly less expressive and doesn't have the same power. I realize those are subjective. By expressive and power I am referring to whether you can declare an idea in your code or whether you have to write the imperative logic to implement an idea (the former meeting my definition of expression or power).

Thanks for the question.