diff --git a/Data/Derive/Arbitrary.hs b/Data/Derive/Arbitrary.hs
--- a/Data/Derive/Arbitrary.hs
+++ b/Data/Derive/Arbitrary.hs
@@ -14,6 +14,7 @@
                     return (Second x1 x2)
             2 -> do x1 <- arbitrary
                     return (Third x1)
+            _ -> error "FATAL ERROR: Arbitrary instance, logic bug"
 
 test :: State
 instance (CoArbitrary s, Arbitrary s, Arbitrary a) => Arbitrary (State s a) where
@@ -49,19 +50,25 @@
     "Symbol" (List [String "-"])])]),App "Lit" (List [App "Int" (List
     [Int 1])])])]])])]),App "Qualifier" (List [App "Case" (List [App
     "Var" (List [App "UnQual" (List [App "Ident" (List [String "x"])])
-    ]),MapCtor (App "Alt" (List [App "PLit" (List [App "Int" (List [
-    CtorIndex])]),App "UnGuardedAlt" (List [App "Do" (List [Concat (
-    List [MapField (App "Generator" (List [App "PVar" (List [App
-    "Ident" (List [Concat (List [String "x",ShowInt FieldIndex])])]),
-    App "Var" (List [App "UnQual" (List [App "Ident" (List [String
-    "arbitrary"])])])])),List [App "Qualifier" (List [App "App" (List
-    [App "Var" (List [App "UnQual" (List [App "Ident" (List [String
-    "return"])])]),App "Paren" (List [Application (Concat (List [List
-    [App "Con" (List [App "UnQual" (List [App "Ident" (List [CtorName]
-    )])])],MapField (App "Var" (List [App "UnQual" (List [App "Ident"
-    (List [Concat (List [String "x",ShowInt FieldIndex])])])]))]))])])
-    ])]])])]),App "BDecls" (List [List []])]))])])]])]),App "BDecls" (
-    List [List []])])])])]
+    ]),Concat (List [MapCtor (App "Alt" (List [App "PLit" (List [App
+    "Int" (List [CtorIndex])]),App "UnGuardedAlt" (List [App "Do" (
+    List [Concat (List [MapField (App "Generator" (List [App "PVar" (
+    List [App "Ident" (List [Concat (List [String "x",ShowInt
+    FieldIndex])])]),App "Var" (List [App "UnQual" (List [App "Ident"
+    (List [String "arbitrary"])])])])),List [App "Qualifier" (List [
+    App "App" (List [App "Var" (List [App "UnQual" (List [App "Ident"
+    (List [String "return"])])]),App "Paren" (List [Application (
+    Concat (List [List [App "Con" (List [App "UnQual" (List [App
+    "Ident" (List [CtorName])])])],MapField (App "Var" (List [App
+    "UnQual" (List [App "Ident" (List [Concat (List [String "x",
+    ShowInt FieldIndex])])])]))]))])])])]])])]),App "BDecls" (List [
+    List []])])),List [App "Alt" (List [App "PWildCard" (List []),App
+    "UnGuardedAlt" (List [App "App" (List [App "Var" (List [App
+    "UnQual" (List [App "Ident" (List [String "error"])])]),App "Lit"
+    (List [App "String" (List [String
+    "FATAL ERROR: Arbitrary instance, logic bug"])])])]),App "BDecls"
+    (List [List []])])]])])])]])]),App "BDecls" (List [List []])])])])
+    ]
 -- GENERATED STOP
 
 custom = customContext context
diff --git a/Data/Derive/Data.hs b/Data/Derive/Data.hs
--- a/Data/Derive/Data.hs
+++ b/Data/Derive/Data.hs
@@ -13,6 +13,7 @@
         0 -> z First
         1 -> const k 1 $ const k 2 $ z Second
         2 -> const k 1 $ z Third
+        i -> error $ ("Data.gunfold for Sample" ++ ", unknown index: ") ++ show i
 
     toConstr x@First{}  = indexConstr (dataTypeOf x) (0+1)
     toConstr x@Second{} = indexConstr (dataTypeOf x) (1+1)
@@ -32,6 +33,7 @@
     gunfold k z c = case constrIndex c - 1 of
         0 -> k $ k $ z Laptop
         1 -> k $ z Desktop
+        i -> error $ "Data.gunfold for Computer, unknown index: " ++ show i
     toConstr x@Laptop{} = indexConstr (dataTypeOf x) 1
     toConstr x@Desktop{} = indexConstr (dataTypeOf x) 2
     dataTypeOf _ = ty
@@ -76,37 +78,51 @@
     "constrIndex"])])]),App "Var" (List [App "UnQual" (List [App
     "Ident" (List [String "c"])])])]),App "QVarOp" (List [App "UnQual"
     (List [App "Symbol" (List [String "-"])])]),App "Lit" (List [App
-    "Int" (List [Int 1])])]),MapCtor (App "Alt" (List [App "PLit" (
-    List [App "Int" (List [CtorIndex])]),App "UnGuardedAlt" (List [
-    Fold (App "InfixApp" (List [Head,App "QVarOp" (List [App "UnQual"
-    (List [App "Symbol" (List [String "$"])])]),Tail])) (Concat (List
-    [MapField (Application (List [App "Var" (List [App "UnQual" (List
-    [App "Ident" (List [String "const"])])]),App "Var" (List [App
-    "UnQual" (List [App "Ident" (List [String "k"])])]),App "Lit" (
-    List [App "Int" (List [FieldIndex])])])),List [App "App" (List [
-    App "Var" (List [App "UnQual" (List [App "Ident" (List [String "z"
-    ])])]),App "Con" (List [App "UnQual" (List [App "Ident" (List [
-    CtorName])])])])]]))]),App "BDecls" (List [List []])]))])]),App
-    "BDecls" (List [List []])])]])]),App "InsDecl" (List [App
-    "FunBind" (List [MapCtor (App "Match" (List [App "Ident" (List [
-    String "toConstr"]),List [App "PAsPat" (List [App "Ident" (List [
-    String "x"]),App "PRec" (List [App "UnQual" (List [App "Ident" (
-    List [CtorName])]),List []])])],App "Nothing" (List []),App
-    "UnGuardedRhs" (List [Application (List [App "Var" (List [App
-    "UnQual" (List [App "Ident" (List [String "indexConstr"])])]),App
-    "Paren" (List [App "App" (List [App "Var" (List [App "UnQual" (
-    List [App "Ident" (List [String "dataTypeOf"])])]),App "Var" (List
-    [App "UnQual" (List [App "Ident" (List [String "x"])])])])]),App
-    "Paren" (List [App "InfixApp" (List [App "Lit" (List [App "Int" (
-    List [CtorIndex])]),App "QVarOp" (List [App "UnQual" (List [App
-    "Symbol" (List [String "+"])])]),App "Lit" (List [App "Int" (List
-    [Int 1])])])])])]),App "BDecls" (List [List []])]))])]),App
-    "InsDecl" (List [App "FunBind" (List [List [App "Match" (List [App
-    "Ident" (List [String "dataTypeOf"]),List [App "PWildCard" (List [
-    ])],App "Nothing" (List []),App "UnGuardedRhs" (List [App "Var" (
-    List [App "UnQual" (List [App "Ident" (List [String "ty"])])])]),
-    App "BDecls" (List [List [App "PatBind" (List [App "PVar" (List [
-    App "Ident" (List [String "ty"])]),App "Nothing" (List []),App
+    "Int" (List [Int 1])])]),Concat (List [MapCtor (App "Alt" (List [
+    App "PLit" (List [App "Int" (List [CtorIndex])]),App
+    "UnGuardedAlt" (List [Fold (App "InfixApp" (List [Head,App
+    "QVarOp" (List [App "UnQual" (List [App "Symbol" (List [String "$"
+    ])])]),Tail])) (Concat (List [MapField (Application (List [App
+    "Var" (List [App "UnQual" (List [App "Ident" (List [String "const"
+    ])])]),App "Var" (List [App "UnQual" (List [App "Ident" (List [
+    String "k"])])]),App "Lit" (List [App "Int" (List [FieldIndex])])]
+    )),List [App "App" (List [App "Var" (List [App "UnQual" (List [App
+    "Ident" (List [String "z"])])]),App "Con" (List [App "UnQual" (
+    List [App "Ident" (List [CtorName])])])])]]))]),App "BDecls" (List
+    [List []])])),List [App "Alt" (List [App "PVar" (List [App "Ident"
+    (List [String "i"])]),App "UnGuardedAlt" (List [App "InfixApp" (
+    List [App "Var" (List [App "UnQual" (List [App "Ident" (List [
+    String "error"])])]),App "QVarOp" (List [App "UnQual" (List [App
+    "Symbol" (List [String "$"])])]),App "InfixApp" (List [App "Paren"
+    (List [App "InfixApp" (List [App "Lit" (List [App "String" (List [
+    Concat (List [String "Data.gunfold for ",DataName])])]),App
+    "QVarOp" (List [App "UnQual" (List [App "Symbol" (List [String
+    "++"])])]),App "Lit" (List [App "String" (List [String
+    ", unknown index: "])])])]),App "QVarOp" (List [App "UnQual" (List
+    [App "Symbol" (List [String "++"])])]),App "App" (List [App "Var"
+    (List [App "UnQual" (List [App "Ident" (List [String "show"])])]),
+    App "Var" (List [App "UnQual" (List [App "Ident" (List [String "i"
+    ])])])])])])]),App "BDecls" (List [List []])])]])])]),App "BDecls"
+    (List [List []])])]])]),App "InsDecl" (List [App "FunBind" (List [
+    MapCtor (App "Match" (List [App "Ident" (List [String "toConstr"])
+    ,List [App "PAsPat" (List [App "Ident" (List [String "x"]),App
+    "PRec" (List [App "UnQual" (List [App "Ident" (List [CtorName])]),
+    List []])])],App "Nothing" (List []),App "UnGuardedRhs" (List [
+    Application (List [App "Var" (List [App "UnQual" (List [App
+    "Ident" (List [String "indexConstr"])])]),App "Paren" (List [App
+    "App" (List [App "Var" (List [App "UnQual" (List [App "Ident" (
+    List [String "dataTypeOf"])])]),App "Var" (List [App "UnQual" (
+    List [App "Ident" (List [String "x"])])])])]),App "Paren" (List [
+    App "InfixApp" (List [App "Lit" (List [App "Int" (List [CtorIndex]
+    )]),App "QVarOp" (List [App "UnQual" (List [App "Symbol" (List [
+    String "+"])])]),App "Lit" (List [App "Int" (List [Int 1])])])])])
+    ]),App "BDecls" (List [List []])]))])]),App "InsDecl" (List [App
+    "FunBind" (List [List [App "Match" (List [App "Ident" (List [
+    String "dataTypeOf"]),List [App "PWildCard" (List [])],App
+    "Nothing" (List []),App "UnGuardedRhs" (List [App "Var" (List [App
+    "UnQual" (List [App "Ident" (List [String "ty"])])])]),App
+    "BDecls" (List [List [App "PatBind" (List [App "PVar" (List [App
+    "Ident" (List [String "ty"])]),App "Nothing" (List []),App
     "UnGuardedRhs" (List [Application (List [App "Var" (List [App
     "UnQual" (List [App "Ident" (List [String "mkDataType"])])]),App
     "SpliceExp" (List [App "ParenSplice" (List [App "Var" (List [App
diff --git a/Data/Derive/Fold.hs b/Data/Derive/Fold.hs
--- a/Data/Derive/Fold.hs
+++ b/Data/Derive/Fold.hs
@@ -14,25 +14,25 @@
 test :: Computer
 
 foldComputer :: (Double -> Int -> a) -> (Int -> a) -> Computer -> a
-foldComputer f1 f2 (Laptop x1 x2) = f1 x1 x2
-foldComputer f1 f2 (Desktop x1) = f2 x1
+foldComputer f _ (Laptop x1 x2) = f x1 x2
+foldComputer _ f (Desktop x1) = f x1
 
 test :: Assoced
 
 foldAssoced :: (typ -> String -> a) -> Assoced typ -> a
-foldAssoced f1 (Assoced x1 x2) = f1 x1 x2
+foldAssoced f (Assoced x1 x2) = f x1 x2
 
 test :: Either
 
 foldEither :: (a -> c) -> (b -> c) -> Either a b -> c
-foldEither f1 f2 (Left x1) = f1 x1
-foldEither f1 f2 (Right x1) = f2 x1
+foldEither f _ (Left x1) = f x1
+foldEither _ f (Right x1) = f x1
 
 test :: Bool
 
 foldBool :: a -> a -> Bool -> a
-foldBool f1 f2 False = f1
-foldBool f1 f2 True = f2
+foldBool f _ False = f
+foldBool _ f True = f
 
 -}
 
@@ -47,13 +47,13 @@
 
 
 mkFold :: DataDecl -> [Decl]
-mkFold d = [TypeSig sl [name n] (foldType d), FunBind $ zipWith f funs $ dataDeclCtors d]
+mkFold d = [TypeSig sl [name n] (foldType d), FunBind $ zipWith f [0..] $ dataDeclCtors d]
     where
         n = "fold" ++ title (dataDeclName d)
-        funs = ['f' : show i | i <- [1..length (dataDeclCtors d)]]
-        f fun c = Match sl (name n) pat Nothing (UnGuardedRhs bod) (BDecls [])
-            where pat = map pVar funs ++ [PParen $ PApp (qname $ ctorDeclName c) (map pVar vars)]
-                  bod = apps (var fun) (map var vars)
+        f i c = Match sl (name n) pat Nothing (UnGuardedRhs bod) (BDecls [])
+            where pat = replicate i PWildCard ++ [pVar "f"] ++ replicate (length (dataDeclCtors d) - i - 1) PWildCard ++
+                        [PParen $ PApp (qname $ ctorDeclName c) (map pVar vars)]
+                  bod = apps (var "f") (map var vars)
                   vars = ['x' : show i | i <- [1..length (ctorDeclFields c)]]
 
 
diff --git a/Data/Derive/Foldable.hs b/Data/Derive/Foldable.hs
--- a/Data/Derive/Foldable.hs
+++ b/Data/Derive/Foldable.hs
@@ -16,19 +16,19 @@
 
 test :: FailList
 instance Foldable (FailList t1) where
-    foldr _f b Zero = b
-    foldr _f b (Fail a1) = b
+    foldr _  b Zero = b
+    foldr _  b (Fail _) = b
     foldr _f b (Const a1 a2) = _f a1 (Data.Foldable.foldr _f b a2)
 
 test :: Sample
 instance Foldable Sample where
-    foldr _f b First = b
+    foldr _  b First = b
     foldr _f b (Second a1 a2) = _f a1 (_f a2 b)
     foldr _f b (Third a1) = _f a1 b
 
 test :: Either
 instance Foldable (Either t1) where
-    foldr _f b (Left a1) = b
+    foldr _  b (Left _) = b
     foldr _f b (Right a1) = _f a1 b
 -}
 
diff --git a/Data/Derive/From.hs b/Data/Derive/From.hs
--- a/Data/Derive/From.hs
+++ b/Data/Derive/From.hs
@@ -16,12 +16,15 @@
 
 fromFirst :: Sample a -> ()
 fromFirst First = ()
+fromFirst _ = error "fromFirst failed, not a First"
 
 fromSecond :: Sample a -> (a, a)
 fromSecond (Second x1 x2) = (x1,x2)
+fromSecond _ = error "fromSecond failed, not a Second"
 
 fromThird :: Sample a -> a
 fromThird (Third x1) = x1
+fromThird _ = error "fromThird failed, not a Third"
 
 -}
 
@@ -34,7 +37,7 @@
 
 
 makeFromCtor :: DataDecl -> CtorDecl -> [Decl]
-makeFromCtor d c = [TypeSig sl [name from] typ, FunBind [match]]
+makeFromCtor d c = [TypeSig sl [name from] typ, FunBind $ match : [defMatch | length (dataDeclCtors d) > 1]]
     where
         n = ctorDeclName c
         from = "from" ++ n
@@ -46,6 +49,9 @@
         pat = (length vars == 0 ? id $ PParen) $ PApp (qname n) (map pVar vars)
         vars = take (length $ ctorDeclFields c) $ map ((:) 'x' . show) [1..]
         rhs = valTuple $ map var vars
+
+        defMatch = Match sl (name from) [PWildCard] Nothing (UnGuardedRhs err) (BDecls [])
+        err = App (var "error") $ Lit $ String $ from ++ " failed, not a " ++ n
 
 
 tyTuple [] = TyCon $ Special UnitCon
diff --git a/Data/Derive/Functor.hs b/Data/Derive/Functor.hs
--- a/Data/Derive/Functor.hs
+++ b/Data/Derive/Functor.hs
@@ -19,8 +19,8 @@
 
 test :: FailList
 instance Functor (FailList t1) where
-    fmap _f Zero = Zero
-    fmap _f (Fail a1) = Fail a1
+    fmap _  Zero = Zero
+    fmap _  (Fail a1) = Fail a1
     fmap _f (Const a1 a2) = Const (_f a1) (fmap _f a2)
 
 test :: State
@@ -29,7 +29,7 @@
 
 test :: Sample
 instance Functor Sample where
-    fmap _f First = First
+    fmap _  First = First
     fmap _f (Second a1 a2) = Second (_f a1) (_f a2)
     fmap _f (Third a1) = Third (_f a1)
 -}
diff --git a/Data/Derive/Internal/Derivation.hs b/Data/Derive/Internal/Derivation.hs
--- a/Data/Derive/Internal/Derivation.hs
+++ b/Data/Derive/Internal/Derivation.hs
@@ -16,14 +16,14 @@
 
 
 derivationDSL :: String -> DSL -> Derivation
-derivationDSL name dsl = Derivation name $ applyDSL dsl . snd
+derivationDSL name dsl = derivationCustomDSL name (const id) dsl
 
 
 derivationCustomDSL :: String -> (FullDataDecl -> [Decl] -> [Decl]) -> DSL -> Derivation
 derivationCustomDSL name custom dsl = Derivation name $
     \d -> case applyDSL dsl $ snd d of
               Left x -> Left x
-              Right x -> Right $ custom d x
+              Right x -> Right $ simplify $ custom d x
 
 
 customSplice :: (FullDataDecl -> Exp -> Exp) -> (FullDataDecl -> [Decl] -> [Decl])
diff --git a/Data/Derive/Is.hs b/Data/Derive/Is.hs
--- a/Data/Derive/Is.hs
+++ b/Data/Derive/Is.hs
@@ -1,37 +1,33 @@
-module Data.Derive.Is where
+module Data.Derive.Is(makeIs) where
 {-
 
-example :: Sample
+test :: Sample
 
-isFirst  (First {}) = True; isFirst  _ | length [First{},Second{},Third{}] > 1 = False
-isSecond (Second{}) = True; isSecond _ | length [First{},Second{},Third{}] > 1 = False
-isThird  (Third {}) = True; isThird  _ | length [First{},Second{},Third{}] > 1 = False
+isFirst :: Sample a -> Bool
+isFirst (First{}) = True ; isFirst _ = False
 
+isSecond :: Sample a -> Bool
+isSecond (Second{}) = True ; isSecond _ = False
+
+isThird :: Sample a -> Bool
+isThird (Third{}) = True ; isThird _ = False
+
 -}
--- GENERATED START
 
-import Data.Derive.DSL.DSL
+import Language.Haskell
 import Data.Derive.Internal.Derivation
 
+
 makeIs :: Derivation
-makeIs = derivationDSL "Is" dslIs
+makeIs = Derivation "Is" $ \(_,d) -> Right $ concatMap (makeIsCtor d) $ dataDeclCtors d
 
-dslIs =
-    MapCtor (App "FunBind" (List [List [App "Match" (List [App "Ident"
-    (List [Concat (List [String "is",CtorName])]),List [App "PParen" (
-    List [App "PRec" (List [App "UnQual" (List [App "Ident" (List [
-    CtorName])]),List []])])],App "Nothing" (List []),App
-    "UnGuardedRhs" (List [App "Con" (List [App "UnQual" (List [App
-    "Ident" (List [String "True"])])])]),App "BDecls" (List [List []])
-    ]),App "Match" (List [App "Ident" (List [Concat (List [String "is"
-    ,CtorName])]),List [App "PWildCard" (List [])],App "Nothing" (List
-    []),App "GuardedRhss" (List [List [App "GuardedRhs" (List [List [
-    App "Qualifier" (List [App "InfixApp" (List [App "App" (List [App
-    "Var" (List [App "UnQual" (List [App "Ident" (List [String
-    "length"])])]),App "List" (List [MapCtor (App "RecConstr" (List [
-    App "UnQual" (List [App "Ident" (List [CtorName])]),List []]))])])
-    ,App "QVarOp" (List [App "UnQual" (List [App "Symbol" (List [
-    String ">"])])]),App "Lit" (List [App "Int" (List [Int 1])])])])],
-    App "Con" (List [App "UnQual" (List [App "Ident" (List [String
-    "False"])])])])]]),App "BDecls" (List [List []])])]]))
--- GENERATED STOP
+
+makeIsCtor :: DataDecl -> CtorDecl -> [Decl]
+makeIsCtor d c =
+        [TypeSig sl [name nam] (dataDeclType d `TyFun` tyCon "Bool")
+        ,FunBind $ match : [defMatch | length (dataDeclCtors d) > 1]]
+    where
+        nam = "is" ++ ctorDeclName c
+        
+        match = Match sl (name nam) [PParen $ PRec (qname $ ctorDeclName c) []] Nothing (UnGuardedRhs $ con "True") (BDecls [])
+        defMatch = Match sl (name nam) [PWildCard] Nothing (UnGuardedRhs $ con "False") (BDecls [])
diff --git a/Data/Derive/Monoid.hs b/Data/Derive/Monoid.hs
--- a/Data/Derive/Monoid.hs
+++ b/Data/Derive/Monoid.hs
@@ -13,6 +13,7 @@
     mappend (First) (First) = First
     mappend (Second x1 x2) (Second y1 y2) = Second (mappend x1 y1) (mappend x2 y2)
     mappend (Third x1) (Third y1) = Third (mappend x1 y1)
+    mappend _ _ | length [First{},Second{},Third{}] > 1 = error "Monoid.mappend: Different constructors for Sample"
 
 -}
 -- GENERATED START
@@ -35,21 +36,34 @@
     "Ident" (List [String "const"])])]),App "Var" (List [App "UnQual"
     (List [App "Ident" (List [String "mempty"])])]),App "Lit" (List [
     App "Int" (List [FieldIndex])])])]))])))])])]),App "BDecls" (List
-    [List []])])]),App "InsDecl" (List [App "FunBind" (List [MapCtor (
-    App "Match" (List [App "Ident" (List [String "mappend"]),List [App
-    "PParen" (List [App "PApp" (List [App "UnQual" (List [App "Ident"
-    (List [CtorName])]),MapField (App "PVar" (List [App "Ident" (List
-    [Concat (List [String "x",ShowInt FieldIndex])])]))])]),App
-    "PParen" (List [App "PApp" (List [App "UnQual" (List [App "Ident"
-    (List [CtorName])]),MapField (App "PVar" (List [App "Ident" (List
-    [Concat (List [String "y",ShowInt FieldIndex])])]))])])],App
-    "Nothing" (List []),App "UnGuardedRhs" (List [Application (Concat
-    (List [List [App "Con" (List [App "UnQual" (List [App "Ident" (
-    List [CtorName])])])],MapField (App "Paren" (List [Application (
-    List [App "Var" (List [App "UnQual" (List [App "Ident" (List [
-    String "mappend"])])]),App "Var" (List [App "UnQual" (List [App
-    "Ident" (List [Concat (List [String "x",ShowInt FieldIndex])])])])
-    ,App "Var" (List [App "UnQual" (List [App "Ident" (List [Concat (
-    List [String "y",ShowInt FieldIndex])])])])])]))]))]),App "BDecls"
-    (List [List []])]))])])])]
+    [List []])])]),App "InsDecl" (List [App "FunBind" (List [Concat (
+    List [MapCtor (App "Match" (List [App "Ident" (List [String
+    "mappend"]),List [App "PParen" (List [App "PApp" (List [App
+    "UnQual" (List [App "Ident" (List [CtorName])]),MapField (App
+    "PVar" (List [App "Ident" (List [Concat (List [String "x",ShowInt
+    FieldIndex])])]))])]),App "PParen" (List [App "PApp" (List [App
+    "UnQual" (List [App "Ident" (List [CtorName])]),MapField (App
+    "PVar" (List [App "Ident" (List [Concat (List [String "y",ShowInt
+    FieldIndex])])]))])])],App "Nothing" (List []),App "UnGuardedRhs"
+    (List [Application (Concat (List [List [App "Con" (List [App
+    "UnQual" (List [App "Ident" (List [CtorName])])])],MapField (App
+    "Paren" (List [Application (List [App "Var" (List [App "UnQual" (
+    List [App "Ident" (List [String "mappend"])])]),App "Var" (List [
+    App "UnQual" (List [App "Ident" (List [Concat (List [String "x",
+    ShowInt FieldIndex])])])]),App "Var" (List [App "UnQual" (List [
+    App "Ident" (List [Concat (List [String "y",ShowInt FieldIndex])])
+    ])])])]))]))]),App "BDecls" (List [List []])])),List [App "Match"
+    (List [App "Ident" (List [String "mappend"]),List [App "PWildCard"
+    (List []),App "PWildCard" (List [])],App "Nothing" (List []),App
+    "GuardedRhss" (List [List [App "GuardedRhs" (List [List [App
+    "Qualifier" (List [App "InfixApp" (List [App "App" (List [App
+    "Var" (List [App "UnQual" (List [App "Ident" (List [String
+    "length"])])]),App "List" (List [MapCtor (App "RecConstr" (List [
+    App "UnQual" (List [App "Ident" (List [CtorName])]),List []]))])])
+    ,App "QVarOp" (List [App "UnQual" (List [App "Symbol" (List [
+    String ">"])])]),App "Lit" (List [App "Int" (List [Int 1])])])])],
+    App "App" (List [App "Var" (List [App "UnQual" (List [App "Ident"
+    (List [String "error"])])]),App "Lit" (List [App "String" (List [
+    Concat (List [String "Monoid.mappend: Different constructors for "
+    ,DataName])])])])])]]),App "BDecls" (List [List []])])]])])])])]
 -- GENERATED STOP
diff --git a/Data/Derive/NFData.hs b/Data/Derive/NFData.hs
--- a/Data/Derive/NFData.hs
+++ b/Data/Derive/NFData.hs
@@ -1,6 +1,6 @@
 module Data.Derive.NFData where
 {-
-import "parallel" Control.Parallel.Strategies
+import "parallel" Control.Parallel.Strategies(NFData, rnf)
 
 example :: Sample
 instance NFData a => NFData (Sample a) where
diff --git a/Data/Derive/Ord.hs b/Data/Derive/Ord.hs
--- a/Data/Derive/Ord.hs
+++ b/Data/Derive/Ord.hs
@@ -14,9 +14,9 @@
             EQ & x = x
             x & _ = x
 
-            tag (First{}) = 0
-            tag (Second{}) = 1
-            tag (Third{}) = 2
+            tag (First{}) = 0 :: Int
+            tag (Second{}) = 1 :: Int
+            tag (Third{}) = 2 :: Int
 
 -}
 -- GENERATED START
@@ -87,6 +87,7 @@
     App "Ident" (List [String "tag"]),List [App "PParen" (List [App
     "PRec" (List [App "UnQual" (List [App "Ident" (List [CtorName])]),
     List []])])],App "Nothing" (List []),App "UnGuardedRhs" (List [App
-    "Lit" (List [App "Int" (List [CtorIndex])])]),App "BDecls" (List [
-    List []])]))])]])])]])])])]
+    "ExpTypeSig" (List [App "Lit" (List [App "Int" (List [CtorIndex])]
+    ),App "TyCon" (List [App "UnQual" (List [App "Ident" (List [String
+    "Int"])])])])]),App "BDecls" (List [List []])]))])]])])]])])])]
 -- GENERATED STOP
diff --git a/Data/Derive/Read.hs b/Data/Derive/Read.hs
--- a/Data/Derive/Read.hs
+++ b/Data/Derive/Read.hs
@@ -38,7 +38,7 @@
 
 test :: Computer
 instance Read Computer where
-    readsPrec p0 r =
+    readsPrec _ r =
         readParen False (\r0 ->
             [ (Laptop x1 x2, r10)
             | ("Laptop", r1) <- lex r0
diff --git a/Data/Derive/Show.hs b/Data/Derive/Show.hs
--- a/Data/Derive/Show.hs
+++ b/Data/Derive/Show.hs
@@ -18,15 +18,15 @@
 
 test :: Sample
 instance Show a => Show (Sample a) where
-    showsPrec p First = showString "First"
+    showsPrec _ First = showString "First"
     showsPrec p (Second x1 x2) = showParen (p > 10) $ showString "Second " . showsPrec 11 x1 . showChar ' ' . showsPrec 11 x2
     showsPrec p (Third x1) = showParen (p > 10) $ showString "Third " . showsPrec 11 x1
 
 test :: Computer
 instance Show Computer where
-    showsPrec p (Laptop x1 x2) =
+    showsPrec _ (Laptop x1 x2) =
         showString "Laptop {weight = " . showsPrec 0 x1 . showString ", speed = " . showsPrec 0 x2 . showChar '}'
-    showsPrec p (Desktop x1) =
+    showsPrec _ (Desktop x1) =
         showString "Desktop {speed = " . showsPrec 0 x1 . showChar '}'
 
 test :: (:*:)
diff --git a/Data/DeriveDSL.hs b/Data/DeriveDSL.hs
--- a/Data/DeriveDSL.hs
+++ b/Data/DeriveDSL.hs
@@ -15,7 +15,7 @@
 
 
 applyDSL :: DSL -> DataDecl -> Either String [Decl]
-applyDSL dsl inp = Right $ simplify $ apply dsl $ toInput inp
+applyDSL dsl inp = Right $ apply dsl $ toInput inp
 
 
 dynamicDSL :: DSL -> Maybe [Decl]
diff --git a/Derive/Test.hs b/Derive/Test.hs
--- a/Derive/Test.hs
+++ b/Derive/Test.hs
@@ -68,11 +68,12 @@
 autoTest :: [Src] -> [DataDecl] -> [Derivation] -> [String]
 autoTest ss ts ds =
     ["{-# LANGUAGE TemplateHaskell,FlexibleInstances,MultiParamTypeClasses #-}"
-    ,"{-# OPTIONS_GHC -fno-warn-missing-fields #-}"
+    ,"{-# OPTIONS_GHC -Wall -fno-warn-missing-fields #-}"
     ,"import Data.DeriveTH"
-    ,"import Derive.TestInstances"] ++
+    ,"import Derive.TestInstances()"] ++
     [prettyPrint i | s <- ss2, i <- srcImportStd s] ++
-    ["main = putStrLn \"Type checking successful\""] ++
+    ["main :: IO ()"
+    ,"main = putStrLn \"Type checking successful\""] ++
     [prettyPrint t | t <- ts2] ++
     ["$(derives [make" ++ derivationName d ++ "] " ++ types ++ ")" | d <- ds2]
     where
diff --git a/Language/Haskell.hs b/Language/Haskell.hs
--- a/Language/Haskell.hs
+++ b/Language/Haskell.hs
@@ -47,6 +47,7 @@
             | op ~= "&&" && y ~= "True" = x
             | x ~= "id" && op ~= "." = y
             | y ~= "id" && op ~= "." = x
+        fExp (InfixApp (Lit (String x)) op (Lit (String y))) | op ~= "++" = Lit $ String $ x ++ y
         fExp (App (App (App flp f) x) y) | flp ~= "flip" = fExp $ appP (fExp $ appP f y) x        
         fExp (App (Paren x@App{}) y) = fExp $ App x y
         fExp (App (Paren (InfixApp x op y)) z) | op ~= "." = fExp $ appP x $ fExp $ appP y z
@@ -71,10 +72,11 @@
             | x ~= "True" = t
             | x ~= "False" = f
         fExp (App (App when b) x)
-            | b ~= "True" = x
-            | b ~= "False" = App (Var $ UnQual $ Ident "return") (Con $ Special $ TupleCon Boxed 0)
+            | when ~= "when" && b ~= "True" = x
+            | when ~= "when" && b ~= "False" = App (Var $ UnQual $ Ident "return") (Con $ Special $ TupleCon Boxed 0)
         fExp (App (Paren (Lambda _ [PVar x] y)) z) | once x2 y = fExp $ subst x2 z y
             where x2 = Var $ UnQual x
+        fExp (App (Paren (Lambda _ [PWildCard] x)) _) = x
         fExp x = x
 
         fTyp (TyApp x y) | x ~= "[]" = TyApp (TyCon (Special ListCon)) y
@@ -92,7 +94,11 @@
             | x ~= "True" = fMatch $ Match sl nam pat sig (UnGuardedRhs bod) decls
         fMatch (Match sl nam [PVar x] sig (UnGuardedRhs (Case (Var (UnQual x2)) [Alt _ pat (UnGuardedAlt y) (BDecls [])])) decls)
             | x == x2 = fMatch $ Match sl nam [PParen pat] sig (UnGuardedRhs y) decls
-        fMatch (Match a b c d e bind) = fBinds (Match a b c d e) bind
+        fMatch o@(Match a b c d e bind) = fBinds (Match a b (transformBi f c) d e) bind
+            where
+                known = nub [x | UnQual x <- universeBi o]
+                f (PVar x) | x `notElem` known = PWildCard
+                f x = x
 
         fDecl (PatBind a b c d bind) = fBinds (PatBind a b c d) bind
         fDecl (FunBind xs) = FunBind $ filter (not . isGuardFalse) xs
diff --git a/derive.cabal b/derive.cabal
--- a/derive.cabal
+++ b/derive.cabal
@@ -1,7 +1,7 @@
 Cabal-Version:  >= 1.6
 Build-Type:     Default
 Name:           derive
-Version:        2.0.0
+Version:        2.0.1
 build-type:     Simple
 Copyright:      2006-2009, Neil Mitchell
 Maintainer:     ndmitchell@gmail.com