add GHC.HetMet.{hetmet_kappa,hetmet_kappa_app}

[ghc-base.git] / Data / List.hs

diff --git a/Data/List.hs b/Data/List.hs
index 7d4cf51..bb71da5 100644 (file)
--- a/Data/List.hs
+++ b/Data/List.hs
@@ -1,4 +1,5 @@
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE CPP, NoImplicitPrelude, MagicHash #-}
+

 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.List
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.List

@@ -14,7 +15,7 @@
 -----------------------------------------------------------------------------
 
 module Data.List
 -----------------------------------------------------------------------------
 
 module Data.List

-   ( 
+   (

 #ifdef __NHC__
      [] (..)
    ,
 #ifdef __NHC__
      [] (..)
    ,

@@ -22,26 +23,31 @@ module Data.List
 
    -- * Basic functions
 
 
    -- * Basic functions
 

-     (++)             -- :: [a] -> [a] -> [a]
-   , head             -- :: [a] -> a
-   , last             -- :: [a] -> a
-   , tail             -- :: [a] -> [a]
+     (++)              -- :: [a] -> [a] -> [a]
+   , head              -- :: [a] -> a
+   , last              -- :: [a] -> a
+   , tail              -- :: [a] -> [a]

    , init              -- :: [a] -> [a]
    , init              -- :: [a] -> [a]

-   , null             -- :: [a] -> Bool
-   , length           -- :: [a] -> Int
+   , null              -- :: [a] -> Bool
+   , length            -- :: [a] -> Int

 
    -- * List transformations
    , map               -- :: (a -> b) -> [a] -> [b]
    , reverse           -- :: [a] -> [a]
 
    , intersperse       -- :: a -> [a] -> [a]
 
    -- * List transformations
    , map               -- :: (a -> b) -> [a] -> [b]
    , reverse           -- :: [a] -> [a]
 
    , intersperse       -- :: a -> [a] -> [a]

+   , intercalate       -- :: [a] -> [[a]] -> [a]

    , transpose         -- :: [[a]] -> [[a]]
    , transpose         -- :: [[a]] -> [[a]]

+   
+   , subsequences      -- :: [a] -> [[a]]
+   , permutations      -- :: [a] -> [[a]]

 
    -- * Reducing lists (folds)
 
 
    -- * Reducing lists (folds)
 

-   , foldl            -- :: (a -> b -> a) -> a -> [b] -> a
-   , foldl'           -- :: (a -> b -> a) -> a -> [b] -> a
-   , foldl1           -- :: (a -> a -> a) -> [a] -> a
+   , foldl             -- :: (a -> b -> a) -> a -> [b] -> a
+   , foldl'            -- :: (a -> b -> a) -> a -> [b] -> a
+   , foldl1            -- :: (a -> a -> a) -> [a] -> a
+   , foldl1'           -- :: (a -> a -> a) -> [a] -> a

    , foldr             -- :: (a -> b -> b) -> b -> [a] -> b
    , foldr1            -- :: (a -> a -> a) -> [a] -> a
 
    , foldr             -- :: (a -> b -> b) -> b -> [a] -> b
    , foldr1            -- :: (a -> a -> a) -> [a] -> a
 

@@ -49,7 +55,7 @@ module Data.List
 
    , concat            -- :: [[a]] -> [a]
    , concatMap         -- :: (a -> [b]) -> [a] -> [b]
 
    , concat            -- :: [[a]] -> [a]
    , concatMap         -- :: (a -> [b]) -> [a] -> [b]

-   , and              -- :: [Bool] -> Bool
+   , and               -- :: [Bool] -> Bool

    , or                -- :: [Bool] -> Bool
    , any               -- :: (a -> Bool) -> [a] -> Bool
    , all               -- :: (a -> Bool) -> [a] -> Bool
    , or                -- :: [Bool] -> Bool
    , any               -- :: (a -> Bool) -> [a] -> Bool
    , all               -- :: (a -> Bool) -> [a] -> Bool

@@ -77,7 +83,7 @@ module Data.List
    , cycle             -- :: [a] -> [a]
 
    -- ** Unfolding
    , cycle             -- :: [a] -> [a]
 
    -- ** Unfolding

-   , unfoldr          -- :: (b -> Maybe (a, b)) -> b -> [a]
+   , unfoldr           -- :: (b -> Maybe (a, b)) -> b -> [a]

 
    -- * Sublists
 
 
    -- * Sublists
 

@@ -91,6 +97,8 @@ module Data.List
    , span              -- :: (a -> Bool) -> [a] -> ([a], [a])
    , break             -- :: (a -> Bool) -> [a] -> ([a], [a])
 
    , span              -- :: (a -> Bool) -> [a] -> ([a], [a])
    , break             -- :: (a -> Bool) -> [a] -> ([a], [a])
 

+   , stripPrefix       -- :: Eq a => [a] -> [a] -> Maybe [a]
+

    , group             -- :: Eq a => [a] -> [[a]]
 
    , inits             -- :: [a] -> [[a]]
    , group             -- :: Eq a => [a] -> [[a]]
 
    , inits             -- :: [a] -> [[a]]

@@ -99,6 +107,7 @@ module Data.List
    -- ** Predicates
    , isPrefixOf        -- :: (Eq a) => [a] -> [a] -> Bool
    , isSuffixOf        -- :: (Eq a) => [a] -> [a] -> Bool
    -- ** Predicates
    , isPrefixOf        -- :: (Eq a) => [a] -> [a] -> Bool
    , isSuffixOf        -- :: (Eq a) => [a] -> [a] -> Bool

+   , isInfixOf         -- :: (Eq a) => [a] -> [a] -> Bool

 
    -- * Searching lists
 
 
    -- * Searching lists
 

@@ -108,26 +117,26 @@ module Data.List
    , lookup            -- :: (Eq a) => a -> [(a,b)] -> Maybe b
 
    -- ** Searching with a predicate
    , lookup            -- :: (Eq a) => a -> [(a,b)] -> Maybe b
 
    -- ** Searching with a predicate

-   , find             -- :: (a -> Bool) -> [a] -> Maybe a
-   , filter           -- :: (a -> Bool) -> [a] -> [a]
+   , find              -- :: (a -> Bool) -> [a] -> Maybe a
+   , filter            -- :: (a -> Bool) -> [a] -> [a]

    , partition         -- :: (a -> Bool) -> [a] -> ([a], [a])
 
    -- * Indexing lists
    -- | These functions treat a list @xs@ as a indexed collection,
    -- with indices ranging from 0 to @'length' xs - 1@.
 
    , partition         -- :: (a -> Bool) -> [a] -> ([a], [a])
 
    -- * Indexing lists
    -- | These functions treat a list @xs@ as a indexed collection,
    -- with indices ranging from 0 to @'length' xs - 1@.
 

-   , (!!)             -- :: [a] -> Int -> a
+   , (!!)              -- :: [a] -> Int -> a

 
 

-   , elemIndex        -- :: (Eq a) => a -> [a] -> Maybe Int
+   , elemIndex         -- :: (Eq a) => a -> [a] -> Maybe Int

    , elemIndices       -- :: (Eq a) => a -> [a] -> [Int]
 
    , elemIndices       -- :: (Eq a) => a -> [a] -> [Int]
 

-   , findIndex        -- :: (a -> Bool) -> [a] -> Maybe Int
+   , findIndex         -- :: (a -> Bool) -> [a] -> Maybe Int

    , findIndices       -- :: (a -> Bool) -> [a] -> [Int]
 
    -- * Zipping and unzipping lists
 
    , zip               -- :: [a] -> [b] -> [(a,b)]
    , findIndices       -- :: (a -> Bool) -> [a] -> [Int]
 
    -- * Zipping and unzipping lists
 
    , zip               -- :: [a] -> [b] -> [(a,b)]

-   , zip3  
+   , zip3

    , zip4, zip5, zip6, zip7
 
    , zipWith           -- :: (a -> b -> c) -> [a] -> [b] -> [c]
    , zip4, zip5, zip6, zip7
 
    , zipWith           -- :: (a -> b -> c) -> [a] -> [b] -> [c]

@@ -141,18 +150,18 @@ module Data.List
    -- * Special lists
 
    -- ** Functions on strings
    -- * Special lists
 
    -- ** Functions on strings

-   , lines            -- :: String   -> [String]
-   , words            -- :: String   -> [String]
+   , lines             -- :: String   -> [String]
+   , words             -- :: String   -> [String]

    , unlines           -- :: [String] -> String
    , unwords           -- :: [String] -> String
 
    -- ** \"Set\" operations
    , unlines           -- :: [String] -> String
    , unwords           -- :: [String] -> String
 
    -- ** \"Set\" operations

-   
+

    , nub               -- :: (Eq a) => [a] -> [a]
 
    , delete            -- :: (Eq a) => a -> [a] -> [a]
    , (\\)              -- :: (Eq a) => [a] -> [a] -> [a]
    , nub               -- :: (Eq a) => [a] -> [a]
 
    , delete            -- :: (Eq a) => a -> [a] -> [a]
    , (\\)              -- :: (Eq a) => [a] -> [a] -> [a]

-   
+

    , union             -- :: (Eq a) => [a] -> [a] -> [a]
    , intersect         -- :: (Eq a) => [a] -> [a] -> [a]
 
    , union             -- :: (Eq a) => [a] -> [a] -> [a]
    , intersect         -- :: (Eq a) => [a] -> [a] -> [a]
 

@@ -165,6 +174,10 @@ module Data.List
    -- ** The \"@By@\" operations
    -- | By convention, overloaded functions have a non-overloaded
    -- counterpart whose name is suffixed with \`@By@\'.
    -- ** The \"@By@\" operations
    -- | By convention, overloaded functions have a non-overloaded
    -- counterpart whose name is suffixed with \`@By@\'.

+   --
+   -- It is often convenient to use these functions together with
+   -- 'Data.Function.on', for instance @'sortBy' ('compare'
+   -- \`on\` 'fst')@.

 
    -- *** User-supplied equality (replacing an @Eq@ context)
    -- | The predicate is assumed to define an equivalence.
 
    -- *** User-supplied equality (replacing an @Eq@ context)
    -- | The predicate is assumed to define an equivalence.

@@ -179,7 +192,7 @@ module Data.List
    -- | The function is assumed to define a total ordering.
    , sortBy            -- :: (a -> a -> Ordering) -> [a] -> [a]
    , insertBy          -- :: (a -> a -> Ordering) -> a -> [a] -> [a]
    -- | The function is assumed to define a total ordering.
    , sortBy            -- :: (a -> a -> Ordering) -> [a] -> [a]
    , insertBy          -- :: (a -> a -> Ordering) -> a -> [a] -> [a]

-   , maximumBy        -- :: (a -> a -> Ordering) -> [a] -> a
+   , maximumBy         -- :: (a -> a -> Ordering) -> [a] -> a

    , minimumBy         -- :: (a -> a -> Ordering) -> [a] -> a
 
    -- ** The \"@generic@\" operations
    , minimumBy         -- :: (a -> a -> Ordering) -> [a] -> a
 
    -- ** The \"@generic@\" operations

@@ -196,11 +209,11 @@ module Data.List
    ) where
 
 #ifdef __NHC__
    ) where
 
 #ifdef __NHC__

-import Prelude hiding (Maybe(..))
+import Prelude

 #endif
 
 import Data.Maybe
 #endif
 
 import Data.Maybe

-import Data.Char       ( isSpace )
+import Data.Char        ( isSpace )

 
 #ifdef __GLASGOW_HASKELL__
 import GHC.Num
 
 #ifdef __GLASGOW_HASKELL__
 import GHC.Num

@@ -214,10 +227,24 @@ infix 5 \\ -- comment to fool cpp
 -- -----------------------------------------------------------------------------
 -- List functions
 
 -- -----------------------------------------------------------------------------
 -- List functions
 

+-- | The 'stripPrefix' function drops the given prefix from a list.
+-- It returns 'Nothing' if the list did not start with the prefix
+-- given, or 'Just' the list after the prefix, if it does.
+--
+-- > stripPrefix "foo" "foobar" == Just "bar"
+-- > stripPrefix "foo" "foo" == Just ""
+-- > stripPrefix "foo" "barfoo" == Nothing
+-- > stripPrefix "foo" "barfoobaz" == Nothing
+stripPrefix :: Eq a => [a] -> [a] -> Maybe [a]
+stripPrefix [] ys = Just ys
+stripPrefix (x:xs) (y:ys)
+ | x == y = stripPrefix xs ys
+stripPrefix _ _ = Nothing
+

 -- | The 'elemIndex' function returns the index of the first element
 -- in the given list which is equal (by '==') to the query element,
 -- or 'Nothing' if there is no such element.
 -- | The 'elemIndex' function returns the index of the first element
 -- in the given list which is equal (by '==') to the query element,
 -- or 'Nothing' if there is no such element.

-elemIndex      :: Eq a => a -> [a] -> Maybe Int
+elemIndex       :: Eq a => a -> [a] -> Maybe Int

 elemIndex x     = findIndex (x==)
 
 -- | The 'elemIndices' function extends 'elemIndex', by returning the
 elemIndex x     = findIndex (x==)
 
 -- | The 'elemIndices' function extends 'elemIndex', by returning the

@@ -228,7 +255,7 @@ elemIndices x   = findIndices (x==)
 -- | The 'find' function takes a predicate and a list and returns the
 -- first element in the list matching the predicate, or 'Nothing' if
 -- there is no such element.
 -- | The 'find' function takes a predicate and a list and returns the
 -- first element in the list matching the predicate, or 'Nothing' if
 -- there is no such element.

-find           :: (a -> Bool) -> [a] -> Maybe a
+find            :: (a -> Bool) -> [a] -> Maybe a

 find p          = listToMaybe . filter p
 
 -- | The 'findIndex' function takes a predicate and a list and returns
 find p          = listToMaybe . filter p
 
 -- | The 'findIndex' function takes a predicate and a list and returns

@@ -246,10 +273,10 @@ findIndices p xs = [ i | (x,i) <- zip xs [0..], p x]
 #else
 -- Efficient definition
 findIndices p ls = loop 0# ls
 #else
 -- Efficient definition
 findIndices p ls = loop 0# ls

-                where
-                  loop _ [] = []
-                  loop n (x:xs) | p x       = I# n : loop (n +# 1#) xs
-                                | otherwise = loop (n +# 1#) xs
+                 where
+                   loop _ [] = []
+                   loop n (x:xs) | p x       = I# n : loop (n +# 1#) xs
+                                 | otherwise = loop (n +# 1#) xs

 #endif  /* USE_REPORT_PRELUDE */
 
 -- | The 'isPrefixOf' function takes two lists and returns 'True'
 #endif  /* USE_REPORT_PRELUDE */
 
 -- | The 'isPrefixOf' function takes two lists and returns 'True'

@@ -265,7 +292,18 @@ isPrefixOf (x:xs) (y:ys)=  x == y && isPrefixOf xs ys
 isSuffixOf              :: (Eq a) => [a] -> [a] -> Bool
 isSuffixOf x y          =  reverse x `isPrefixOf` reverse y
 
 isSuffixOf              :: (Eq a) => [a] -> [a] -> Bool
 isSuffixOf x y          =  reverse x `isPrefixOf` reverse y
 

--- | The 'nub' function removes duplicate elements from a list.
+-- | The 'isInfixOf' function takes two lists and returns 'True'
+-- iff the first list is contained, wholly and intact,
+-- anywhere within the second.
+--
+-- Example:
+--
+-- >isInfixOf "Haskell" "I really like Haskell." == True
+-- >isInfixOf "Ial" "I really like Haskell." == False
+isInfixOf               :: (Eq a) => [a] -> [a] -> Bool
+isInfixOf needle haystack = any (isPrefixOf needle) (tails haystack)
+
+-- | /O(n^2)/. The 'nub' function removes duplicate elements from a list.

 -- In particular, it keeps only the first occurrence of each element.
 -- (The name 'nub' means \`essence\'.)
 -- It is a special case of 'nubBy', which allows the programmer to supply
 -- In particular, it keeps only the first occurrence of each element.
 -- (The name 'nub' means \`essence\'.)
 -- It is a special case of 'nubBy', which allows the programmer to supply

@@ -275,28 +313,28 @@ nub                     :: (Eq a) => [a] -> [a]
 nub                     =  nubBy (==)
 #else
 -- stolen from HBC
 nub                     =  nubBy (==)
 #else
 -- stolen from HBC

-nub l                   = nub' l []            -- '
+nub l                   = nub' l []             -- '

   where
   where

-    nub' [] _          = []                    -- '
-    nub' (x:xs) ls                             -- '
-       | x `elem` ls   = nub' xs ls            -- '
-       | otherwise     = x : nub' xs (x:ls)    -- '
+    nub' [] _           = []                    -- '
+    nub' (x:xs) ls                              -- '
+        | x `elem` ls   = nub' xs ls            -- '
+        | otherwise     = x : nub' xs (x:ls)    -- '

 #endif
 
 -- | The 'nubBy' function behaves just like 'nub', except it uses a
 -- user-supplied equality predicate instead of the overloaded '=='
 -- function.
 #endif
 
 -- | The 'nubBy' function behaves just like 'nub', except it uses a
 -- user-supplied equality predicate instead of the overloaded '=='
 -- function.

-nubBy                  :: (a -> a -> Bool) -> [a] -> [a]
+nubBy                   :: (a -> a -> Bool) -> [a] -> [a]

 #ifdef USE_REPORT_PRELUDE
 nubBy eq []             =  []
 nubBy eq (x:xs)         =  x : nubBy eq (filter (\ y -> not (eq x y)) xs)
 #else
 nubBy eq l              = nubBy' l []
   where
 #ifdef USE_REPORT_PRELUDE
 nubBy eq []             =  []
 nubBy eq (x:xs)         =  x : nubBy eq (filter (\ y -> not (eq x y)) xs)
 #else
 nubBy eq l              = nubBy' l []
   where

-    nubBy' [] _                = []
+    nubBy' [] _         = []

     nubBy' (y:ys) xs
     nubBy' (y:ys) xs

-       | elem_by eq y xs = nubBy' ys xs 
-       | otherwise      = y : nubBy' ys (y:xs)
+       | elem_by eq y xs = nubBy' ys xs
+       | otherwise       = y : nubBy' ys (y:xs)

 
 -- Not exported:
 -- Note that we keep the call to `eq` with arguments in the
 
 -- Not exported:
 -- Note that we keep the call to `eq` with arguments in the

@@ -304,8 +342,8 @@ nubBy eq l              = nubBy' l []
 -- 'xs' is the list of things we've seen so far, 
 -- 'y' is the potential new element
 elem_by :: (a -> a -> Bool) -> a -> [a] -> Bool
 -- 'xs' is the list of things we've seen so far, 
 -- 'y' is the potential new element
 elem_by :: (a -> a -> Bool) -> a -> [a] -> Bool

-elem_by _  _ []                =  False
-elem_by eq y (x:xs)    =  x `eq` y || elem_by eq y xs
+elem_by _  _ []         =  False
+elem_by eq y (x:xs)     =  y `eq` x || elem_by eq y xs

 #endif
 
 
 #endif
 
 

@@ -335,8 +373,8 @@ deleteBy eq x (y:ys)    = if x `eq` y then ys else y : deleteBy eq x ys
 -- It is a special case of 'deleteFirstsBy', which allows the programmer
 -- to supply their own equality test.
 
 -- It is a special case of 'deleteFirstsBy', which allows the programmer
 -- to supply their own equality test.
 

-(\\)                   :: (Eq a) => [a] -> [a] -> [a]
-(\\)                   =  foldl (flip delete)
+(\\)                    :: (Eq a) => [a] -> [a] -> [a]
+(\\)                    =  foldl (flip delete)

 
 -- | The 'union' function returns the list union of the two lists.
 -- For example,
 
 -- | The 'union' function returns the list union of the two lists.
 -- For example,

@@ -349,8 +387,8 @@ deleteBy eq x (y:ys)    = if x `eq` y then ys else y : deleteBy eq x ys
 -- It is a special case of 'unionBy', which allows the programmer to supply
 -- their own equality test.
 
 -- It is a special case of 'unionBy', which allows the programmer to supply
 -- their own equality test.
 

-union                  :: (Eq a) => [a] -> [a] -> [a]
-union                  = unionBy (==)
+union                   :: (Eq a) => [a] -> [a] -> [a]
+union                   = unionBy (==)

 
 -- | The 'unionBy' function is the non-overloaded version of 'union'.
 unionBy                 :: (a -> a -> Bool) -> [a] -> [a] -> [a]
 
 -- | The 'unionBy' function is the non-overloaded version of 'union'.
 unionBy                 :: (a -> a -> Bool) -> [a] -> [a] -> [a]

@@ -362,6 +400,9 @@ unionBy eq xs ys        =  xs ++ foldl (flip (deleteBy eq)) (nubBy eq ys) xs
 -- > [1,2,3,4] `intersect` [2,4,6,8] == [2,4]
 --
 -- If the first list contains duplicates, so will the result.
 -- > [1,2,3,4] `intersect` [2,4,6,8] == [2,4]
 --
 -- If the first list contains duplicates, so will the result.

+--
+-- > [1,2,2,3,4] `intersect` [6,4,4,2] == [2,2,4]
+--

 -- It is a special case of 'intersectBy', which allows the programmer to
 -- supply their own equality test.
 
 -- It is a special case of 'intersectBy', which allows the programmer to
 -- supply their own equality test.
 

@@ -370,6 +411,8 @@ intersect               =  intersectBy (==)
 
 -- | The 'intersectBy' function is the non-overloaded version of 'intersect'.
 intersectBy             :: (a -> a -> Bool) -> [a] -> [a] -> [a]
 
 -- | The 'intersectBy' function is the non-overloaded version of 'intersect'.
 intersectBy             :: (a -> a -> Bool) -> [a] -> [a] -> [a]

+intersectBy _  [] _     =  []
+intersectBy _  _  []    =  []

 intersectBy eq xs ys    =  [x | x <- xs, any (eq x) ys]
 
 -- | The 'intersperse' function takes an element and a list and
 intersectBy eq xs ys    =  [x | x <- xs, any (eq x) ys]
 
 -- | The 'intersperse' function takes an element and a list and

@@ -378,20 +421,34 @@ intersectBy eq xs ys    =  [x | x <- xs, any (eq x) ys]
 --
 -- > intersperse ',' "abcde" == "a,b,c,d,e"
 
 --
 -- > intersperse ',' "abcde" == "a,b,c,d,e"
 

-intersperse            :: a -> [a] -> [a]
+intersperse             :: a -> [a] -> [a]

 intersperse _   []      = []
 intersperse _   []      = []

-intersperse _   [x]     = [x]
-intersperse sep (x:xs)  = x : sep : intersperse sep xs
+intersperse sep (x:xs)  = x : prependToAll sep xs
+
+
+-- Not exported:
+-- We want to make every element in the 'intersperse'd list available
+-- as soon as possible to avoid space leaks. Experiments suggested that
+-- a separate top-level helper is more efficient than a local worker.
+prependToAll            :: a -> [a] -> [a]
+prependToAll _   []     = []
+prependToAll sep (x:xs) = sep : x : prependToAll sep xs
+
+-- | 'intercalate' @xs xss@ is equivalent to @('concat' ('intersperse' xs xss))@.
+-- It inserts the list @xs@ in between the lists in @xss@ and concatenates the
+-- result.
+intercalate :: [a] -> [[a]] -> [a]
+intercalate xs xss = concat (intersperse xs xss)

 
 -- | The 'transpose' function transposes the rows and columns of its argument.
 -- For example,
 --
 -- > transpose [[1,2,3],[4,5,6]] == [[1,4],[2,5],[3,6]]
 
 
 -- | The 'transpose' function transposes the rows and columns of its argument.
 -- For example,
 --
 -- > transpose [[1,2,3],[4,5,6]] == [[1,4],[2,5],[3,6]]
 

-transpose              :: [[a]] -> [[a]]
-transpose []            = []
-transpose ([]  : xss)   = transpose xss
-transpose ((x:xs) : xss) = (x : [h | (h:t) <- xss]) : transpose (xs : [ t | (h:t) <- xss])
+transpose               :: [[a]] -> [[a]]
+transpose []             = []
+transpose ([]   : xss)   = transpose xss
+transpose ((x:xs) : xss) = (x : [h | (h:_) <- xss]) : transpose (xs : [ t | (_:t) <- xss])

 
 
 -- | The 'partition' function takes a predicate a list and returns
 
 
 -- | The 'partition' function takes a predicate a list and returns

@@ -400,10 +457,11 @@ transpose ((x:xs) : xss) = (x : [h | (h:t) <- xss]) : transpose (xs : [ t | (h:t
 --
 -- > partition p xs == (filter p xs, filter (not . p) xs)
 
 --
 -- > partition p xs == (filter p xs, filter (not . p) xs)
 

-partition              :: (a -> Bool) -> [a] -> ([a],[a])
+partition               :: (a -> Bool) -> [a] -> ([a],[a])

 {-# INLINE partition #-}
 partition p xs = foldr (select p) ([],[]) xs
 
 {-# INLINE partition #-}
 partition p xs = foldr (select p) ([],[]) xs
 

+select :: (a -> Bool) -> a -> ([a], [a]) -> ([a], [a])

 select p x ~(ts,fs) | p x       = (x:ts,fs)
                     | otherwise = (ts, x:fs)
 
 select p x ~(ts,fs) | p x       = (x:ts,fs)
                     | otherwise = (ts, x:fs)
 

@@ -412,30 +470,30 @@ select p x ~(ts,fs) | p x       = (x:ts,fs)
 -- an accumulating parameter from left to right, and returning a final
 -- value of this accumulator together with the new list.
 mapAccumL :: (acc -> x -> (acc, y)) -- Function of elt of input list
 -- an accumulating parameter from left to right, and returning a final
 -- value of this accumulator together with the new list.
 mapAccumL :: (acc -> x -> (acc, y)) -- Function of elt of input list

-                                   -- and accumulator, returning new
-                                   -- accumulator and elt of result list
-         -> acc            -- Initial accumulator 
-         -> [x]            -- Input list
-         -> (acc, [y])     -- Final accumulator and result list
-mapAccumL _ s []       =  (s, [])
-mapAccumL f s (x:xs)   =  (s'',y:ys)
-                          where (s', y ) = f s x
-                                (s'',ys) = mapAccumL f s' xs
+                                    -- and accumulator, returning new
+                                    -- accumulator and elt of result list
+          -> acc            -- Initial accumulator 
+          -> [x]            -- Input list
+          -> (acc, [y])     -- Final accumulator and result list
+mapAccumL _ s []        =  (s, [])
+mapAccumL f s (x:xs)    =  (s'',y:ys)
+                           where (s', y ) = f s x
+                                 (s'',ys) = mapAccumL f s' xs

 
 -- | The 'mapAccumR' function behaves like a combination of 'map' and
 -- 'foldr'; it applies a function to each element of a list, passing
 -- an accumulating parameter from right to left, and returning a final
 -- value of this accumulator together with the new list.
 
 -- | The 'mapAccumR' function behaves like a combination of 'map' and
 -- 'foldr'; it applies a function to each element of a list, passing
 -- an accumulating parameter from right to left, and returning a final
 -- value of this accumulator together with the new list.

-mapAccumR :: (acc -> x -> (acc, y))    -- Function of elt of input list
-                                       -- and accumulator, returning new
-                                       -- accumulator and elt of result list
-           -> acc              -- Initial accumulator
-           -> [x]              -- Input list
-           -> (acc, [y])               -- Final accumulator and result list
-mapAccumR _ s []       =  (s, [])
-mapAccumR f s (x:xs)   =  (s'', y:ys)
-                          where (s'',y ) = f s' x
-                                (s', ys) = mapAccumR f s xs
+mapAccumR :: (acc -> x -> (acc, y))     -- Function of elt of input list
+                                        -- and accumulator, returning new
+                                        -- accumulator and elt of result list
+            -> acc              -- Initial accumulator
+            -> [x]              -- Input list
+            -> (acc, [y])               -- Final accumulator and result list
+mapAccumR _ s []        =  (s, [])
+mapAccumR f s (x:xs)    =  (s'', y:ys)
+                           where (s'',y ) = f s' x
+                                 (s', ys) = mapAccumR f s xs

 
 -- | The 'insert' function takes an element and a list and inserts the
 -- element into the list at the last position where it is still less
 
 -- | The 'insert' function takes an element and a list and inserts the
 -- element into the list at the last position where it is still less

@@ -454,6 +512,8 @@ insertBy cmp x ys@(y:ys')
      GT -> y : insertBy cmp x ys'
      _  -> x : ys
 
      GT -> y : insertBy cmp x ys'
      _  -> x : ys
 

+#ifdef __GLASGOW_HASKELL__
+

 -- | 'maximum' returns the maximum value from a list,
 -- which must be non-empty, finite, and of an ordered type.
 -- It is a special case of 'Data.List.maximumBy', which allows the
 -- | 'maximum' returns the maximum value from a list,
 -- which must be non-empty, finite, and of an ordered type.
 -- It is a special case of 'Data.List.maximumBy', which allows the

@@ -462,14 +522,17 @@ maximum                 :: (Ord a) => [a] -> a
 maximum []              =  errorEmptyList "maximum"
 maximum xs              =  foldl1 max xs
 
 maximum []              =  errorEmptyList "maximum"
 maximum xs              =  foldl1 max xs
 

-{-# RULES "maximumInt" maximum = maximumInt #-}
+{-# RULES
+  "maximumInt"     maximum = (strictMaximum :: [Int]     -> Int);
+  "maximumInteger" maximum = (strictMaximum :: [Integer] -> Integer)
+ #-}

 
 -- We can't make the overloaded version of maximum strict without
 -- changing its semantics (max might not be strict), but we can for
 -- the version specialised to 'Int'.
 
 -- We can't make the overloaded version of maximum strict without
 -- changing its semantics (max might not be strict), but we can for
 -- the version specialised to 'Int'.

-maximumInt             :: [Int] -> Int
-maximumInt []           =  errorEmptyList "maximum"
-maximumInt xs           =  foldl1' max xs
+strictMaximum           :: (Ord a) => [a] -> a
+strictMaximum []        =  errorEmptyList "maximum"
+strictMaximum xs        =  foldl1' max xs

 
 -- | 'minimum' returns the minimum value from a list,
 -- which must be non-empty, finite, and of an ordered type.
 
 -- | 'minimum' returns the minimum value from a list,
 -- which must be non-empty, finite, and of an ordered type.

@@ -479,33 +542,38 @@ minimum                 :: (Ord a) => [a] -> a
 minimum []              =  errorEmptyList "minimum"
 minimum xs              =  foldl1 min xs
 
 minimum []              =  errorEmptyList "minimum"
 minimum xs              =  foldl1 min xs
 

-{-# RULES "minimumInt" minimum = minimumInt #-}
+{-# RULES
+  "minimumInt"     minimum = (strictMinimum :: [Int]     -> Int);
+  "minimumInteger" minimum = (strictMinimum :: [Integer] -> Integer)
+ #-}
+
+strictMinimum           :: (Ord a) => [a] -> a
+strictMinimum []        =  errorEmptyList "minimum"
+strictMinimum xs        =  foldl1' min xs

 
 

-minimumInt             :: [Int] -> Int
-minimumInt []           =  errorEmptyList "minimum"
-minimumInt xs           =  foldl1' min xs
+#endif /* __GLASGOW_HASKELL__ */

 
 -- | The 'maximumBy' function takes a comparison function and a list
 -- and returns the greatest element of the list by the comparison function.
 -- The list must be finite and non-empty.
 
 -- | The 'maximumBy' function takes a comparison function and a list
 -- and returns the greatest element of the list by the comparison function.
 -- The list must be finite and non-empty.

-maximumBy              :: (a -> a -> Ordering) -> [a] -> a
-maximumBy _ []         =  error "List.maximumBy: empty list"
-maximumBy cmp xs       =  foldl1 max xs
-                       where
-                          max x y = case cmp x y of
-                                       GT -> x
-                                       _  -> y
+maximumBy               :: (a -> a -> Ordering) -> [a] -> a
+maximumBy _ []          =  error "List.maximumBy: empty list"
+maximumBy cmp xs        =  foldl1 maxBy xs
+                        where
+                           maxBy x y = case cmp x y of
+                                       GT -> x
+                                       _  -> y

 
 -- | The 'minimumBy' function takes a comparison function and a list
 -- and returns the least element of the list by the comparison function.
 -- The list must be finite and non-empty.
 
 -- | The 'minimumBy' function takes a comparison function and a list
 -- and returns the least element of the list by the comparison function.
 -- The list must be finite and non-empty.

-minimumBy              :: (a -> a -> Ordering) -> [a] -> a
-minimumBy _ []         =  error "List.minimumBy: empty list"
-minimumBy cmp xs       =  foldl1 min xs
-                       where
-                          min x y = case cmp x y of
-                                       GT -> y
-                                       _  -> x
+minimumBy               :: (a -> a -> Ordering) -> [a] -> a
+minimumBy _ []          =  error "List.minimumBy: empty list"
+minimumBy cmp xs        =  foldl1 minBy xs
+                        where
+                           minBy x y = case cmp x y of
+                                       GT -> y
+                                       _  -> x

 
 -- | The 'genericLength' function is an overloaded version of 'length'.  In
 -- particular, instead of returning an 'Int', it returns any type which is
 
 -- | The 'genericLength' function is an overloaded version of 'length'.  In
 -- particular, instead of returning an 'Int', it returns any type which is

@@ -514,129 +582,138 @@ genericLength           :: (Num i) => [b] -> i
 genericLength []        =  0
 genericLength (_:l)     =  1 + genericLength l
 
 genericLength []        =  0
 genericLength (_:l)     =  1 + genericLength l
 

+{-# RULES
+  "genericLengthInt"     genericLength = (strictGenericLength :: [a] -> Int);
+  "genericLengthInteger" genericLength = (strictGenericLength :: [a] -> Integer);
+ #-}
+
+strictGenericLength     :: (Num i) => [b] -> i
+strictGenericLength l   =  gl l 0
+                        where
+                           gl [] a     = a
+                           gl (_:xs) a = let a' = a + 1 in a' `seq` gl xs a'
+

 -- | The 'genericTake' function is an overloaded version of 'take', which
 -- accepts any 'Integral' value as the number of elements to take.
 -- | The 'genericTake' function is an overloaded version of 'take', which
 -- accepts any 'Integral' value as the number of elements to take.

-genericTake            :: (Integral i) => i -> [a] -> [a]
-genericTake 0 _         =  []
+genericTake             :: (Integral i) => i -> [a] -> [a]
+genericTake n _ | n <= 0 = []

 genericTake _ []        =  []
 genericTake _ []        =  []

-genericTake n (x:xs) | n > 0  =  x : genericTake (n-1) xs
-genericTake _  _        =  error "List.genericTake: negative argument"
+genericTake n (x:xs)    =  x : genericTake (n-1) xs

 
 -- | The 'genericDrop' function is an overloaded version of 'drop', which
 -- accepts any 'Integral' value as the number of elements to drop.
 
 -- | The 'genericDrop' function is an overloaded version of 'drop', which
 -- accepts any 'Integral' value as the number of elements to drop.

-genericDrop            :: (Integral i) => i -> [a] -> [a]
-genericDrop 0 xs        =  xs
+genericDrop             :: (Integral i) => i -> [a] -> [a]
+genericDrop n xs | n <= 0 = xs

 genericDrop _ []        =  []
 genericDrop _ []        =  []

-genericDrop n (_:xs) | n > 0  =  genericDrop (n-1) xs
-genericDrop _ _                =  error "List.genericDrop: negative argument"
+genericDrop n (_:xs)    =  genericDrop (n-1) xs
+

 
 -- | The 'genericSplitAt' function is an overloaded version of 'splitAt', which
 -- accepts any 'Integral' value as the position at which to split.
 genericSplitAt          :: (Integral i) => i -> [b] -> ([b],[b])
 
 -- | The 'genericSplitAt' function is an overloaded version of 'splitAt', which
 -- accepts any 'Integral' value as the position at which to split.
 genericSplitAt          :: (Integral i) => i -> [b] -> ([b],[b])

-genericSplitAt 0 xs     =  ([],xs)
+genericSplitAt n xs | n <= 0 =  ([],xs)

 genericSplitAt _ []     =  ([],[])
 genericSplitAt _ []     =  ([],[])

-genericSplitAt n (x:xs) | n > 0  =  (x:xs',xs'') where
-                               (xs',xs'') = genericSplitAt (n-1) xs
-genericSplitAt _ _      =  error "List.genericSplitAt: negative argument"
+genericSplitAt n (x:xs) =  (x:xs',xs'') where
+    (xs',xs'') = genericSplitAt (n-1) xs

 
 -- | The 'genericIndex' function is an overloaded version of '!!', which
 -- accepts any 'Integral' value as the index.
 genericIndex :: (Integral a) => [b] -> a -> b
 genericIndex (x:_)  0 = x
 
 -- | The 'genericIndex' function is an overloaded version of '!!', which
 -- accepts any 'Integral' value as the index.
 genericIndex :: (Integral a) => [b] -> a -> b
 genericIndex (x:_)  0 = x

-genericIndex (_:xs) n 
+genericIndex (_:xs) n

  | n > 0     = genericIndex xs (n-1)
  | otherwise = error "List.genericIndex: negative argument."
 genericIndex _ _      = error "List.genericIndex: index too large."
 
 -- | The 'genericReplicate' function is an overloaded version of 'replicate',
 -- which accepts any 'Integral' value as the number of repetitions to make.
  | n > 0     = genericIndex xs (n-1)
  | otherwise = error "List.genericIndex: negative argument."
 genericIndex _ _      = error "List.genericIndex: index too large."
 
 -- | The 'genericReplicate' function is an overloaded version of 'replicate',
 -- which accepts any 'Integral' value as the number of repetitions to make.

-genericReplicate       :: (Integral i) => i -> a -> [a]
-genericReplicate n x   =  genericTake n (repeat x)
+genericReplicate        :: (Integral i) => i -> a -> [a]
+genericReplicate n x    =  genericTake n (repeat x)

 
 -- | The 'zip4' function takes four lists and returns a list of
 -- quadruples, analogous to 'zip'.
 
 -- | The 'zip4' function takes four lists and returns a list of
 -- quadruples, analogous to 'zip'.

-zip4                   :: [a] -> [b] -> [c] -> [d] -> [(a,b,c,d)]
-zip4                   =  zipWith4 (,,,)
+zip4                    :: [a] -> [b] -> [c] -> [d] -> [(a,b,c,d)]
+zip4                    =  zipWith4 (,,,)

 
 -- | The 'zip5' function takes five lists and returns a list of
 -- five-tuples, analogous to 'zip'.
 
 -- | The 'zip5' function takes five lists and returns a list of
 -- five-tuples, analogous to 'zip'.

-zip5                   :: [a] -> [b] -> [c] -> [d] -> [e] -> [(a,b,c,d,e)]
-zip5                   =  zipWith5 (,,,,)
+zip5                    :: [a] -> [b] -> [c] -> [d] -> [e] -> [(a,b,c,d,e)]
+zip5                    =  zipWith5 (,,,,)

 
 -- | The 'zip6' function takes six lists and returns a list of six-tuples,
 -- analogous to 'zip'.
 
 -- | The 'zip6' function takes six lists and returns a list of six-tuples,
 -- analogous to 'zip'.

-zip6                   :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> 
+zip6                    :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] ->

                               [(a,b,c,d,e,f)]
                               [(a,b,c,d,e,f)]

-zip6                   =  zipWith6 (,,,,,)
+zip6                    =  zipWith6 (,,,,,)

 
 -- | The 'zip7' function takes seven lists and returns a list of
 -- seven-tuples, analogous to 'zip'.
 
 -- | The 'zip7' function takes seven lists and returns a list of
 -- seven-tuples, analogous to 'zip'.

-zip7                   :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] ->
+zip7                    :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] ->

                               [g] -> [(a,b,c,d,e,f,g)]
                               [g] -> [(a,b,c,d,e,f,g)]

-zip7                   =  zipWith7 (,,,,,,)
+zip7                    =  zipWith7 (,,,,,,)

 
 -- | The 'zipWith4' function takes a function which combines four
 -- elements, as well as four lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.
 
 -- | The 'zipWith4' function takes a function which combines four
 -- elements, as well as four lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.

-zipWith4               :: (a->b->c->d->e) -> [a]->[b]->[c]->[d]->[e]
+zipWith4                :: (a->b->c->d->e) -> [a]->[b]->[c]->[d]->[e]

 zipWith4 z (a:as) (b:bs) (c:cs) (d:ds)
 zipWith4 z (a:as) (b:bs) (c:cs) (d:ds)

-                       =  z a b c d : zipWith4 z as bs cs ds
-zipWith4 _ _ _ _ _     =  []
+                        =  z a b c d : zipWith4 z as bs cs ds
+zipWith4 _ _ _ _ _      =  []

 
 -- | The 'zipWith5' function takes a function which combines five
 -- elements, as well as five lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.
 
 -- | The 'zipWith5' function takes a function which combines five
 -- elements, as well as five lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.

-zipWith5               :: (a->b->c->d->e->f) -> 
+zipWith5                :: (a->b->c->d->e->f) ->

                            [a]->[b]->[c]->[d]->[e]->[f]
 zipWith5 z (a:as) (b:bs) (c:cs) (d:ds) (e:es)
                            [a]->[b]->[c]->[d]->[e]->[f]
 zipWith5 z (a:as) (b:bs) (c:cs) (d:ds) (e:es)

-                       =  z a b c d e : zipWith5 z as bs cs ds es
-zipWith5 _ _ _ _ _ _   = []
+                        =  z a b c d e : zipWith5 z as bs cs ds es
+zipWith5 _ _ _ _ _ _    = []

 
 -- | The 'zipWith6' function takes a function which combines six
 -- elements, as well as six lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.
 
 -- | The 'zipWith6' function takes a function which combines six
 -- elements, as well as six lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.

-zipWith6               :: (a->b->c->d->e->f->g) ->
+zipWith6                :: (a->b->c->d->e->f->g) ->

                            [a]->[b]->[c]->[d]->[e]->[f]->[g]
 zipWith6 z (a:as) (b:bs) (c:cs) (d:ds) (e:es) (f:fs)
                            [a]->[b]->[c]->[d]->[e]->[f]->[g]
 zipWith6 z (a:as) (b:bs) (c:cs) (d:ds) (e:es) (f:fs)

-                       =  z a b c d e f : zipWith6 z as bs cs ds es fs
-zipWith6 _ _ _ _ _ _ _ = []
+                        =  z a b c d e f : zipWith6 z as bs cs ds es fs
+zipWith6 _ _ _ _ _ _ _  = []

 
 -- | The 'zipWith7' function takes a function which combines seven
 -- elements, as well as seven lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.
 
 -- | The 'zipWith7' function takes a function which combines seven
 -- elements, as well as seven lists and returns a list of their point-wise
 -- combination, analogous to 'zipWith'.

-zipWith7               :: (a->b->c->d->e->f->g->h) ->
+zipWith7                :: (a->b->c->d->e->f->g->h) ->

                            [a]->[b]->[c]->[d]->[e]->[f]->[g]->[h]
 zipWith7 z (a:as) (b:bs) (c:cs) (d:ds) (e:es) (f:fs) (g:gs)
                            [a]->[b]->[c]->[d]->[e]->[f]->[g]->[h]
 zipWith7 z (a:as) (b:bs) (c:cs) (d:ds) (e:es) (f:fs) (g:gs)

-                  =  z a b c d e f g : zipWith7 z as bs cs ds es fs gs
+                   =  z a b c d e f g : zipWith7 z as bs cs ds es fs gs

 zipWith7 _ _ _ _ _ _ _ _ = []
 
 -- | The 'unzip4' function takes a list of quadruples and returns four
 -- lists, analogous to 'unzip'.
 zipWith7 _ _ _ _ _ _ _ _ = []
 
 -- | The 'unzip4' function takes a list of quadruples and returns four
 -- lists, analogous to 'unzip'.

-unzip4                 :: [(a,b,c,d)] -> ([a],[b],[c],[d])
-unzip4                 =  foldr (\(a,b,c,d) ~(as,bs,cs,ds) ->
-                                       (a:as,b:bs,c:cs,d:ds))
-                                ([],[],[],[])
+unzip4                  :: [(a,b,c,d)] -> ([a],[b],[c],[d])
+unzip4                  =  foldr (\(a,b,c,d) ~(as,bs,cs,ds) ->
+                                        (a:as,b:bs,c:cs,d:ds))
+                                 ([],[],[],[])

 
 -- | The 'unzip5' function takes a list of five-tuples and returns five
 -- lists, analogous to 'unzip'.
 
 -- | The 'unzip5' function takes a list of five-tuples and returns five
 -- lists, analogous to 'unzip'.

-unzip5                 :: [(a,b,c,d,e)] -> ([a],[b],[c],[d],[e])
-unzip5                 =  foldr (\(a,b,c,d,e) ~(as,bs,cs,ds,es) ->
-                                       (a:as,b:bs,c:cs,d:ds,e:es))
-                                ([],[],[],[],[])
+unzip5                  :: [(a,b,c,d,e)] -> ([a],[b],[c],[d],[e])
+unzip5                  =  foldr (\(a,b,c,d,e) ~(as,bs,cs,ds,es) ->
+                                        (a:as,b:bs,c:cs,d:ds,e:es))
+                                 ([],[],[],[],[])

 
 -- | The 'unzip6' function takes a list of six-tuples and returns six
 -- lists, analogous to 'unzip'.
 
 -- | The 'unzip6' function takes a list of six-tuples and returns six
 -- lists, analogous to 'unzip'.

-unzip6                 :: [(a,b,c,d,e,f)] -> ([a],[b],[c],[d],[e],[f])
-unzip6                 =  foldr (\(a,b,c,d,e,f) ~(as,bs,cs,ds,es,fs) ->
-                                       (a:as,b:bs,c:cs,d:ds,e:es,f:fs))
-                                ([],[],[],[],[],[])
+unzip6                  :: [(a,b,c,d,e,f)] -> ([a],[b],[c],[d],[e],[f])
+unzip6                  =  foldr (\(a,b,c,d,e,f) ~(as,bs,cs,ds,es,fs) ->
+                                        (a:as,b:bs,c:cs,d:ds,e:es,f:fs))
+                                 ([],[],[],[],[],[])

 
 -- | The 'unzip7' function takes a list of seven-tuples and returns
 -- seven lists, analogous to 'unzip'.
 
 -- | The 'unzip7' function takes a list of seven-tuples and returns
 -- seven lists, analogous to 'unzip'.

-unzip7         :: [(a,b,c,d,e,f,g)] -> ([a],[b],[c],[d],[e],[f],[g])
-unzip7         =  foldr (\(a,b,c,d,e,f,g) ~(as,bs,cs,ds,es,fs,gs) ->
-                               (a:as,b:bs,c:cs,d:ds,e:es,f:fs,g:gs))
-                        ([],[],[],[],[],[],[])
+unzip7          :: [(a,b,c,d,e,f,g)] -> ([a],[b],[c],[d],[e],[f],[g])
+unzip7          =  foldr (\(a,b,c,d,e,f,g) ~(as,bs,cs,ds,es,fs,gs) ->
+                                (a:as,b:bs,c:cs,d:ds,e:es,f:fs,g:gs))
+                         ([],[],[],[],[],[],[])

 
 
 -- | The 'deleteFirstsBy' function takes a predicate and two lists and
 
 
 -- | The 'deleteFirstsBy' function takes a predicate and two lists and

@@ -653,13 +730,13 @@ deleteFirstsBy eq       =  foldl (flip (deleteBy eq))
 --
 -- It is a special case of 'groupBy', which allows the programmer to supply
 -- their own equality test.
 --
 -- It is a special case of 'groupBy', which allows the programmer to supply
 -- their own equality test.

-group                  :: Eq a => [a] -> [[a]]
+group                   :: Eq a => [a] -> [[a]]

 group                   =  groupBy (==)
 
 -- | The 'groupBy' function is the non-overloaded version of 'group'.
 group                   =  groupBy (==)
 
 -- | The 'groupBy' function is the non-overloaded version of 'group'.

-groupBy                :: (a -> a -> Bool) -> [a] -> [[a]]
-groupBy _  []          =  []
-groupBy eq (x:xs)      =  (x:ys) : groupBy eq zs
+groupBy                 :: (a -> a -> Bool) -> [a] -> [[a]]
+groupBy _  []           =  []
+groupBy eq (x:xs)       =  (x:ys) : groupBy eq zs

                            where (ys,zs) = span (eq x) xs
 
 -- | The 'inits' function returns all initial segments of the argument,
                            where (ys,zs) = span (eq x) xs
 
 -- | The 'inits' function returns all initial segments of the argument,

@@ -667,18 +744,53 @@ groupBy eq (x:xs) =  (x:ys) : groupBy eq zs
 --
 -- > inits "abc" == ["","a","ab","abc"]
 --
 --
 -- > inits "abc" == ["","a","ab","abc"]
 --

-inits                  :: [a] -> [[a]]
-inits []               =  [[]]
-inits (x:xs)           =  [[]] ++ map (x:) (inits xs)
+-- Note that 'inits' has the following strictness property:
+-- @inits _|_ = [] : _|_@
+inits                   :: [a] -> [[a]]
+inits xs                =  [] : case xs of
+                                  []      -> []
+                                  x : xs' -> map (x :) (inits xs')

 
 -- | The 'tails' function returns all final segments of the argument,
 -- longest first.  For example,
 --
 -- > tails "abc" == ["abc", "bc", "c",""]
 --
 
 -- | The 'tails' function returns all final segments of the argument,
 -- longest first.  For example,
 --
 -- > tails "abc" == ["abc", "bc", "c",""]
 --

-tails                  :: [a] -> [[a]]
-tails []               =  [[]]
-tails xxs@(_:xs)       =  xxs : tails xs
+-- Note that 'tails' has the following strictness property:
+-- @tails _|_ = _|_ : _|_@
+tails                   :: [a] -> [[a]]
+tails xs                =  xs : case xs of
+                                  []      -> []
+                                  _ : xs' -> tails xs'
+
+-- | The 'subsequences' function returns the list of all subsequences of the argument.
+--
+-- > subsequences "abc" == ["","a","b","ab","c","ac","bc","abc"]
+subsequences            :: [a] -> [[a]]
+subsequences xs         =  [] : nonEmptySubsequences xs
+
+-- | The 'nonEmptySubsequences' function returns the list of all subsequences of the argument,
+--   except for the empty list.
+--
+-- > nonEmptySubsequences "abc" == ["a","b","ab","c","ac","bc","abc"]
+nonEmptySubsequences         :: [a] -> [[a]]
+nonEmptySubsequences []      =  []
+nonEmptySubsequences (x:xs)  =  [x] : foldr f [] (nonEmptySubsequences xs)
+  where f ys r = ys : (x : ys) : r
+
+
+-- | The 'permutations' function returns the list of all permutations of the argument.
+--
+-- > permutations "abc" == ["abc","bac","cba","bca","cab","acb"]
+permutations            :: [a] -> [[a]]
+permutations xs0        =  xs0 : perms xs0 []
+  where
+    perms []     _  = []
+    perms (t:ts) is = foldr interleave (perms ts (t:is)) (permutations is)
+      where interleave    xs     r = let (_,zs) = interleave' id xs r in zs
+            interleave' _ []     r = (ts, r)
+            interleave' f (y:ys) r = let (us,zs) = interleave' (f . (y:)) ys r
+                                     in  (y:us, f (t:y:us) : zs)

 
 
 ------------------------------------------------------------------------------
 
 
 ------------------------------------------------------------------------------

@@ -697,10 +809,50 @@ sort = sortBy compare
 sortBy cmp = foldr (insertBy cmp) []
 #else
 
 sortBy cmp = foldr (insertBy cmp) []
 #else
 

+{-
+GHC's mergesort replaced by a better implementation, 24/12/2009.
+This code originally contributed to the nhc12 compiler by Thomas Nordin
+in 2002.  Rumoured to have been based on code by Lennart Augustsson, e.g.
+    http://www.mail-archive.com/haskell@haskell.org/msg01822.html
+and possibly to bear similarities to a 1982 paper by Richard O'Keefe:
+"A smooth applicative merge sort".
+
+Benchmarks show it to be often 2x the speed of the previous implementation.
+Fixes ticket http://hackage.haskell.org/trac/ghc/ticket/2143
+-}
+
+sort = sortBy compare
+sortBy cmp = mergeAll . sequences
+  where
+    sequences (a:b:xs)
+      | a `cmp` b == GT = descending b [a]  xs
+      | otherwise       = ascending  b (a:) xs
+    sequences xs = [xs]
+
+    descending a as (b:bs)
+      | a `cmp` b == GT = descending b (a:as) bs
+    descending a as bs  = (a:as): sequences bs
+
+    ascending a as (b:bs)
+      | a `cmp` b /= GT = ascending b (\ys -> as (a:ys)) bs
+    ascending a as bs   = as [a]: sequences bs
+
+    mergeAll [x] = x
+    mergeAll xs  = mergeAll (mergePairs xs)
+
+    mergePairs (a:b:xs) = merge a b: mergePairs xs
+    mergePairs xs       = xs
+
+    merge as@(a:as') bs@(b:bs')
+      | a `cmp` b == GT = b:merge as  bs'
+      | otherwise       = a:merge as' bs
+    merge [] bs         = bs
+    merge as []         = as
+
+{-

 sortBy cmp l = mergesort cmp l
 sort l = mergesort compare l
 
 sortBy cmp l = mergesort cmp l
 sort l = mergesort compare l
 

-{-

 Quicksort replaced by mergesort, 14/5/2002.
 
 From: Ian Lynagh <igloo@earth.li>
 Quicksort replaced by mergesort, 14/5/2002.
 
 From: Ian Lynagh <igloo@earth.li>

@@ -741,24 +893,23 @@ func            100000           sorted        sort        5831.47
 func            100000           sorted        mergesort   2.23
 func            100000           revsorted     sort        5872.34
 func            100000           revsorted     mergesort   2.24
 func            100000           sorted        mergesort   2.23
 func            100000           revsorted     sort        5872.34
 func            100000           revsorted     mergesort   2.24

--}

 
 mergesort :: (a -> a -> Ordering) -> [a] -> [a]
 mergesort cmp = mergesort' cmp . map wrap
 
 mergesort' :: (a -> a -> Ordering) -> [[a]] -> [a]
 
 mergesort :: (a -> a -> Ordering) -> [a] -> [a]
 mergesort cmp = mergesort' cmp . map wrap
 
 mergesort' :: (a -> a -> Ordering) -> [[a]] -> [a]

-mergesort' cmp [] = []
-mergesort' cmp [xs] = xs
+mergesort' _   [] = []
+mergesort' _   [xs] = xs

 mergesort' cmp xss = mergesort' cmp (merge_pairs cmp xss)
 
 merge_pairs :: (a -> a -> Ordering) -> [[a]] -> [[a]]
 mergesort' cmp xss = mergesort' cmp (merge_pairs cmp xss)
 
 merge_pairs :: (a -> a -> Ordering) -> [[a]] -> [[a]]

-merge_pairs cmp [] = []
-merge_pairs cmp [xs] = [xs]
+merge_pairs _   [] = []
+merge_pairs _   [xs] = [xs]

 merge_pairs cmp (xs:ys:xss) = merge cmp xs ys : merge_pairs cmp xss
 
 merge :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
 merge_pairs cmp (xs:ys:xss) = merge cmp xs ys : merge_pairs cmp xss
 
 merge :: (a -> a -> Ordering) -> [a] -> [a] -> [a]

-merge cmp xs [] = xs
-merge cmp [] ys = ys
+merge _   [] ys = ys
+merge _   xs [] = xs

 merge cmp (x:xs) (y:ys)
  = case x `cmp` y of
         GT -> y : merge cmp (x:xs)   ys
 merge cmp (x:xs) (y:ys)
  = case x `cmp` y of
         GT -> y : merge cmp (x:xs)   ys

@@ -767,8 +918,9 @@ merge cmp (x:xs) (y:ys)
 wrap :: a -> [a]
 wrap x = [x]
 
 wrap :: a -> [a]
 wrap x = [x]
 

-{-
-OLD: qsort version
+
+
+OLDER: qsort version

 
 -- qsort is stable and does not concatenate.
 qsort :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
 
 -- qsort is stable and does not concatenate.
 qsort :: (a -> a -> Ordering) -> [a] -> [a] -> [a]

@@ -784,7 +936,7 @@ qpart cmp x [] rlt rge r =
     rqsort cmp rlt (x:rqsort cmp rge r)
 qpart cmp x (y:ys) rlt rge r =
     case cmp x y of
     rqsort cmp rlt (x:rqsort cmp rge r)
 qpart cmp x (y:ys) rlt rge r =
     case cmp x y of

-       GT -> qpart cmp x ys (y:rlt) rge r
+        GT -> qpart cmp x ys (y:rlt) rge r

         _  -> qpart cmp x ys rlt (y:rge) r
 
 -- rqsort is as qsort but anti-stable, i.e. reverses equal elements
         _  -> qpart cmp x ys rlt (y:rge) r
 
 -- rqsort is as qsort but anti-stable, i.e. reverses equal elements

@@ -798,8 +950,8 @@ rqpart cmp x [] rle rgt r =
     qsort cmp rle (x:qsort cmp rgt r)
 rqpart cmp x (y:ys) rle rgt r =
     case cmp y x of
     qsort cmp rle (x:qsort cmp rgt r)
 rqpart cmp x (y:ys) rle rgt r =
     case cmp y x of

-       GT -> rqpart cmp x ys rle (y:rgt) r
-       _  -> rqpart cmp x ys (y:rle) rgt r
+        GT -> rqpart cmp x ys rle (y:rgt) r
+        _  -> rqpart cmp x ys (y:rle) rgt r

 -}
 
 #endif /* USE_REPORT_PRELUDE */
 -}
 
 #endif /* USE_REPORT_PRELUDE */

@@ -821,7 +973,12 @@ rqpart cmp x (y:ys) rle rgt r =
 --
 -- > f' (f x y) = Just (x,y)
 -- > f' z       = Nothing
 --
 -- > f' (f x y) = Just (x,y)
 -- > f' z       = Nothing

-
+--
+-- A simple use of unfoldr:
+--
+-- > unfoldr (\b -> if b == 0 then Nothing else Just (b, b-1)) 10
+-- >  [10,9,8,7,6,5,4,3,2,1]
+--

 unfoldr      :: (b -> Maybe (a, b)) -> b -> [a]
 unfoldr f b  =
   case f b of
 unfoldr      :: (b -> Maybe (a, b)) -> b -> [a]
 unfoldr f b  =
   case f b of

@@ -832,14 +989,22 @@ unfoldr f b  =
 
 -- | A strict version of 'foldl'.
 foldl'           :: (a -> b -> a) -> a -> [b] -> a
 
 -- | A strict version of 'foldl'.
 foldl'           :: (a -> b -> a) -> a -> [b] -> a

+#ifdef __GLASGOW_HASKELL__
+foldl' f z0 xs0 = lgo z0 xs0
+    where lgo z []     = z
+          lgo z (x:xs) = let z' = f z x in z' `seq` lgo z' xs
+#else

 foldl' f a []     = a
 foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs
 foldl' f a []     = a
 foldl' f a (x:xs) = let a' = f a x in a' `seq` foldl' f a' xs

+#endif

 
 

+#ifdef __GLASGOW_HASKELL__

 -- | 'foldl1' is a variant of 'foldl' that has no starting value argument,
 -- and thus must be applied to non-empty lists.
 foldl1                  :: (a -> a -> a) -> [a] -> a
 foldl1 f (x:xs)         =  foldl f x xs
 foldl1 _ []             =  errorEmptyList "foldl1"
 -- | 'foldl1' is a variant of 'foldl' that has no starting value argument,
 -- and thus must be applied to non-empty lists.
 foldl1                  :: (a -> a -> a) -> [a] -> a
 foldl1 f (x:xs)         =  foldl f x xs
 foldl1 _ []             =  errorEmptyList "foldl1"

+#endif /* __GLASGOW_HASKELL__ */

 
 -- | A strict version of 'foldl1'
 foldl1'                  :: (a -> a -> a) -> [a] -> a
 
 -- | A strict version of 'foldl1'
 foldl1'                  :: (a -> a -> a) -> [a] -> a

@@ -859,14 +1024,14 @@ sum                     :: (Num a) => [a] -> a
 -- | The 'product' function computes the product of a finite list of numbers.
 product                 :: (Num a) => [a] -> a
 #ifdef USE_REPORT_PRELUDE
 -- | The 'product' function computes the product of a finite list of numbers.
 product                 :: (Num a) => [a] -> a
 #ifdef USE_REPORT_PRELUDE

-sum                     =  foldl (+) 0  
+sum                     =  foldl (+) 0

 product                 =  foldl (*) 1
 #else
 product                 =  foldl (*) 1
 #else

-sum    l       = sum' l 0
+sum     l       = sum' l 0

   where
     sum' []     a = a
     sum' (x:xs) a = sum' xs (a+x)
   where
     sum' []     a = a
     sum' (x:xs) a = sum' xs (a+x)

-product        l       = prod l 1
+product l       = prod l 1

   where
     prod []     a = a
     prod (x:xs) a = prod xs (a*x)
   where
     prod []     a = a
     prod (x:xs) a = prod xs (a*x)

@@ -877,18 +1042,31 @@ product  l       = prod l 1
 
 -- | 'lines' breaks a string up into a list of strings at newline
 -- characters.  The resulting strings do not contain newlines.
 
 -- | 'lines' breaks a string up into a list of strings at newline
 -- characters.  The resulting strings do not contain newlines.

-lines                  :: String -> [String]
-lines ""               =  []
-lines s                        =  let (l, s') = break (== '\n') s
-                          in  l : case s' of
-                                       []      -> []
-                                       (_:s'') -> lines s''
+lines                   :: String -> [String]
+lines ""                =  []
+#ifdef __GLASGOW_HASKELL__
+-- Somehow GHC doesn't detect the selector thunks in the below code,
+-- so s' keeps a reference to the first line via the pair and we have
+-- a space leak (cf. #4334).
+-- So we need to make GHC see the selector thunks with a trick.
+lines s                 =  cons (case break (== '\n') s of
+                                    (l, s') -> (l, case s' of
+                                                    []      -> []
+                                                    _:s''   -> lines s''))
+  where
+    cons ~(h, t)        =  h : t
+#else
+lines s                 =  let (l, s') = break (== '\n') s
+                           in  l : case s' of
+                                        []      -> []
+                                        (_:s'') -> lines s''
+#endif

 
 -- | 'unlines' is an inverse operation to 'lines'.
 -- It joins lines, after appending a terminating newline to each.
 
 -- | 'unlines' is an inverse operation to 'lines'.
 -- It joins lines, after appending a terminating newline to each.

-unlines                        :: [String] -> String
+unlines                 :: [String] -> String

 #ifdef USE_REPORT_PRELUDE
 #ifdef USE_REPORT_PRELUDE

-unlines                        =  concatMap (++ "\n")
+unlines                 =  concatMap (++ "\n")

 #else
 -- HBC version (stolen)
 -- here's a more efficient version
 #else
 -- HBC version (stolen)
 -- here's a more efficient version

@@ -898,24 +1076,31 @@ unlines (l:ls) = l ++ '\n' : unlines ls
 
 -- | 'words' breaks a string up into a list of words, which were delimited
 -- by white space.
 
 -- | 'words' breaks a string up into a list of words, which were delimited
 -- by white space.

-words                  :: String -> [String]
-words s                        =  case dropWhile {-partain:Char.-}isSpace s of
-                               "" -> []
-                               s' -> w : words s''
-                                     where (w, s'') = 
+words                   :: String -> [String]
+words s                 =  case dropWhile {-partain:Char.-}isSpace s of
+                                "" -> []
+                                s' -> w : words s''
+                                      where (w, s'') =

                                              break {-partain:Char.-}isSpace s'
 
 -- | 'unwords' is an inverse operation to 'words'.
 -- It joins words with separating spaces.
                                              break {-partain:Char.-}isSpace s'
 
 -- | 'unwords' is an inverse operation to 'words'.
 -- It joins words with separating spaces.

-unwords                        :: [String] -> String
+unwords                 :: [String] -> String

 #ifdef USE_REPORT_PRELUDE
 #ifdef USE_REPORT_PRELUDE

-unwords []             =  ""
-unwords ws             =  foldr1 (\w s -> w ++ ' ':s) ws
+unwords []              =  ""
+unwords ws              =  foldr1 (\w s -> w ++ ' ':s) ws

 #else
 -- HBC version (stolen)
 -- here's a more efficient version
 #else
 -- HBC version (stolen)
 -- here's a more efficient version

-unwords []             =  ""
-unwords [w]            = w
-unwords (w:ws)         = w ++ ' ' : unwords ws
+unwords []              =  ""
+unwords [w]             = w
+unwords (w:ws)          = w ++ ' ' : unwords ws

 #endif
 #endif

-#endif  /* __GLASGOW_HASKELL__ */
+
+#else  /* !__GLASGOW_HASKELL__ */
+
+errorEmptyList :: String -> a
+errorEmptyList fun =
+  error ("Prelude." ++ fun ++ ": empty list")
+
+#endif /* !__GLASGOW_HASKELL__ */