[project @ 2005-07-19 17:33:23 by ross]

improved definition of gfold for Seq a

diff --git a/Data/Sequence.hs b/Data/Sequence.hs
index 6ffded0..7ff4403 100644 (file)
--- a/Data/Sequence.hs
+++ b/Data/Sequence.hs
@@ -143,10 +143,18 @@ INSTANCE_TYPEABLE1(Seq,seqTc,"Seq")
 
 #if __GLASGOW_HASKELL__
 instance Data a => Data (Seq a) where
-       gfoldl f z xs   = z fromList `f` toList xs
+       gfoldl f z      = gfoldSeq f z id
        toConstr _      = error "toConstr"
        gunfold _ _     = error "gunfold"
        dataTypeOf _    = mkNorepType "Data.Sequence.Seq"
+
+-- Treat the type as consisting of constructors of arity 0, 1, 2, ...
+gfoldSeq :: Data a => (forall a b. Data a => c (a -> b) -> a -> c b) ->
+       (forall g. g -> c g) -> (Seq a -> r) -> Seq a -> c r
+gfoldSeq f z k s = case viewr s of
+       EmptyR -> z (k empty)
+       xs :> x -> gfoldSeq f z (snoc k) xs `f` x
+  where        snoc k xs x = k (xs |> x)
 #endif
 
 -- Finger trees