tests: add custom Arbitrary (Maybe Text) that doesn't generate Just mempty

Author: sorki

hnix-store-remote/tests/NixSerializerSpec.hs

@@ -62,9 +62,7 @@ spec = parallel $ do
     prop "Bool" $ roundtripS bool
     prop "ByteString" $ roundtripS byteString
     prop "Text" $ roundtripS text
-    prop "Maybe Text"
-      $ forAll (arbitrary `suchThat` (/= Just ""))
-      $ roundtripS maybeText
+    prop "Maybe Text" $ roundtripS maybeText
     prop "UTCTime" $ roundtripS @UTCTime @() time
 
   describe "Combinators" $ do

hnix-store-tests/src/Data/Text/Arbitrary.hs

@@ -2,9 +2,15 @@
 module Data.Text.Arbitrary () where
 
 import Data.Text (Text)
-import Test.QuickCheck (Arbitrary(..))
+import Test.QuickCheck (Arbitrary(..), frequency, suchThat)
 import qualified Data.Text
 
 instance Arbitrary Text where
   arbitrary = Data.Text.pack <$> arbitrary
   shrink xs = Data.Text.pack <$> shrink (Data.Text.unpack xs)
+
+instance {-# OVERLAPPING #-} Arbitrary (Maybe Text) where
+  arbitrary = frequency
+    [ (1, pure Nothing)
+    , (3, Just <$> arbitrary `suchThat` (/= mempty))
+    ]