Monoid instance doesn't obey the concatenation law

slist.cabal

@@ -68,6 +68,8 @@ test-suite slist-test
   main-is:             Spec.hs
 
   build-depends:       slist
+                     , hedgehog
+                     , hedgehog-classes
 
   ghc-options:         -threaded
                        -rtsopts
@@ -82,4 +84,4 @@ test-suite slist-doctest
   build-depends:       doctest
                      , Glob
 
-  ghc-options:         -threaded
+  ghc-options:         -threaded

src/Slist.hs

@@ -266,7 +266,7 @@ instance Monoid (Slist a) where
       where
         -- foldr :: (a -> ([a], Size) -> ([a], Size)) -> ([a], Size) -> [Slist a] -> ([a], Size)
         f :: Slist a -> ([a], Size) -> ([a], Size)
-        f (Slist l s) (xL, !xS) = (xL ++ l, s + xS)
+        f (Slist l s) (xL, !xS) = (l ++ xL, s + xS)
     {-# INLINE mconcat #-}
 
 instance Functor Slist where

test/Spec.hs

@@ -1,4 +1,27 @@
 module Main (main) where
 
+import Slist as  SL
+import           Hedgehog
+import           Hedgehog.Main (defaultMain)
+import           Hedgehog.Classes
+import qualified Hedgehog.Gen as Gen
+import qualified Hedgehog.Range as Range
+
 main :: IO ()
-main = putStrLn ("Test suite not yet implemented" :: String)
+main = defaultMain $
+  lawsCheck <$>
+    [ functorLaws genFiniteSlist
+    , applicativeLaws genFiniteSlist
+    , monadLaws genFiniteSlist
+    , foldableLaws genFiniteSlist
+    , traversableLaws genFiniteSlist
+    , alternativeLaws genFiniteSlist
+    , semigroupLaws (genFiniteSlist genInt)
+    , monoidLaws (genFiniteSlist genInt)
+    ]
+
+genInt :: Gen Int
+genInt = Gen.int (Range.linear 0 100)
+
+genFiniteSlist :: Gen a -> Gen (Slist a)
+genFiniteSlist gen = slist <$> Gen.list (Range.linear 0 100) gen