grains: Change return type to Maybe

exercises/grains/Grains.hs

@@ -0,0 +1,5 @@
+module Grains (square, total) where
+
+square = undefined
+
+total = undefined

exercises/grains/example.hs

@@ -1,7 +1,12 @@
 module Grains (square, total) where
 
-square :: Int -> Integer
-square = (2 ^) . pred
+import Data.Maybe (fromJust)
+
+square :: Integer -> Maybe Integer
+square x
+    | x < 1     = Nothing
+    | x > 64    = Nothing
+    | otherwise = Just . (2^) . pred $ x
 
 total :: Integer
-total = pred $ 2 ^ (64 :: Int)
+total = sum . map (fromJust . square) $ [1..64]

exercises/grains/grains_test.hs

@@ -19,22 +19,31 @@ main = exitProperly $ runTestTT $ TestList
 i :: Integral a => a -> Integer
 i = fromIntegral
 
+mi :: Integral a => Maybe a -> Maybe Integer
+mi = fmap fromIntegral
+
 grainsTests :: [Test]
 grainsTests =
   [ testCase "square 1" $
-    1 @=? i (square 1)
+    Just 1 @=? mi (square 1)
   , testCase "square 2" $
-    2 @=? i (square 2)
+    Just 2 @=? mi (square 2)
   , testCase "square 3" $
-    4 @=? i (square 3)
+    Just 4 @=? mi (square 3)
   , testCase "square 4" $
-    8 @=? i (square 4)
+    Just 8 @=? mi (square 4)
   , testCase "square 16" $
-    32768 @=? i (square 16)
+    Just 32768 @=? mi (square 16)
   , testCase "square 32" $
-    2147483648 @=? i (square 32)
+    Just 2147483648 @=? mi (square 32)
   , testCase "square 64" $
-    9223372036854775808 @=? i (square 64)
+    Just 9223372036854775808 @=? mi (square 64)
+  , testCase "square negative" $
+    Nothing @=? mi (square (-1))
+  , testCase "square 0" $
+    Nothing @=? mi (square 0)
+  , testCase "square bigger than 64" $
+    Nothing @=? mi (square 65)
   , testCase "total grains" $
     18446744073709551615 @=? i total
   ]