> Pervasives.compare currently returns an int. Intuitively it would be 
> more appropriate for it to return a union type such as the following.
> type comparison_result = Less | Equals | Greater
> What are the reasons behind the present design choice?

It's a historical error.  If I were to do it again, I'd use a sum type
such as your "comparison_result".  The current solution allows to use
(-) (integer subtraction) as the comparison predicate for *small*
integer arguments, but this doesn't work as a general integer
comparison because of subtraction wrapping around for large arguments.
So, there are really no benefits to encode the result of a 3-way
comparison as an "int".

- Xavier Leroy