2 files changed, 58 insertions, 1 deletions

diff --git a/lib/Data/Packed/Vector.hs b/lib/Data/Packed/Vector.hs
index eaf4b9c..23fe37f 100644
--- a/lib/Data/Packed/Vector.hs
+++ b/lib/Data/Packed/Vector.hs
@@ -22,7 +22,8 @@ module Data.Packed.Vector (
     subVector, takesV, join,
     mapVector, zipVector, zipVectorWith, unzipVector, unzipVectorWith,
     mapVectorM, mapVectorM_, mapVectorWithIndexM, mapVectorWithIndexM_,
-    foldLoop, foldVector, foldVectorG, foldVectorWithIndex
+    foldLoop, foldVector, foldVectorG, foldVectorWithIndex,
+    successive_, successive
 ) where
 
 import Data.Packed.Internal.Vector
@@ -82,4 +83,54 @@ zipVector = zipVectorWith (,)
 unzipVector :: (Storable a, Storable b, Storable (a,b)) => Vector (a,b) -> (Vector a,Vector b)
 unzipVector = unzipVectorWith id
 
+-------------------------------------------------------------------
+
+newtype State s a = State { runState :: s -> (a,s) }
+
+instance Monad (State s) where
+    return a = State $ \s -> (a,s)
+    m >>= f = State $ \s -> let (a,s') = runState m s
+                            in runState (f a) s'
+
+state_get :: State s s
+state_get = State $ \s -> (s,s)
+
+state_put :: s -> State s ()
+state_put s = State $ \_ -> ((),s)
+
+evalState :: State s a -> s -> a
+evalState m s = fst $ runState m s
+
+newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
+
+instance Monad m => Monad (MaybeT m) where
+    return a = MaybeT $ return $ Just a
+    m >>= f  = MaybeT $ do
+                        res <- runMaybeT m
+                        case res of
+                                 Nothing -> return Nothing
+                                 Just r  -> runMaybeT (f r)
+    fail _   = MaybeT $ return Nothing
+
+lift_maybe m = MaybeT $ do
+                        res <- m
+                        return $ Just res
+
+-- | apply a test to successive elements of a vector, evaluates to true iff test passes for all pairs
+successive_ :: Storable a => (a -> a -> Bool) -> Vector a -> Bool
+successive_ t v = maybe False (\_ -> True) $ evalState (runMaybeT (mapVectorM_ step (subVector 1 (dim v - 1) v))) (v @> 0)
+   where step e = do
+                  ep <- lift_maybe $ state_get
+                  if t e ep
+                     then lift_maybe $ state_put e
+                     else (fail "successive_ test failed")
+
+-- | operate on successive elements of a vector and return the resulting vector, whose length 1 less than that of the input
+successive :: (Storable a, Storable b) => (a -> a -> b) -> Vector a -> Vector b
+successive f v = evalState (mapVectorM step (subVector 1 (dim v - 1) v)) (v @> 0)
+   where step e = do
+                  ep <- state_get
+                  state_put e
+                  return $ f ep e
 
+-------------------------------------------------------------------
diff --git a/lib/Numeric/LinearAlgebra/Tests.hs b/lib/Numeric/LinearAlgebra/Tests.hs
index 0df29a8..a44c273 100644
--- a/lib/Numeric/LinearAlgebra/Tests.hs
+++ b/lib/Numeric/LinearAlgebra/Tests.hs
@@ -300,6 +300,11 @@ conjuTest m = mapVector conjugate (flatten (trans m)) == flatten (ctrans m)
 
 ---------------------------------------------------------------------
 
+succTest = utest "successive" $ successive_ (<) (fromList [1 :: Double,2,3,4]) == True 
+                                && successive_ (<) (fromList [1 :: Double,3,2,4]) == False
+
+---------------------------------------------------------------------
+
 
 -- | All tests must pass with a maximum dimension of about 20
 --  (some tests may fail with bigger sizes due to precision loss).
@@ -466,6 +471,7 @@ runTests n = do
         , normsMTest
         , sumprodTest
         , chainTest
+        , succTest
         ]
     return ()