Perhaps a silly question, but why does the return value from unbox appear (in my F# Interactive session) to be typed as obj instead of the concrete type int? As far as I can understand (trying to apply existing knowledge from C#) if it's typed as obj then it's still boxed. Example follows:
> (unbox<int> >> box<int>) 42;;
val it : obj = 42
> 42;;
val it : int = 42
Function composition (f >> g) v means g (f (v)), so you're actually calling box<int> at the end (and the call to unbox<int> is not necessary):
> box<int> (unbox<int> 42);;
val it : obj = 42
> box<int> 42;;
val it : obj = 42
The types are box : 'T -> obj and unbox : obj -> 'T, so the functions convert between boxed (objects) and value types (int). You can call unbox<int> 42, because F# automatically inserts conversion from int to obj when calling a function.