refactor: make ExamOccurrenceCapacity a newtype

use pattern synonyms for convenience, so usage doesn't change

src/Model/Types/Exam.hs

@@ -19,7 +19,7 @@ module Model.Types.Exam
   , _examOccurrenceMappingRule
   , _examOccurrenceMappingMapping
   , traverseExamOccurrenceMapping
-  , ExamOccurrenceCapacity(..)
+  , ExamOccurrenceCapacity(.., Unrestricted, Restricted)
   , _examOccurrenceCapacityIso
   , ExamGrade(..)
   , numberGrade
@@ -231,15 +231,16 @@ traverseExamOccurrenceMapping = _examOccurrenceMappingMapping . iso Map.toList (
 -- 
 -- Maybe doesn't work, because the 'Ord' instance puts 'Nothing' below 0
 -- instead of above every other number.
-data ExamOccurrenceCapacity = Unrestricted | Restricted Natural
-  deriving (Show, Eq)
+newtype ExamOccurrenceCapacity = EOCapacity (Maybe Natural)
+    deriving stock (Show)
+    deriving (Eq, Ord) via (NTop (Maybe Natural))
 
--- | Unrestricted is bigger then everything else, otherwise use instance from 'Natural'.
-instance Ord ExamOccurrenceCapacity where
-  compare Unrestricted Unrestricted     = EQ
-  compare Unrestricted (Restricted _n)  = GT
-  compare (Restricted _n) Unrestricted  = LT
-  compare (Restricted a) (Restricted b) = compare a b
+pattern Unrestricted :: ExamOccurrenceCapacity
+pattern Unrestricted = EOCapacity Nothing
+pattern Restricted :: Natural -> ExamOccurrenceCapacity
+pattern Restricted n = EOCapacity (Just n)
+
+{-# COMPLETE Unrestricted, Restricted #-}
 
 -- | Addition monoid with 'Unrestricted' interpreted as infinity.
 instance Semigroup ExamOccurrenceCapacity where