Add property tests for lerp

To test the fix for #164.

linear.cabal

@@ -129,7 +129,9 @@ test-suite doctests
 test-suite test
   type:           exitcode-stdio-1.0
   main-is:        Test.hs
-  other-modules:  Unit.Binary
+  other-modules:  Prop.Quaternion
+                  Prop.V3
+                  Unit.Binary
                   Unit.Plucker
                   Unit.V
   ghc-options:    -Wall -threaded
@@ -141,8 +143,10 @@ test-suite test
     deepseq,
     test-framework >= 0.8,
     test-framework-hunit >= 0.3,
+    test-framework-quickcheck2 >= 0.3,
     HUnit >= 1.2.5,
     linear,
+    QuickCheck >= 2.5,
     reflection,
     vector
   default-language: Haskell2010

tests/Prop/Quaternion.hs

@@ -0,0 +1,28 @@
+{-# OPTIONS_GHC -Wno-orphans #-}
+module Prop.Quaternion (tests) where
+
+import Linear.Quaternion (Quaternion(..))
+import Linear.Epsilon (nearZero)
+import Linear.Vector (lerp)
+import Test.Framework (Test, testGroup)
+import Test.Framework.Providers.QuickCheck2 (testProperty)
+import Test.QuickCheck (Arbitrary(..))
+
+import Prop.V3 ()
+
+instance Arbitrary a => Arbitrary (Quaternion a) where
+  arbitrary = Quaternion <$> arbitrary <*> arbitrary
+
+prop_lerp0 :: Quaternion Double -> Quaternion Double -> Bool
+prop_lerp0 a b = nearZero (lerp 0 a b - a)
+
+prop_lerp1 :: Quaternion Double -> Quaternion Double -> Bool
+prop_lerp1 a b = nearZero (lerp 1 a b - b)
+
+tests :: [Test]
+tests =
+  [ testGroup "lerp"
+    [ testProperty "lerp 0 a b == a" prop_lerp0
+    , testProperty "lerp 1 a b == b" prop_lerp1
+    ]
+  ]

tests/Prop/V3.hs

@@ -0,0 +1,8 @@
+{-# OPTIONS_GHC -Wno-orphans #-}
+module Prop.V3 () where
+
+import Linear.V3 (V3(..))
+import Test.QuickCheck (Arbitrary(..))
+
+instance Arbitrary a => Arbitrary (V3 a) where
+  arbitrary = V3 <$> arbitrary <*> arbitrary <*> arbitrary

tests/Test.hs

@@ -1,14 +1,20 @@
 {-# LANGUAGE CPP #-}
 module Main (main) where
+
 import Test.Framework (defaultMain, testGroup, Test)
 import Test.Framework.Providers.HUnit
+
+import qualified Prop.Quaternion
 import qualified Unit.Binary
 import qualified Unit.Plucker
 import qualified Unit.V
 
 tests :: [Test]
 tests =
-  [ testGroup "Unit tests"
+  [ testGroup "Property tests"
+    [ testGroup "Quaternion" Prop.Quaternion.tests
+    ]
+  , testGroup "Unit tests"
     [ testGroup "Binary" $ hUnitTestToTests Unit.Binary.tests
     , testGroup "Plucker" $ hUnitTestToTests Unit.Plucker.tests
     , testGroup "V" $ hUnitTestToTests Unit.V.tests