chapter 15: added semigroup exercise for AccumulateBoth type which implements another Semigroup instance for Validtion type

Author: mukeshsoni

chapter15/exercises/semigroup/13_accumulate_both.hs

@@ -0,0 +1,28 @@
+import Test.QuickCheck (arbitrary, Arbitrary, quickCheck, oneof)
+import Data.Semigroup (Semigroup, (<>))
+import SemigroupLaws 
+
+data Validation a b =
+    Failure a | Success b
+    deriving (Eq, Show)
+
+newtype AccumulateBoth a b =
+    AccumulateBoth (Validation a b)
+    deriving (Eq, Show)
+
+instance (Semigroup a, Semigroup b) =>
+    Semigroup (AccumulateBoth a b) where
+        (AccumulateBoth (Success a)) <> (AccumulateBoth (Success b)) = AccumulateBoth (Success (a <> b))
+        (AccumulateBoth (Failure a)) <> (AccumulateBoth (Failure b)) = AccumulateBoth (Failure (a <> b))
+        (AccumulateBoth (Failure a)) <> (AccumulateBoth (Success _)) = AccumulateBoth (Failure a)
+        (AccumulateBoth (Success _)) <> (AccumulateBoth (Failure b)) = AccumulateBoth (Failure b)
+
+instance (Arbitrary a, Arbitrary b) => Arbitrary (AccumulateBoth a b) where
+    arbitrary = do
+        x <- arbitrary
+        y <- arbitrary
+        oneof [return (AccumulateBoth (Success x)), return (AccumulateBoth (Failure y))]
+
+type ValidationAssoc = AccumulateBoth String String -> AccumulateBoth String String -> AccumulateBoth String String -> Bool
+
+main = quickCheck (assocLaw :: ValidationAssoc)