, span -- :: (a -> Bool) -> [a] -> ([a], [a])
, break -- :: (a -> Bool) -> [a] -> ([a], [a])
- , split -- :: Eq a => a -> [a] -> [[a]]
-
, group -- :: Eq a => [a] -> [[a]]
, inits -- :: [a] -> [[a]]
-- ** 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.
deleteFirstsBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
deleteFirstsBy eq = foldl (flip (deleteBy eq))
--- | 'split' @x xs@ divides the list @xs@ at every occurrence of @x@ and
--- removes those occurrences.
---
--- Example:
---
--- > split 'i' "mississippi" -> ["m","ss","ss","pp",""]
--- > split 'i' [] -> [""]
-split :: Eq a => a -> [a] -> [[a]]
-split x xs = let (f,r) = break (==x) xs
- in f : case r of
- [] -> []
- (_:ys) -> split x ys
-
-- | The 'group' function takes a list and returns a list of lists such
-- that the concatenation of the result is equal to the argument. Moreover,
-- each sublist in the result contains only equal elements. For example,
--
-- > 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