diff --git a/src/Data/NestTuple/TH.hs b/src/Data/NestTuple/TH.hs
--- a/src/Data/NestTuple/TH.hs
+++ b/src/Data/NestTuple/TH.hs
@@ -51,7 +51,7 @@
     -> DecsQ
 mkNestableInstances n = return $ map nestableInstance [2..n]
   where
-    nestableInstance w = InstanceD
+    nestableInstance w = instD
         []
         (AppT (ConT (mkName "Nestable")) tupT)
         [ tySynInst (mkName "Nested") [tupT] (foldNest VarT mkPairT $ toNest vars)
diff --git a/src/Language/Syntactic/Functional/Sharing.hs b/src/Language/Syntactic/Functional/Sharing.hs
--- a/src/Language/Syntactic/Functional/Sharing.hs
+++ b/src/Language/Syntactic/Functional/Sharing.hs
@@ -273,6 +273,12 @@
     chooseEnvSub env (s :$ b) = chooseEnvSub env s `mplus` chooseEnv env b
     chooseEnvSub _ _ = Nothing
 
+-- If `codeMotionM` loops forever, the reason may be that `castExprCM` is
+-- broken. If `castExprCM` fails to cast even when it should, it means that
+-- we can get into situations where `substitute` returns the same expression
+-- unchanged. This in turn means that `codeMotionM` will loop, since it calls
+-- itself with `codeMotionM iface $ substitute iface b x a`.
+
 codeMotionM :: forall sym m a
     .  ( Equality sym
        , BindingDomain sym
diff --git a/src/Language/Syntactic/Functional/Tuple/TH.hs b/src/Language/Syntactic/Functional/Tuple/TH.hs
--- a/src/Language/Syntactic/Functional/Tuple/TH.hs
+++ b/src/Language/Syntactic/Functional/Tuple/TH.hs
@@ -55,7 +55,7 @@
 deriveSyntacticForTuples internalPred mkDomain extraConstraint n = return $
     map deriveSyntacticForTuple [3..n]
   where
-    deriveSyntacticForTuple w = InstanceD
+    deriveSyntacticForTuple w = instD
         ( concat
             [ map (classPred ''Syntactic ConT . return) varsT
             , concatMap internalPred $ map (AppT (ConT ''Internal)) varsT
diff --git a/src/Language/Syntactic/TH.hs b/src/Language/Syntactic/TH.hs
--- a/src/Language/Syntactic/TH.hs
+++ b/src/Language/Syntactic/TH.hs
@@ -5,6 +5,10 @@
 
 
 
+#if __GLASGOW_HASKELL__ < 710
+import Control.Applicative
+#endif
+
 import Language.Haskell.TH
 
 import Data.Hash (hashInt, combine)
@@ -20,6 +24,12 @@
 conName (RecC name args)    = (name, length args)
 conName (InfixC _ name _)   = (name, 2)
 conName (ForallC _ _ c)     = conName c
+#if __GLASGOW_HASKELL__ >= 800
+conName (GadtC [n] as _)    = (n, length as)
+conName (RecGadtC [n] as _) = (n, length as)
+  -- I don't know what it means when a `GadtC` and `RecGadtC` don't have
+  -- singleton lists of names
+#endif
 
 -- | Description of class methods
 data Method
@@ -40,9 +50,9 @@
     -> [Method]  -- ^ Methods
     -> DecsQ
 deriveClass cxt ty clHead methods = do
-    t@(TyConI (DataD _ _ _ cs _)) <- reify ty
+    Just cs <- viewDataDef <$> reify ty
     return
-      [ InstanceD cxt clHead $
+      [ instD cxt clHead $
           [ FunD method (clauses ++ extra)
             | MatchingMethod method mkClause extra <- methods
             , let clauses = [ mkClause c i nm ar | (i,c) <- zip [0..] cs
@@ -90,7 +100,7 @@
     :: Name  -- ^ Type name
     -> DecsQ
 deriveEquality ty = do
-    TyConI (DataD _ _ _ cs _) <- reify ty
+    Just cs <- viewDataDef <$> reify ty
     let equalFallThrough = if length cs > 1
           then [Clause [WildP, WildP] (NormalB $ ConE 'False) []]
           else []
@@ -180,6 +190,27 @@
 
 -- Using `__GLASGOW_HASKELL__` instead of `MIN_VERSION_template_haskell`,
 -- because the latter doesn't work when the package is compiled with `-f-th`.
+
+-- | Construct an instance declaration
+instD
+    :: Cxt    -- ^ Context
+    -> Type   -- ^ Instance
+    -> [Dec]  -- ^ Methods, etc.
+    -> Dec
+#if __GLASGOW_HASKELL__ >= 800
+instD = InstanceD Nothing
+#else
+instD = InstanceD
+#endif
+
+-- | Get the constructors of a data type definition
+viewDataDef :: Info -> Maybe [Con]
+#if __GLASGOW_HASKELL__ >= 800
+viewDataDef (TyConI (DataD _ _ _ _ cs _)) = Just cs
+#else
+viewDataDef (TyConI (DataD _ _ _ cs _)) = Just cs
+#endif
+viewDataDef _ = Nothing
 
 -- | Portable method for constructing a 'Pred' of the form @(t1 ~ t2)@
 eqPred :: Type -> Type -> Pred
diff --git a/syntactic.cabal b/syntactic.cabal
--- a/syntactic.cabal
+++ b/syntactic.cabal
@@ -1,5 +1,5 @@
 Name:           syntactic
-Version:        3.6
+Version:        3.6.1
 Synopsis:       Generic representation and manipulation of abstract syntax
 Description:    The library provides a generic representation of type-indexed abstract syntax trees
                 (or indexed data types in general). It also permits the definition of open syntax