Haskell: Sets

Given a finite Set type (not Data.Set), find a map Set -> Int that is bijective! (Let's try injective first, how would you do that?)

The definition of Set I have in mind is this:
data Set = Empty | Set [Set]
But if you want to use some other equivalent definition go ahead.

#define SET(type, init_size) type * data[size]
...
typedef SET(int, 10000) muh_set;