unfoldr -text

unfoldr :: (b -> Maybe (a, b)) -> b -> [a]
base Data.List
The unfoldr function is a `dual' to foldr: while foldr reduces a list to a summary value, unfoldr builds a list from a seed value. The function takes the element and returns Nothing if it is done producing the list or returns Just (a,b), in which case, a is a prepended to the list and b is used as the next element in a recursive call. For example, > iterate f == unfoldr (\x -> Just (x, f x)) In some cases, unfoldr can undo a foldr operation: > unfoldr f' (foldr f z xs) == xs if the following holds: > 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 :: (a -> Maybe (Char, a)) -> a -> ByteString
bytestring Data.ByteString.Lazy.Char8
O(n) The unfoldr function is analogous to the List 'unfoldr'. unfoldr builds a ByteString from a seed value. The function takes the element and returns Nothing if it is done producing the ByteString or returns Just (a,b), in which case, a is a prepending to the ByteString and b is used as the next element in a recursive call.
unfoldr :: (a -> Maybe (Char, a)) -> a -> ByteString
bytestring Data.ByteString.Char8
O(n), unfoldr function is analogous to the List 'unfoldr'. unfoldr builds a ByteString from a seed value. The function takes the element and returns Nothing if it is done producing the ByteString or returns Just (a,b), in which case, a is the next character in the string, and b is the seed value for further production. Examples: > unfoldr (\x -> if x <= '9' then Just (x, succ x) else Nothing) '0' == "0123456789"
unfoldr :: (a -> Maybe (Word8, a)) -> a -> ByteString
bytestring Data.ByteString.Lazy
O(n) The unfoldr function is analogous to the List 'unfoldr'. unfoldr builds a ByteString from a seed value. The function takes the element and returns Nothing if it is done producing the ByteString or returns Just (a,b), in which case, a is a prepending to the ByteString and b is used as the next element in a recursive call.
unfoldr :: (a -> Maybe (Word8, a)) -> a -> ByteString
bytestring Data.ByteString
O(n), unfoldr function is analogous to the List 'unfoldr'. unfoldr builds a ByteString from a seed value. The function takes the element and returns Nothing if it is done producing the ByteString or returns Just (a,b), in which case, a is the next byte in the string, and b is the seed value for further production. Examples: > unfoldr (\x -> if x <= 5 then Just (x, x + 1) else Nothing) 0 > == pack [0, 1, 2, 3, 4, 5]
unfoldr :: (b -> Maybe (a, b)) -> b -> Seq a
containers Data.Sequence
Builds a sequence from a seed value. Takes time linear in the number of generated elements. WARNING: If the number of generated elements is infinite, this method will not terminate.
unfoldrN :: Int -> (a -> Maybe (Char, a)) -> a -> (ByteString, Maybe a)
bytestring Data.ByteString.Char8
O(n) Like unfoldr, unfoldrN builds a ByteString from a seed value. However, the length of the result is limited by the first argument to unfoldrN. This function is more efficient than unfoldr when the maximum length of the result is known. The following equation relates unfoldrN and unfoldr: > unfoldrN n f s == take n (unfoldr f s)
unfoldrN :: Int -> (a -> Maybe (Word8, a)) -> a -> (ByteString, Maybe a)
bytestring Data.ByteString
O(n) Like unfoldr, unfoldrN builds a ByteString from a seed value. However, the length of the result is limited by the first argument to unfoldrN. This function is more efficient than unfoldr when the maximum length of the result is known. The following equation relates unfoldrN and unfoldr: > fst (unfoldrN n f s) == take n (unfoldr f s)
primUnfoldrBounded :: BoundedPrim b -> (a -> Maybe (b, a)) -> a -> Builder
bytestring Data.ByteString.Builder.Prim
Create a Builder that encodes a sequence generated from a seed value using a BoundedPrim for each sequence element.
primUnfoldrFixed :: FixedPrim b -> (a -> Maybe (b, a)) -> a -> Builder
bytestring Data.ByteString.Builder.Prim
Encode a list of values represented as an unfoldr with a FixedPrim.