Monoid-instance for Load

src/Model/Types.hs

@@ -85,6 +85,19 @@ data Load -- = ByTutorial { countsToLoad :: Bool } | ByProportion { load :: Rati
   deriving (Show, Read, Eq)
 derivePersistField "Load"
 
+instance Semigroup Load where
+  (Load byTut prop) <> (Load byTut' prop') = Load byTut'' (prop + prop')
+    where
+      byTut''
+        | Nothing <- byTut  = byTut'
+        | Nothing <- byTut' = byTut
+        | Just a <- byTut
+        , Just b <- byTut'  = Just $ a || b
+
+instance Monoid Load where
+  mempty = Load Nothing 0
+  mappend = (<>)
+
 {- Use (is _ByTutorial) instead of this unneeded definition:
   isByTutorial :: Load -> Bool
   isByTutorial (ByTutorial {}) = True