Add property tests to resistor-color-duo

Closes exercism#867

exercises/resistor-color-duo/.meta/hints.md

@@ -8,3 +8,32 @@ value :: (Color, Color) -> Int
 
 that only accepts two colors and produces the resistor's numeric value
 component of its resistance.
+
+**Bonus - Property Testing:**
+
+This exercise is a good opportunity to write some [property tests](https://en.wikipedia.org/wiki/Property_testing). If you haven't written any before here are some resources:
+
+1. [School of Haskell](https://www.schoolofhaskell.com/user/pbv/an-introduction-to-quickcheck-testing).
+2. [Real world Haskell](http://book.realworldhaskell.org/read/testing-and-quality-assurance.html)
+
+We have provided one sample property test in the [test file](test/Tests.hs). We use the [QuickCheck](https://hackage.haskell.org/package/QuickCheck) library, but there are [others](http://hackage.haskell.org/package/hedgehog) as well, feel free to play around.
+
+Few properties that can be tested:
+
+* `value (x, y) >= 10` for all x except Black and all y
+
+* `show (value (x, y)) == reverse (show (value (y, x)))` for all x, y except Black.
+
+* (Color, Color) is isomorphic to [0..99] (or: all colors have a unique value).
+
+* Associativity: Kind of tricky because you can't get the value of a single Color:
+
+```haskell
+value (k, x) <= value (k, y) && value (k, y) <= value (k, z)
+  ⇒ value (k, x) <= value (k, z)
+
+value (x, k) <= value (y, k) && value (y, k) <= value (z, k)
+  ⇒ value (k, x) <= value (k, z)
+```
+
+Can you think of any other properties?

exercises/resistor-color-duo/examples/success-standard/package.yaml

@@ -14,3 +14,4 @@ tests:
     dependencies:
       - resistor-color-duo
       - hspec
+      - QuickCheck

exercises/resistor-color-duo/package.yaml

@@ -1,5 +1,5 @@
 name: resistor-color-duo
-version: 2.1.0.3
+version: 2.1.0.4
 
 dependencies:
   - base
@@ -19,3 +19,4 @@ tests:
     dependencies:
       - resistor-color-duo
       - hspec
+      - QuickCheck

exercises/resistor-color-duo/test/Tests.hs

@@ -1,19 +1,32 @@
 {-# OPTIONS_GHC -fno-warn-type-defaults #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
 {-# LANGUAGE RecordWildCards #-}
 
-import Data.Foldable     (for_)
-import Test.Hspec        (Spec, describe, it, shouldBe)
-import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)
+import Data.Foldable              (for_)
+import Test.Hspec                 (Spec, describe, context, it, shouldBe)
+import Test.Hspec.Runner          (configFastFail, defaultConfig, hspecWith)
+import Test.QuickCheck            (property)
+import Test.QuickCheck.Gen        (oneof)
+import Test.QuickCheck.Arbitrary
 
 import ResistorColors (Color(..), value)
 
+instance Arbitrary Color where
+  arbitrary = oneof $ map return [Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White]
+
 main :: IO ()
 main = hspecWith defaultConfig {configFastFail = True} specs
 
 specs :: Spec
-specs = describe "value" $ for_ cases test
-  where
+specs = describe "value" $ do
+  context "equality tests" $ for_ cases test
 
+  context "property tests" $
+    it "all values starting with Black are single digit" $ property $
+      \color -> value (Black, color) < 10
+    -- Add more property tests here
+
+  where
     test Case{..} = it explanation assertion
       where
         explanation = unwords [show input, "-", description]