1 files changed, 30 insertions, 0 deletions
diff --git a/testsuite/tests/deriving/should_run/drvrun003.hs b/testsuite/tests/deriving/should_run/drvrun003.hs
new file mode 100644
index 0000000000..bb7486159e
--- /dev/null
+++ b/testsuite/tests/deriving/should_run/drvrun003.hs
@@ -0,0 +1,30 @@
+-- !!! Deriving Show/Read for nullary constructors.
+module Main(main) where
+
+data A = B | C deriving ( Show, Read )
+
+data Opt = N | Y A deriving (Show, Read)
+
+x = Y B
+
+{-
+ If the Haskell report's specification of how Show instances
+ are to be derived is followed to the letter, the code for
+ a nullary constructor would put parens around the constructor
+ when (showsPrec 10) is used. This would cause
+
+      Y A
+
+ to be showed as
+ 
+      Y (A)
+
+ Overkill, so ghc's derived Show code treats nullary
+ constructors specially.
+-}
+
+main = do
+  print x
+  print ((read (show x))::Opt)
+  print ((read "Y (B)")::Opt)
+