1 files changed, 45 insertions, 0 deletions
diff --git a/users/grfn/xanthous/test/Xanthous/RandomSpec.hs b/users/grfn/xanthous/test/Xanthous/RandomSpec.hs
new file mode 100644
index 000000000000..c88bd9562928
--- /dev/null
+++ b/users/grfn/xanthous/test/Xanthous/RandomSpec.hs
@@ -0,0 +1,45 @@
+--------------------------------------------------------------------------------
+module Xanthous.RandomSpec (main, test) where
+--------------------------------------------------------------------------------
+import Test.Prelude
+--------------------------------------------------------------------------------
+import Control.Monad.Random
+--------------------------------------------------------------------------------
+import           Xanthous.Random
+import           Xanthous.Orphans ()
+import qualified Data.Interval as Interval
+import           Data.Interval (Interval, Extended (Finite), (<=..<=))
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Random"
+  [ testGroup "chooseSubset"
+    [ testProperty "chooses a subset"
+      $ \(l :: [Int]) (Positive (r :: Double)) -> randomTest $ do
+        ss <- chooseSubset r l
+        pure $ all (`elem` l) ss
+    ]
+  , testGroup "chooseRange"
+    [ testProperty "chooses in the range"
+      $ \(rng :: Interval Int) ->
+        not (Interval.null rng)
+        ==> randomTest ( do
+                chooseRange rng >>= \case
+                  Just r -> pure
+                           . counterexample (show r)
+                           $ r `Interval.member` rng
+                  Nothing -> pure $ property Discard
+            )
+    , testProperty "nonEmpty range is never empty"
+      $ \ (lower :: Int) (NonZero diff) -> randomTest $ do
+        let upper = lower + diff
+        r <- chooseRange (Finite lower <=..<= Finite upper)
+        pure $ isJust r
+
+    ]
+  ]
+  where
+    randomTest prop = evalRandT prop . mkStdGen =<< arbitrary