Add example usage of explicit generators

examples/generic-random-examples.cabal

@@ -13,3 +13,15 @@ executable generic-example
     QuickCheck,
     generic-random
   default-language: Haskell2010
+
+executable text-example
+  main-is: text.hs
+  ghc-options: -Wall
+  if impl(ghc < 7.10)
+    ghc-options: -fcontext-stack=30
+  build-depends:
+    base,
+    QuickCheck,
+    text,
+    generic-random
+  default-language: Haskell2010

examples/text.hs

@@ -0,0 +1,46 @@
+-- An example of generic Arbitrary instances with explicit generators
+-- for some types of fields for which there is no 'Arbitrary' instance
+-- or the existing one is unsatisfactory.
+--
+-- For example, Postgres can't handle strings containing NUL characters
+-- see https://github.com/lpsmith/postgresql-simple/issues/223
+-- so applications may want to generate data without them.
+
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE FlexibleInstances #-}
+
+import Data.Text (Text, pack, unpack)
+import GHC.Generics (Generic)
+import Test.QuickCheck
+
+import Generic.Random
+
+instance Arbitrary Text where
+  arbitrary = pack <$> arbitrary
+  shrink = fmap pack . shrink . unpack
+
+data R = R
+  { name :: Text
+  , address :: Text
+  } deriving (Show, Generic)
+
+instance Arbitrary R where
+  arbitrary = genericArbitrarySingleG gens
+    where
+      gens = (pack . filter (/= '\NUL') <$> arbitrary) :@ Nil
+
+  shrink = genericShrink
+
+newtype Bugged a = Bugged a deriving Show
+
+instance Arbitrary (Bugged R) where
+  arbitrary = Bugged <$> genericArbitrarySingle
+  shrink (Bugged r) = Bugged <$> shrink r
+
+main :: IO ()
+main = do
+  sample (arbitrary :: Gen R)
+  let prop_nameNullFree r = all (/= '\NUL') (unpack (name r))
+      qc prop = quickCheckWith stdArgs{maxSuccess = 1000} prop
+  qc prop_nameNullFree
+  qc $ expectFailure $ \(Bugged r) -> prop_nameNullFree r