Cover our utility functons with unit tests

flake.nix

@@ -67,6 +67,7 @@
             nixfmt
             hpack
             hlint
+            haskellPackages.doctest
           ];
           inputsFrom = builtins.attrValues self.packages.${system};
         };

src/HCat/Internal.hs

@@ -1,8 +1,30 @@
 -- | Internal module in order to facilitate testability.
 module HCat.Internal where
 
+-- | @parseArgs@ takes a list of strings and returns a single FilePath if there was exactly one element.
+--
+-- >>> parseArgs ["foo"]
+-- Right "foo"
 parseArgs :: [String] -> Either String FilePath
 parseArgs args = case args of
     [] -> Left "No filename given!"
     [arg] -> Right arg
     _ -> Left "Only a single file is supported"
+
+-- | @chunksOf n@ splits a list into chunks of at most @n@ items.
+--
+-- >>> chunksOf 3 "abcdefgh"
+-- ["abc","def","gh"]
+--
+-- >>> chunksOf 0 "abcdefgh"
+-- []
+--
+-- >>> chunksOf (-1) "abcdefgh"
+-- []
+chunksOf :: Int -> [a] -> [[a]]
+chunksOf _ [] = []
+chunksOf n xs@(_ : _)
+    | n <= 0 = []
+    | otherwise =
+        let (chunk, rest) = splitAt n xs
+         in chunk : chunksOf n rest

test/HCatSpec.hs

@@ -2,9 +2,11 @@
 
 module HCatSpec (spec) where
 
-import HCat.Internal (parseArgs)
+import HCat.Internal (chunksOf, parseArgs)
 
 import Test.Hspec
+import Test.Hspec.QuickCheck (prop)
+import Test.QuickCheck (Positive (getPositive))
 
 spec :: Spec
 spec = do
@@ -13,3 +15,13 @@ spec = do
       parseArgs ["foo"] `shouldBe` Right "foo"
       parseArgs [] `shouldBe` Left "No filename given!"
       parseArgs ["foo", "bar"] `shouldBe` Left "Only a single file is supported"
+  describe "chunksOf" $ do
+    prop "each chunk contains at most N items" $ \n xs ->
+      let chunkLengths = length <$> chunksOf n (xs :: [Int])
+       in case chunkLengths of
+            [] -> pure ()
+            ns -> maximum ns `shouldSatisfy` (<= n)
+
+    prop "the sum of all lengths is equal to the length of the input" $ \n xs ->
+      let chunkLengths = length <$> chunksOf (getPositive n) (xs :: [Int])
+       in sum chunkLengths `shouldBe` length xs