1 files changed, 79 insertions, 0 deletions
diff --git a/testsuite/tests/gadt/karl1.hs b/testsuite/tests/gadt/karl1.hs
new file mode 100644
index 0000000000..e3af7adb1d
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+++ b/testsuite/tests/gadt/karl1.hs
@@ -0,0 +1,79 @@
+{-# LANGUAGE GADTs, KindSignatures #-}
+
+-- See Trac #301
+-- This particular one doesn't use GADTs per se, 
+-- but it does use dictionaries in constructors 
+
+module Expr1 where
+
+data Expr :: * -> * where	-- Not a GADT at all
+  Const :: Show a => a -> Expr a
+		-- Note the Show constraint here
+  Var   :: Var a -> Expr a
+
+newtype Var a = V String
+
+instance Show (Var a) where show (V s) = s
+
+--------------------------
+e1 :: Expr Int
+e1 = Const 42
+
+e2 :: Expr Bool
+e2 = Const True
+
+e3 :: Expr Integer
+e3 = Var (V "mersenne100")
+
+--------------------------
+eval :: Expr a -> a
+eval (Const c) = c
+eval (Var v) = error ("free variable `" ++ shows v "'")
+
+{-
+    Up to here, everything works nicely:
+
+    \begin{verbatim}
+    *Expr0> eval e1
+    42
+    *Expr0> eval e2
+    True
+    *Expr1> eval e3
+    *** Exception: free variable `mersenne100'
+    \end{verbatim}
+
+    But let us now try to define a |shows| function.
+
+    In the following, without the type signature we get:
+    \begin{verbatim}
+    *Expr1> :t showsExpr
+    showsExpr :: forall a. (Show a) => Expr a -> String -> String
+    *Expr1> showsExpr e1 ""
+    "42"
+    *Expr1> showsExpr e2 ""
+    "True"
+    *Expr1> showsExpr e3 ""
+    "mersenne100"
+    \end{verbatim}
+
+    However, in the last case, the instance |Show Integer| was not used,
+    so should not have been required.
+    Therefore I would expect it to work as it is now, i.e.,
+    with the type signature:
+-}
+
+showsExpr :: Expr a -> ShowS
+showsExpr (Const c) = shows c
+showsExpr (Var v) = shows v
+
+{- 
+
+We used to get a complaint about the |Const| alternative (then line
+63) that documents that the constraint in the type of |Const| must
+have been ignored:
+
+    No instance for (Show a)
+      arising from use of `shows' at Expr1.lhs:63:22-26
+    Probable fix: add (Show a) to the type signature(s) for `showsExpr'
+    In the definition of `showsExpr': showsExpr (Const c) = shows c
+-}