[project @ 2004-02-26 18:06:51 by ralf]

Simplified kind-polymorphic boilerplate stuff.

diff --git a/Data/Generics/Aliases.hs b/Data/Generics/Aliases.hs
index 3c03f56..3c66298 100644 (file)
--- a/Data/Generics/Aliases.hs
+++ b/Data/Generics/Aliases.hs
@@ -336,6 +336,15 @@ gunfoldR c f = gmapM (const f) $ fromConstr c
 ------------------------------------------------------------------------------
 
 
+
+-- | Flexible type extension
+ext1 :: (Data a, Typeable1 t)
+     => c a
+     -> (forall a. Data a => c (t a))
+     -> c a
+ext1 def ext = maybe def id (cast0to1 ext)
+
+
 -- | Type extension of transformations for unary type constructors
 ext1T :: (Data d, Typeable1 t)
       => (forall d. Data d => d -> d)

diff --git a/Data/Generics/Basics.hs b/Data/Generics/Basics.hs
index e7c8c76..89738d3 100644 (file)
--- a/Data/Generics/Basics.hs
+++ b/Data/Generics/Basics.hs
@@ -25,8 +25,8 @@ module Data.Generics.Basics (
                toConstr,       -- :: a -> Constr
                fromConstr,     -- :: Constr -> a
                dataTypeOf,     -- :: a -> DataType
-               ext1,           -- type extension for unary type constructors
-               ext2            -- type extension for binary type constructors
+               cast0to1,       -- mediate types and unary type constructors
+               cast0to2        -- mediate types and binary type constructors
             ),
 
        -- * Constructor representations
@@ -145,25 +145,23 @@ fold.
 
 ------------------------------------------------------------------------------
 --
--- Type extension for unary and binary type constructors
+-- Mediate types and type constructors
 --
 ------------------------------------------------------------------------------
 
-  -- | Type extension for unary type constructors
-  ext1 :: Typeable1 t
-       => c a
-       -> (forall a. Data a => c (t a))
-       -> c a
+  -- | Mediate types and unary type constructors
+  cast0to1 :: Typeable1 t
+           => (forall a. Data a => c (t a))
+           -> Maybe (c a)
+  cast0to1 _ = Nothing
 
-  ext1 def ext = def
+  -- | Mediate types and binary type constructors
+  cast0to2 :: Typeable2 t
+           => (forall a b. (Data a, Data b) => c (t a b))
+           -> Maybe (c a)
+  cast0to2 _ = Nothing
 
 
-  -- | Type extension for binary type constructors
-  ext2 :: Typeable2 t
-       => c a
-       -> (forall a b. (Data a, Data b) => c (t a b)) -> c a
-  ext2 def ext = def
-
 
 ------------------------------------------------------------------------------
 --
@@ -578,8 +576,7 @@ instance Data a => Data [a] where
                    1 -> []
                    2 -> undefined:undefined
   dataTypeOf _ = listDataType
-  ext1 def ext = maybe def id (cast1 ext)
-
+  cast0to1     = cast1
 
 --
 -- The gmaps are given as an illustration.
@@ -611,7 +608,7 @@ instance Data a => Data (Maybe a) where
                    1 -> Nothing
                    2 -> Just undefined
   dataTypeOf _ = maybeDataType
-  ext1 def ext = maybe def id (cast1 ext)
+  cast0to1     = cast1
 
 
 --
@@ -628,7 +625,7 @@ instance (Data a, Data b) => Data (a,b) where
   fromConstr c = case conIndex c of
                    1 -> (undefined,undefined)
   dataTypeOf _ = productDataType
-  ext2 def ext = maybe def id (cast2 ext)
+  cast0to2     = cast2
 
 
 --
@@ -675,7 +672,7 @@ instance (Data a, Data b) => Data (Either a b) where
                    1 -> Left undefined
                    2 -> Right undefined
   dataTypeOf _ = eitherDataType
-  ext2 def ext = maybe def id (cast2 ext)
+  cast0to2     = cast2
 
 
 {-
@@ -703,4 +700,4 @@ instance (Data a, Data b) => Data (a -> b) where
   toConstr _   = FunConstr
   fromConstr _ = undefined
   dataTypeOf _ = FunType
-  ext2 def ext = maybe def id (cast2 ext)
+  cast0to2     = cast2