Data/Text/Lazy.hs

    {-# OPTIONS_GHC -fno-warn-orphans #-}
    {-# LANGUAGE BangPatterns, MagicHash, CPP, TypeFamilies #-}
    #if __GLASGOW_HASKELL__ >= 702
    {-# LANGUAGE Trustworthy #-}
    #endif
    -- Using TemplateHaskell in text unconditionally is unacceptable, as
    -- it's a GHC boot library. TemplateHaskellQuotes was added in 8.0, so
    -- this would seem to be a problem. However, GHC's policy of only
    -- needing to be able to compile itself from the last few releases
    -- allows us to use full-fat TH on older versions, while using THQ for
    -- GHC versions that may be used for bootstrapping.
    #if __GLASGOW_HASKELL__ >= 800
    {-# LANGUAGE TemplateHaskellQuotes #-}
    #else
    {-# LANGUAGE TemplateHaskell #-}
    #endif

    -- |
    -- Module      : Data.Text.Lazy
    -- Copyright   : (c) 2009, 2010, 2012 Bryan O'Sullivan
    --
    -- License     : BSD-style
    -- Maintainer  : bos@serpentine.com
    -- Portability : GHC
    --
    -- A time and space-efficient implementation of Unicode text using
    -- lists of packed arrays.
    --
    -- /Note/: Read below the synopsis for important notes on the use of
    -- this module.
    --
    -- The representation used by this module is suitable for high
    -- performance use and for streaming large quantities of data.  It
    -- provides a means to manipulate a large body of text without
    -- requiring that the entire content be resident in memory.
    --
    -- Some operations, such as 'concat', 'append', 'reverse' and 'cons',
    -- have better time complexity than their "Data.Text" equivalents, due
    -- to the underlying representation being a list of chunks. For other
    -- operations, lazy 'Text's are usually within a few percent of strict
    -- ones, but often with better heap usage if used in a streaming
    -- fashion. For data larger than available memory, or if you have
    -- tight memory constraints, this module will be the only option.
    --
    -- This module is intended to be imported @qualified@, to avoid name
    -- clashes with "Prelude" functions.  eg.
    --
    -- > import qualified Data.Text.Lazy as L

    module Data.Text.Lazy
        (
        -- * Fusion
        -- $fusion

        -- * Acceptable data
        -- $replacement

        -- * Types
          Text

        -- * Creation and elimination
        , pack
        , unpack
        , singleton
        , empty
        , fromChunks
        , toChunks
        , toStrict
        , fromStrict
        , foldrChunks
        , foldlChunks

        -- * Basic interface
        , cons
        , snoc
        , append
        , uncons
        , unsnoc
        , head
        , last
        , tail
        , init
        , null
        , length
        , compareLength

        -- * Transformations
        , map
        , intercalate
        , intersperse
        , transpose
        , reverse
        , replace

        -- ** Case conversion
        -- $case
        , toCaseFold
        , toLower
        , toUpper
        , toTitle

        -- ** Justification
        , justifyLeft
        , justifyRight
        , center

        -- * Folds
        , foldl
        , foldl'
        , foldl1
        , foldl1'
        , foldr
        , foldr1

        -- ** Special folds
        , concat
        , concatMap
        , any
        , all
        , maximum
        , minimum

        -- * Construction

        -- ** Scans
        , scanl
        , scanl1
        , scanr
        , scanr1

        -- ** Accumulating maps
        , mapAccumL
        , mapAccumR

        -- ** Generation and unfolding
        , repeat
        , replicate
        , cycle
        , iterate
        , unfoldr
        , unfoldrN

        -- * Substrings

        -- ** Breaking strings
        , take
        , takeEnd
        , drop
        , dropEnd
        , takeWhile
        , takeWhileEnd
        , dropWhile
        , dropWhileEnd
        , dropAround
        , strip
        , stripStart
        , stripEnd
        , splitAt
        , span
        , breakOn
        , breakOnEnd
        , break
        , group
        , groupBy
        , inits
        , tails

        -- ** Breaking into many substrings
        -- $split
        , splitOn
        , split
        , chunksOf
        -- , breakSubstring

        -- ** Breaking into lines and words
        , lines
        , words
        , unlines
        , unwords

        -- * Predicates
        , isPrefixOf
        , isSuffixOf
        , isInfixOf

        -- ** View patterns
        , stripPrefix
        , stripSuffix
        , commonPrefixes

        -- * Searching
        , filter
        , find
        , breakOnAll
        , partition

        -- , findSubstring

        -- * Indexing
        , index
        , count

        -- * Zipping and unzipping
        , zip
        , zipWith

        -- -* Ordered text
        -- , sort
        ) where

    import Prelude (Char, Bool(..), Maybe(..), String,
                    Eq(..), Ord(..), Ordering(..), Read(..), Show(..),
                    (&&), (||), (+), (-), (.), ($), (++),
                    error, flip, fmap, fromIntegral, not, otherwise, quot)
    import qualified Prelude as P
    import Control.DeepSeq (NFData(..))
    import Data.Int (Int64)
    import qualified Data.List as L
    import Data.Char (isSpace)
    import Data.Data (Data(gfoldl, toConstr, gunfold, dataTypeOf), constrIndex,
                      Constr, mkConstr, DataType, mkDataType, Fixity(Prefix))
    import Data.Binary (Binary(get, put))
    import Data.Monoid (Monoid(..))
    #if MIN_VERSION_base(4,9,0)
    import Data.Semigroup (Semigroup(..))
    #endif
    import Data.String (IsString(..))
    import qualified Data.Text as T
    import qualified Data.Text.Internal as T
    import qualified Data.Text.Internal.Fusion.Common as S
    import qualified Data.Text.Unsafe as T
    import qualified Data.Text.Internal.Lazy.Fusion as S
    import Data.Text.Internal.Fusion.Types (PairS(..))
    import Data.Text.Internal.Lazy.Fusion (stream, unstream)
    import Data.Text.Internal.Lazy (Text(..), chunk, empty, foldlChunks,
                                    foldrChunks, smallChunkSize)
    import Data.Text.Internal (firstf, safe, text)
    import Data.Text.Lazy.Encoding (decodeUtf8', encodeUtf8)
    import qualified Data.Text.Internal.Functions as F
    import Data.Text.Internal.Lazy.Search (indices)
    #if __GLASGOW_HASKELL__ >= 702
    import qualified GHC.CString as GHC
    #else
    import qualified GHC.Base as GHC
    #endif
    #if MIN_VERSION_base(4,7,0)
    import qualified GHC.Exts as Exts
    #endif
    import GHC.Prim (Addr#)
    import qualified Language.Haskell.TH.Lib as TH
    import qualified Language.Haskell.TH.Syntax as TH
    #if MIN_VERSION_base(4,7,0)
    import Text.Printf (PrintfArg, formatArg, formatString)
    #endif

    -- $fusion
    --
    -- Most of the functions in this module are subject to /fusion/,
    -- meaning that a pipeline of such functions will usually allocate at
    -- most one 'Text' value.
    --
    -- As an example, consider the following pipeline:
    --
    -- > import Data.Text.Lazy as T
    -- > import Data.Text.Lazy.Encoding as E
    -- > import Data.ByteString.Lazy (ByteString)
    -- >
    -- > countChars :: ByteString -> Int
    -- > countChars = T.length . T.toUpper . E.decodeUtf8
    --
    -- From the type signatures involved, this looks like it should
    -- allocate one 'ByteString' value, and two 'Text' values. However,
    -- when a module is compiled with optimisation enabled under GHC, the
    -- two intermediate 'Text' values will be optimised away, and the
    -- function will be compiled down to a single loop over the source
    -- 'ByteString'.
    --
    -- Functions that can be fused by the compiler are documented with the
    -- phrase \"Subject to fusion\".

    -- $replacement
    --
    -- A 'Text' value is a sequence of Unicode scalar values, as defined
    -- in
    -- <http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf#page=35 §3.9, definition D76 of the Unicode 5.2 standard >.
    -- As such, a 'Text' cannot contain values in the range U+D800 to
    -- U+DFFF inclusive. Haskell implementations admit all Unicode code
    -- points
    -- (<http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf#page=13 §3.4, definition D10 >)
    -- as 'Char' values, including code points from this invalid range.
    -- This means that there are some 'Char' values that are not valid
    -- Unicode scalar values, and the functions in this module must handle
    -- those cases.
    --
    -- Within this module, many functions construct a 'Text' from one or
    -- more 'Char' values. Those functions will substitute 'Char' values
    -- that are not valid Unicode scalar values with the replacement
    -- character \"&#xfffd;\" (U+FFFD).  Functions that perform this
    -- inspection and replacement are documented with the phrase
    -- \"Performs replacement on invalid scalar values\".
    --
    -- (One reason for this policy of replacement is that internally, a
    -- 'Text' value is represented as packed UTF-16 data. Values in the
    -- range U+D800 through U+DFFF are used by UTF-16 to denote surrogate
    -- code points, and so cannot be represented. The functions replace
    -- invalid scalar values, instead of dropping them, as a security
    -- measure. For details, see
    -- <http://unicode.org/reports/tr36/#Deletion_of_Noncharacters Unicode Technical Report 36, §3.5 >.)

    equal :: Text -> Text -> Bool
    equal Empty Empty = True
    equal Empty _     = False
    equal _ Empty     = False
    equal (Chunk a as) (Chunk b bs) =
        case compare lenA lenB of
          LT -> a == (T.takeWord16 lenA b) &&
                as `equal` Chunk (T.dropWord16 lenA b) bs
          EQ -> a == b && as `equal` bs
          GT -> T.takeWord16 lenB a == b &&
                Chunk (T.dropWord16 lenB a) as `equal` bs
      where lenA = T.lengthWord16 a
            lenB = T.lengthWord16 b

    instance Eq Text where
        (==) = equal
        {-# INLINE (==) #-}

    instance Ord Text where
        compare = compareText

    compareText :: Text -> Text -> Ordering
    compareText Empty Empty = EQ
    compareText Empty _     = LT
    compareText _     Empty = GT
    compareText (Chunk a0 as) (Chunk b0 bs) = outer a0 b0
     where
      outer ta@(T.Text arrA offA lenA) tb@(T.Text arrB offB lenB) = go 0 0
       where
        go !i !j
          | i >= lenA = compareText as (chunk (T.Text arrB (offB+j) (lenB-j)) bs)
          | j >= lenB = compareText (chunk (T.Text arrA (offA+i) (lenA-i)) as) bs
          | a < b     = LT
          | a > b     = GT
          | otherwise = go (i+di) (j+dj)
          where T.Iter a di = T.iter ta i
                T.Iter b dj = T.iter tb j

    instance Show Text where
        showsPrec p ps r = showsPrec p (unpack ps) r

    instance Read Text where
        readsPrec p str = [(pack x,y) | (x,y) <- readsPrec p str]

    #if MIN_VERSION_base(4,9,0)
    -- | Non-orphan 'Semigroup' instance only defined for
    -- @base-4.9.0.0@ and later; orphan instances for older GHCs are
    -- provided by
    -- the [semigroups](http://hackage.haskell.org/package/semigroups)
    -- package
    --
    -- @since 1.2.2.0
    instance Semigroup Text where
        (<>) = append
    #endif

    instance Monoid Text where
        mempty  = empty
    #if MIN_VERSION_base(4,9,0)
        mappend = (<>) -- future-proof definition
    #else
        mappend = append
    #endif
        mconcat = concat

    instance IsString Text where
        fromString = pack

    #if MIN_VERSION_base(4,7,0)
    -- | @since 1.2.0.0
    instance Exts.IsList Text where
        type Item Text = Char
        fromList       = pack
        toList         = unpack
    #endif

    instance NFData Text where
        rnf Empty        = ()
        rnf (Chunk _ ts) = rnf ts

    -- | @since 1.2.1.0
    instance Binary Text where
        put t = put (encodeUtf8 t)
        get   = do
          bs <- get
          case decodeUtf8' bs of
            P.Left exn -> P.fail (P.show exn)
            P.Right a -> P.return a

    -- | This instance preserves data abstraction at the cost of inefficiency.
    -- We omit reflection services for the sake of data abstraction.
    --
    -- This instance was created by copying the updated behavior of
    -- @"Data.Text".@'Data.Text.Text'
    instance Data Text where
      gfoldl f z txt = z pack `f` (unpack txt)
      toConstr _     = packConstr
      gunfold k z c  = case constrIndex c of
        1 -> k (z pack)
        _ -> error "Data.Text.Lazy.Text.gunfold"
      dataTypeOf _   = textDataType

    -- | This instance has similar considerations to the 'Data' instance:
    -- it preserves abstraction at the cost of inefficiency.
    --
    -- @since 1.2.4.0
    instance TH.Lift Text where
      lift = TH.appE (TH.varE 'pack) . TH.stringE . unpack
    #if MIN_VERSION_template_haskell(2,17,0)
      liftTyped = TH.unsafeCodeCoerce . TH.lift
    #elif MIN_VERSION_template_haskell(2,16,0)
      liftTyped = TH.unsafeTExpCoerce . TH.lift
    #endif

    #if MIN_VERSION_base(4,7,0)
    -- | Only defined for @base-4.7.0.0@ and later
    --
    -- @since 1.2.2.0
    instance PrintfArg Text where
      formatArg txt = formatString $ unpack txt
    #endif

    packConstr :: Constr
    packConstr = mkConstr textDataType "pack" [] Prefix

    textDataType :: DataType
    textDataType = mkDataType "Data.Text.Lazy.Text" [packConstr]

    -- | /O(n)/ Convert a 'String' into a 'Text'.
    --
    -- Subject to fusion.  Performs replacement on invalid scalar values.
    pack :: String -> Text
    pack = unstream . S.streamList . L.map safe
    {-# INLINE [1] pack #-}

    -- | /O(n)/ Convert a 'Text' into a 'String'.
    -- Subject to fusion.
    unpack :: Text -> String
    unpack t = S.unstreamList (stream t)
    {-# INLINE [1] unpack #-}

    -- | /O(n)/ Convert a literal string into a Text.
    unpackCString# :: Addr# -> Text
    unpackCString# addr# = unstream (S.streamCString# addr#)
    {-# NOINLINE unpackCString# #-}

    {-# RULES "TEXT literal" forall a.
        unstream (S.streamList (L.map safe (GHC.unpackCString# a)))
          = unpackCString# a #-}

    {-# RULES "TEXT literal UTF8" forall a.
        unstream (S.streamList (L.map safe (GHC.unpackCStringUtf8# a)))
          = unpackCString# a #-}

    {-# RULES "LAZY TEXT empty literal"
        unstream (S.streamList (L.map safe []))
          = Empty #-}

    {-# RULES "LAZY TEXT empty literal" forall a.
        unstream (S.streamList (L.map safe [a]))
          = Chunk (T.singleton a) Empty #-}

    -- | /O(1)/ Convert a character into a Text.  Subject to fusion.
    -- Performs replacement on invalid scalar values.
    singleton :: Char -> Text
    singleton c = Chunk (T.singleton c) Empty
    {-# INLINE [1] singleton #-}

    {-# RULES
    "LAZY TEXT singleton -> fused" [~1] forall c.
        singleton c = unstream (S.singleton c)
    "LAZY TEXT singleton -> unfused" [1] forall c.
        unstream (S.singleton c) = singleton c
      #-}

    -- | /O(c)/ Convert a list of strict 'T.Text's into a lazy 'Text'.
    fromChunks :: [T.Text] -> Text
    fromChunks cs = L.foldr chunk Empty cs

    -- | /O(n)/ Convert a lazy 'Text' into a list of strict 'T.Text's.
    toChunks :: Text -> [T.Text]
    toChunks cs = foldrChunks (:) [] cs

    -- | /O(n)/ Convert a lazy 'Text' into a strict 'T.Text'.
    toStrict :: Text -> T.Text
    toStrict t = T.concat (toChunks t)
    {-# INLINE [1] toStrict #-}

    -- | /O(c)/ Convert a strict 'T.Text' into a lazy 'Text'.
    fromStrict :: T.Text -> Text
    fromStrict t = chunk t Empty
    {-# INLINE [1] fromStrict #-}

    -- -----------------------------------------------------------------------------
    -- * Basic functions

    -- | /O(1)/ Adds a character to the front of a 'Text'.  Subject to fusion.
    cons :: Char -> Text -> Text
    cons c t = Chunk (T.singleton c) t
    {-# INLINE [1] cons #-}

    infixr 5 `cons`

    {-# RULES
    "LAZY TEXT cons -> fused" [~1] forall c t.
        cons c t = unstream (S.cons c (stream t))
    "LAZY TEXT cons -> unfused" [1] forall c t.
        unstream (S.cons c (stream t)) = cons c t
     #-}

    -- | /O(n)/ Adds a character to the end of a 'Text'.  This copies the
    -- entire array in the process, unless fused.  Subject to fusion.
    snoc :: Text -> Char -> Text
    snoc t c = foldrChunks Chunk (singleton c) t
    {-# INLINE [1] snoc #-}

    {-# RULES
    "LAZY TEXT snoc -> fused" [~1] forall t c.
        snoc t c = unstream (S.snoc (stream t) c)
    "LAZY TEXT snoc -> unfused" [1] forall t c.
        unstream (S.snoc (stream t) c) = snoc t c
     #-}

    -- | /O(n\/c)/ Appends one 'Text' to another.  Subject to fusion.
    append :: Text -> Text -> Text
    append xs ys = foldrChunks Chunk ys xs
    {-# INLINE [1] append #-}

    {-# RULES
    "LAZY TEXT append -> fused" [~1] forall t1 t2.
        append t1 t2 = unstream (S.append (stream t1) (stream t2))
    "LAZY TEXT append -> unfused" [1] forall t1 t2.
        unstream (S.append (stream t1) (stream t2)) = append t1 t2
     #-}

    -- | /O(1)/ Returns the first character and rest of a 'Text', or
    -- 'Nothing' if empty. Subject to fusion.
    uncons :: Text -> Maybe (Char, Text)
    uncons Empty        = Nothing
    uncons (Chunk t ts) = Just (T.unsafeHead t, ts')
      where ts' | T.compareLength t 1 == EQ = ts
                | otherwise                 = Chunk (T.unsafeTail t) ts
    {-# INLINE uncons #-}

    -- | /O(1)/ Returns the first character of a 'Text', which must be
    -- non-empty.  Subject to fusion.
    head :: Text -> Char
    head t = S.head (stream t)
    {-# INLINE head #-}

    -- | /O(1)/ Returns all characters after the head of a 'Text', which
    -- must be non-empty.  Subject to fusion.
    tail :: Text -> Text
    tail (Chunk t ts) = chunk (T.tail t) ts
    tail Empty        = emptyError "tail"
    {-# INLINE [1] tail #-}

    {-# RULES
    "LAZY TEXT tail -> fused" [~1] forall t.
        tail t = unstream (S.tail (stream t))
    "LAZY TEXT tail -> unfused" [1] forall t.
        unstream (S.tail (stream t)) = tail t
     #-}

    -- | /O(n\/c)/ Returns all but the last character of a 'Text', which must
    -- be non-empty.  Subject to fusion.
    init :: Text -> Text
    init (Chunk t0 ts0) = go t0 ts0
        where go t (Chunk t' ts) = Chunk t (go t' ts)
              go t Empty         = chunk (T.init t) Empty
    init Empty = emptyError "init"
    {-# INLINE [1] init #-}

    {-# RULES
    "LAZY TEXT init -> fused" [~1] forall t.
        init t = unstream (S.init (stream t))
    "LAZY TEXT init -> unfused" [1] forall t.
        unstream (S.init (stream t)) = init t
     #-}

    -- | /O(n\/c)/ Returns the 'init' and 'last' of a 'Text', or 'Nothing' if
    -- empty.
    --
    -- * It is no faster than using 'init' and 'last'.
    --
    -- @since 1.2.3.0
    unsnoc :: Text -> Maybe (Text, Char)
    unsnoc Empty          = Nothing
    unsnoc ts@(Chunk _ _) = Just (init ts, last ts)
    {-# INLINE unsnoc #-}

    -- | /O(1)/ Tests whether a 'Text' is empty or not.  Subject to
    -- fusion.
    null :: Text -> Bool
    null Empty = True
    null _     = False
    {-# INLINE [1] null #-}

    {-# RULES
    "LAZY TEXT null -> fused" [~1] forall t.
        null t = S.null (stream t)
    "LAZY TEXT null -> unfused" [1] forall t.
        S.null (stream t) = null t
     #-}

    -- | /O(1)/ Tests whether a 'Text' contains exactly one character.
    -- Subject to fusion.
    isSingleton :: Text -> Bool
    isSingleton = S.isSingleton . stream
    {-# INLINE isSingleton #-}

    -- | /O(n\/c)/ Returns the last character of a 'Text', which must be
    -- non-empty.  Subject to fusion.
    last :: Text -> Char
    last Empty        = emptyError "last"
    last (Chunk t ts) = go t ts
        where go _ (Chunk t' ts') = go t' ts'
              go t' Empty         = T.last t'
    {-# INLINE [1] last #-}

    {-# RULES
    "LAZY TEXT last -> fused" [~1] forall t.
        last t = S.last (stream t)
    "LAZY TEXT last -> unfused" [1] forall t.
        S.last (stream t) = last t
      #-}

    -- | /O(n)/ Returns the number of characters in a 'Text'.
    -- Subject to fusion.
    length :: Text -> Int64
    length = foldlChunks go 0
        where go l t = l + fromIntegral (T.length t)
    {-# INLINE [1] length #-}

    {-# RULES
    "LAZY TEXT length -> fused" [~1] forall t.
        length t = S.length (stream t)
    "LAZY TEXT length -> unfused" [1] forall t.
        S.length (stream t) = length t
     #-}

    -- | /O(n)/ Compare the count of characters in a 'Text' to a number.
    -- Subject to fusion.
    --
    -- This function gives the same answer as comparing against the result
    -- of 'length', but can short circuit if the count of characters is
    -- greater than the number, and hence be more efficient.
    compareLength :: Text -> Int64 -> Ordering
    compareLength t n = S.compareLengthI (stream t) n
    {-# INLINE [1] compareLength #-}

    -- We don't apply those otherwise appealing length-to-compareLength
    -- rewrite rules here, because they can change the strictness
    -- properties of code.

    -- | /O(n)/ 'map' @f@ @t@ is the 'Text' obtained by applying @f@ to
    -- each element of @t@.  Subject to fusion.  Performs replacement on
    -- invalid scalar values.
    map :: (Char -> Char) -> Text -> Text
    map f t = unstream (S.map (safe . f) (stream t))
    {-# INLINE [1] map #-}

    -- | /O(n)/ The 'intercalate' function takes a 'Text' and a list of
    -- 'Text's and concatenates the list after interspersing the first
    -- argument between each element of the list.
    intercalate :: Text -> [Text] -> Text
    intercalate t = concat . (F.intersperse t)
    {-# INLINE intercalate #-}

    -- | /O(n)/ The 'intersperse' function takes a character and places it
    -- between the characters of a 'Text'.  Subject to fusion.  Performs
    -- replacement on invalid scalar values.
    intersperse :: Char -> Text -> Text
    intersperse c t = unstream (S.intersperse (safe c) (stream t))
    {-# INLINE intersperse #-}

    -- | /O(n)/ Left-justify a string to the given length, using the
    -- specified fill character on the right. Subject to fusion.  Performs
    -- replacement on invalid scalar values.
    --
    -- Examples:
    --
    -- > justifyLeft 7 'x' "foo"    == "fooxxxx"
    -- > justifyLeft 3 'x' "foobar" == "foobar"
    justifyLeft :: Int64 -> Char -> Text -> Text
    justifyLeft k c t
        | len >= k  = t
        | otherwise = t `append` replicateChar (k-len) c
      where len = length t
    {-# INLINE [1] justifyLeft #-}

    {-# RULES
    "LAZY TEXT justifyLeft -> fused" [~1] forall k c t.
        justifyLeft k c t = unstream (S.justifyLeftI k c (stream t))
    "LAZY TEXT justifyLeft -> unfused" [1] forall k c t.
        unstream (S.justifyLeftI k c (stream t)) = justifyLeft k c t
      #-}

    -- | /O(n)/ Right-justify a string to the given length, using the
    -- specified fill character on the left.  Performs replacement on
    -- invalid scalar values.
    --
    -- Examples:
    --
    -- > justifyRight 7 'x' "bar"    == "xxxxbar"
    -- > justifyRight 3 'x' "foobar" == "foobar"
    justifyRight :: Int64 -> Char -> Text -> Text
    justifyRight k c t
        | len >= k  = t
        | otherwise = replicateChar (k-len) c `append` t
      where len = length t
    {-# INLINE justifyRight #-}

    -- | /O(n)/ Center a string to the given length, using the specified
    -- fill character on either side.  Performs replacement on invalid
    -- scalar values.
    --
    -- Examples:
    --
    -- > center 8 'x' "HS" = "xxxHSxxx"
    center :: Int64 -> Char -> Text -> Text
    center k c t
        | len >= k  = t
        | otherwise = replicateChar l c `append` t `append` replicateChar r c
      where len = length t
            d   = k - len
            r   = d `quot` 2
            l   = d - r
    {-# INLINE center #-}

    -- | /O(n)/ The 'transpose' function transposes the rows and columns
    -- of its 'Text' argument.  Note that this function uses 'pack',
    -- 'unpack', and the list version of transpose, and is thus not very
    -- efficient.
    transpose :: [Text] -> [Text]
    transpose ts = L.map (\ss -> Chunk (T.pack ss) Empty)
                         (L.transpose (L.map unpack ts))
    -- TODO: make this fast

    -- | /O(n)/ 'reverse' @t@ returns the elements of @t@ in reverse order.
    reverse :: Text -> Text
    reverse = rev Empty
      where rev a Empty        = a
            rev a (Chunk t ts) = rev (Chunk (T.reverse t) a) ts

    -- | /O(m+n)/ Replace every non-overlapping occurrence of @needle@ in
    -- @haystack@ with @replacement@.
    --
    -- This function behaves as though it was defined as follows:
    --
    -- @
    -- replace needle replacement haystack =
    --   'intercalate' replacement ('splitOn' needle haystack)
    -- @
    --
    -- As this suggests, each occurrence is replaced exactly once.  So if
    -- @needle@ occurs in @replacement@, that occurrence will /not/ itself
    -- be replaced recursively:
    --
    -- > replace "oo" "foo" "oo" == "foo"
    --
    -- In cases where several instances of @needle@ overlap, only the
    -- first one will be replaced:
    --
    -- > replace "ofo" "bar" "ofofo" == "barfo"
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    replace :: Text
            -- ^ @needle@ to search for.  If this string is empty, an
            -- error will occur.
            -> Text
            -- ^ @replacement@ to replace @needle@ with.
            -> Text
            -- ^ @haystack@ in which to search.
            -> Text
    replace s d = intercalate d . splitOn s
    {-# INLINE replace #-}

    -- ----------------------------------------------------------------------------
    -- ** Case conversions (folds)

    -- $case
    --
    -- With Unicode text, it is incorrect to use combinators like @map
    -- toUpper@ to case convert each character of a string individually.
    -- Instead, use the whole-string case conversion functions from this
    -- module.  For correctness in different writing systems, these
    -- functions may map one input character to two or three output
    -- characters.

    -- | /O(n)/ Convert a string to folded case.  Subject to fusion.
    --
    -- This function is mainly useful for performing caseless (or case
    -- insensitive) string comparisons.
    --
    -- A string @x@ is a caseless match for a string @y@ if and only if:
    --
    -- @toCaseFold x == toCaseFold y@
    --
    -- The result string may be longer than the input string, and may
    -- differ from applying 'toLower' to the input string.  For instance,
    -- the Armenian small ligature men now (U+FB13) is case folded to the
    -- bigram men now (U+0574 U+0576), while the micro sign (U+00B5) is
    -- case folded to the Greek small letter letter mu (U+03BC) instead of
    -- itself.
    toCaseFold :: Text -> Text
    toCaseFold t = unstream (S.toCaseFold (stream t))
    {-# INLINE toCaseFold #-}

    -- | /O(n)/ Convert a string to lower case, using simple case
    -- conversion.  Subject to fusion.
    --
    -- The result string may be longer than the input string.  For
    -- instance, the Latin capital letter I with dot above (U+0130) maps
    -- to the sequence Latin small letter i (U+0069) followed by combining
    -- dot above (U+0307).
    toLower :: Text -> Text
    toLower t = unstream (S.toLower (stream t))
    {-# INLINE toLower #-}

    -- | /O(n)/ Convert a string to upper case, using simple case
    -- conversion.  Subject to fusion.
    --
    -- The result string may be longer than the input string.  For
    -- instance, the German eszett (U+00DF) maps to the two-letter
    -- sequence SS.
    toUpper :: Text -> Text
    toUpper t = unstream (S.toUpper (stream t))
    {-# INLINE toUpper #-}

    -- | /O(n)/ Convert a string to title case, using simple case
    -- conversion.  Subject to fusion.
    --
    -- The first letter of the input is converted to title case, as is
    -- every subsequent letter that immediately follows a non-letter.
    -- Every letter that immediately follows another letter is converted
    -- to lower case.
    --
    -- The result string may be longer than the input string. For example,
    -- the Latin small ligature &#xfb02; (U+FB02) is converted to the
    -- sequence Latin capital letter F (U+0046) followed by Latin small
    -- letter l (U+006C).
    --
    -- /Note/: this function does not take language or culture specific
    -- rules into account. For instance, in English, different style
    -- guides disagree on whether the book name \"The Hill of the Red
    -- Fox\" is correctly title cased&#x2014;but this function will
    -- capitalize /every/ word.
    --
    -- @since 1.0.0.0
    toTitle :: Text -> Text
    toTitle t = unstream (S.toTitle (stream t))
    {-# INLINE toTitle #-}

    -- | /O(n)/ 'foldl', applied to a binary operator, a starting value
    -- (typically the left-identity of the operator), and a 'Text',
    -- reduces the 'Text' using the binary operator, from left to right.
    -- Subject to fusion.
    foldl :: (a -> Char -> a) -> a -> Text -> a
    foldl f z t = S.foldl f z (stream t)
    {-# INLINE foldl #-}

    -- | /O(n)/ A strict version of 'foldl'.
    -- Subject to fusion.
    foldl' :: (a -> Char -> a) -> a -> Text -> a
    foldl' f z t = S.foldl' f z (stream t)
    {-# INLINE foldl' #-}

    -- | /O(n)/ A variant of 'foldl' that has no starting value argument,
    -- and thus must be applied to a non-empty 'Text'.  Subject to fusion.
    foldl1 :: (Char -> Char -> Char) -> Text -> Char
    foldl1 f t = S.foldl1 f (stream t)
    {-# INLINE foldl1 #-}

    -- | /O(n)/ A strict version of 'foldl1'.  Subject to fusion.
    foldl1' :: (Char -> Char -> Char) -> Text -> Char
    foldl1' f t = S.foldl1' f (stream t)
    {-# INLINE foldl1' #-}

    -- | /O(n)/ 'foldr', applied to a binary operator, a starting value
    -- (typically the right-identity of the operator), and a 'Text',
    -- reduces the 'Text' using the binary operator, from right to left.
    -- Subject to fusion.
    foldr :: (Char -> a -> a) -> a -> Text -> a
    foldr f z t = S.foldr f z (stream t)
    {-# INLINE foldr #-}

    -- | /O(n)/ A variant of 'foldr' that has no starting value argument,
    -- and thus must be applied to a non-empty 'Text'.  Subject to
    -- fusion.
    foldr1 :: (Char -> Char -> Char) -> Text -> Char
    foldr1 f t = S.foldr1 f (stream t)
    {-# INLINE foldr1 #-}

    -- | /O(n)/ Concatenate a list of 'Text's.
    concat :: [Text] -> Text
    concat = to
      where
        go Empty        css = to css
        go (Chunk c cs) css = Chunk c (go cs css)
        to []               = Empty
        to (cs:css)         = go cs css
    {-# INLINE concat #-}

    -- | /O(n)/ Map a function over a 'Text' that results in a 'Text', and
    -- concatenate the results.
    concatMap :: (Char -> Text) -> Text -> Text
    concatMap f = concat . foldr ((:) . f) []
    {-# INLINE concatMap #-}

    -- | /O(n)/ 'any' @p@ @t@ determines whether any character in the
    -- 'Text' @t@ satisfies the predicate @p@. Subject to fusion.
    any :: (Char -> Bool) -> Text -> Bool
    any p t = S.any p (stream t)
    {-# INLINE any #-}

    -- | /O(n)/ 'all' @p@ @t@ determines whether all characters in the
    -- 'Text' @t@ satisfy the predicate @p@. Subject to fusion.
    all :: (Char -> Bool) -> Text -> Bool
    all p t = S.all p (stream t)
    {-# INLINE all #-}

    -- | /O(n)/ 'maximum' returns the maximum value from a 'Text', which
    -- must be non-empty. Subject to fusion.
    maximum :: Text -> Char
    maximum t = S.maximum (stream t)
    {-# INLINE maximum #-}

    -- | /O(n)/ 'minimum' returns the minimum value from a 'Text', which
    -- must be non-empty. Subject to fusion.
    minimum :: Text -> Char
    minimum t = S.minimum (stream t)
    {-# INLINE minimum #-}

    -- | /O(n)/ 'scanl' is similar to 'foldl', but returns a list of
    -- successive reduced values from the left. Subject to fusion.
    -- Performs replacement on invalid scalar values.
    --
    -- > scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
    --
    -- Note that
    --
    -- > last (scanl f z xs) == foldl f z xs.
    scanl :: (Char -> Char -> Char) -> Char -> Text -> Text
    scanl f z t = unstream (S.scanl g z (stream t))
        where g a b = safe (f a b)
    {-# INLINE scanl #-}

    -- | /O(n)/ 'scanl1' is a variant of 'scanl' that has no starting
    -- value argument.  Performs replacement on invalid scalar values.
    --
    -- > scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
    scanl1 :: (Char -> Char -> Char) -> Text -> Text
    scanl1 f t0 = case uncons t0 of
                    Nothing -> empty
                    Just (t,ts) -> scanl f t ts
    {-# INLINE scanl1 #-}

    -- | /O(n)/ 'scanr' is the right-to-left dual of 'scanl'.  Performs
    -- replacement on invalid scalar values.
    --
    -- > scanr f v == reverse . scanl (flip f) v . reverse
    scanr :: (Char -> Char -> Char) -> Char -> Text -> Text
    scanr f v = reverse . scanl g v . reverse
        where g a b = safe (f b a)

    -- | /O(n)/ 'scanr1' is a variant of 'scanr' that has no starting
    -- value argument.  Performs replacement on invalid scalar values.
    scanr1 :: (Char -> Char -> Char) -> Text -> Text
    scanr1 f t | null t    = empty
               | otherwise = scanr f (last t) (init t)

    -- | /O(n)/ Like a combination of 'map' and 'foldl''. Applies a
    -- function to each element of a 'Text', passing an accumulating
    -- parameter from left to right, and returns a final 'Text'.  Performs
    -- replacement on invalid scalar values.
    mapAccumL :: (a -> Char -> (a,Char)) -> a -> Text -> (a, Text)
    mapAccumL f = go
      where
        go z (Chunk c cs)    = (z'', Chunk c' cs')
            where (z',  c')  = T.mapAccumL f z c
                  (z'', cs') = go z' cs
        go z Empty           = (z, Empty)
    {-# INLINE mapAccumL #-}

    -- | The 'mapAccumR' function behaves like a combination of 'map' and
    -- a strict 'foldr'; it applies a function to each element of a
    -- 'Text', passing an accumulating parameter from right to left, and
    -- returning a final value of this accumulator together with the new
    -- 'Text'.  Performs replacement on invalid scalar values.
    mapAccumR :: (a -> Char -> (a,Char)) -> a -> Text -> (a, Text)
    mapAccumR f = go
      where
        go z (Chunk c cs)   = (z'', Chunk c' cs')
            where (z'', c') = T.mapAccumR f z' c
                  (z', cs') = go z cs
        go z Empty          = (z, Empty)
    {-# INLINE mapAccumR #-}

    -- | @'repeat' x@ is an infinite 'Text', with @x@ the value of every
    -- element.
    --
    -- @since 1.2.0.5
    repeat :: Char -> Text
    repeat c = let t = Chunk (T.replicate smallChunkSize (T.singleton c)) t
                in t

    -- | /O(n*m)/ 'replicate' @n@ @t@ is a 'Text' consisting of the input
    -- @t@ repeated @n@ times.
    replicate :: Int64 -> Text -> Text
    replicate n t
        | null t || n <= 0 = empty
        | isSingleton t    = replicateChar n (head t)
        | otherwise        = concat (rep 0)
        where rep !i | i >= n    = []
                     | otherwise = t : rep (i+1)
    {-# INLINE [1] replicate #-}

    -- | 'cycle' ties a finite, non-empty 'Text' into a circular one, or
    -- equivalently, the infinite repetition of the original 'Text'.
    --
    -- @since 1.2.0.5
    cycle :: Text -> Text
    cycle Empty = emptyError "cycle"
    cycle t     = let t' = foldrChunks Chunk t' t
                   in t'

    -- | @'iterate' f x@ returns an infinite 'Text' of repeated applications
    -- of @f@ to @x@:
    --
    -- > iterate f x == [x, f x, f (f x), ...]
    --
    -- @since 1.2.0.5
    iterate :: (Char -> Char) -> Char -> Text
    iterate f c = let t c' = Chunk (T.singleton c') (t (f c'))
                   in t c

    -- | /O(n)/ 'replicateChar' @n@ @c@ is a 'Text' of length @n@ with @c@ the
    -- value of every element. Subject to fusion.
    replicateChar :: Int64 -> Char -> Text
    replicateChar n c = unstream (S.replicateCharI n (safe c))
    {-# INLINE replicateChar #-}

    {-# RULES
    "LAZY TEXT replicate/singleton -> replicateChar" [~1] forall n c.
        replicate n (singleton c) = replicateChar n c
    "LAZY TEXT replicate/unstream/singleton -> replicateChar" [~1] forall n c.
        replicate n (unstream (S.singleton c)) = replicateChar n c
      #-}

    -- | /O(n)/, where @n@ is the length of the result. The 'unfoldr'
    -- function is analogous to the List 'L.unfoldr'. 'unfoldr' builds a
    -- 'Text' from a seed value. The function takes the element and
    -- returns 'Nothing' if it is done producing the 'Text', otherwise
    -- 'Just' @(a,b)@.  In this case, @a@ is the next 'Char' in the
    -- string, and @b@ is the seed value for further production.
    -- Subject to fusion.
    -- Performs replacement on invalid scalar values.
    unfoldr :: (a -> Maybe (Char,a)) -> a -> Text
    unfoldr f s = unstream (S.unfoldr (firstf safe . f) s)
    {-# INLINE unfoldr #-}

    -- | /O(n)/ Like 'unfoldr', 'unfoldrN' builds a 'Text' from a seed
    -- value. However, the length of the result should be limited by the
    -- first argument to 'unfoldrN'. This function is more efficient than
    -- 'unfoldr' when the maximum length of the result is known and
    -- correct, otherwise its performance is similar to 'unfoldr'.
    -- Subject to fusion.
    -- Performs replacement on invalid scalar values.
    unfoldrN :: Int64 -> (a -> Maybe (Char,a)) -> a -> Text
    unfoldrN n f s = unstream (S.unfoldrN n (firstf safe . f) s)
    {-# INLINE unfoldrN #-}

    -- | /O(n)/ 'take' @n@, applied to a 'Text', returns the prefix of the
    -- 'Text' of length @n@, or the 'Text' itself if @n@ is greater than
    -- the length of the Text. Subject to fusion.
    take :: Int64 -> Text -> Text
    take i _ | i <= 0 = Empty
    take i t0         = take' i t0
      where take' 0 _            = Empty
            take' _ Empty        = Empty
            take' n (Chunk t ts)
                | n < len   = Chunk (T.take (fromIntegral n) t) Empty
                | otherwise = Chunk t (take' (n - len) ts)
                where len = fromIntegral (T.length t)
    {-# INLINE [1] take #-}

    {-# RULES
    "LAZY TEXT take -> fused" [~1] forall n t.
        take n t = unstream (S.take n (stream t))
    "LAZY TEXT take -> unfused" [1] forall n t.
        unstream (S.take n (stream t)) = take n t
      #-}

    -- | /O(n)/ 'takeEnd' @n@ @t@ returns the suffix remaining after
    -- taking @n@ characters from the end of @t@.
    --
    -- Examples:
    --
    -- > takeEnd 3 "foobar" == "bar"
    --
    -- @since 1.1.1.0
    takeEnd :: Int64 -> Text -> Text
    takeEnd n t0
        | n <= 0    = empty
        | otherwise = takeChunk n empty . L.reverse . toChunks $ t0
      where takeChunk _ acc [] = acc
            takeChunk i acc (t:ts)
              | i <= l    = chunk (T.takeEnd (fromIntegral i) t) acc
              | otherwise = takeChunk (i-l) (Chunk t acc) ts
              where l = fromIntegral (T.length t)

    -- | /O(n)/ 'drop' @n@, applied to a 'Text', returns the suffix of the
    -- 'Text' after the first @n@ characters, or the empty 'Text' if @n@
    -- is greater than the length of the 'Text'. Subject to fusion.
    drop :: Int64 -> Text -> Text
    drop i t0
        | i <= 0    = t0
        | otherwise = drop' i t0
      where drop' 0 ts           = ts
            drop' _ Empty        = Empty
            drop' n (Chunk t ts)
                | n < len   = Chunk (T.drop (fromIntegral n) t) ts
                | otherwise = drop' (n - len) ts
                where len   = fromIntegral (T.length t)
    {-# INLINE [1] drop #-}

    {-# RULES
    "LAZY TEXT drop -> fused" [~1] forall n t.
        drop n t = unstream (S.drop n (stream t))
    "LAZY TEXT drop -> unfused" [1] forall n t.
        unstream (S.drop n (stream t)) = drop n t
      #-}

    -- | /O(n)/ 'dropEnd' @n@ @t@ returns the prefix remaining after
    -- dropping @n@ characters from the end of @t@.
    --
    -- Examples:
    --
    -- > dropEnd 3 "foobar" == "foo"
    --
    -- @since 1.1.1.0
    dropEnd :: Int64 -> Text -> Text
    dropEnd n t0
        | n <= 0    = t0
        | otherwise = dropChunk n . L.reverse . toChunks $ t0
      where dropChunk _ [] = empty
            dropChunk m (t:ts)
              | m >= l    = dropChunk (m-l) ts
              | otherwise = fromChunks . L.reverse $
                            T.dropEnd (fromIntegral m) t : ts
              where l = fromIntegral (T.length t)

    -- | /O(n)/ 'dropWords' @n@ returns the suffix with @n@ 'Word16'
    -- values dropped, or the empty 'Text' if @n@ is greater than the
    -- number of 'Word16' values present.
    dropWords :: Int64 -> Text -> Text
    dropWords i t0
        | i <= 0    = t0
        | otherwise = drop' i t0
      where drop' 0 ts           = ts
            drop' _ Empty        = Empty
            drop' n (Chunk (T.Text arr off len) ts)
                | n < len'  = chunk (text arr (off+n') (len-n')) ts
                | otherwise = drop' (n - len') ts
                where len'  = fromIntegral len
                      n'    = fromIntegral n

    -- | /O(n)/ 'takeWhile', applied to a predicate @p@ and a 'Text',
    -- returns the longest prefix (possibly empty) of elements that
    -- satisfy @p@.  Subject to fusion.
    takeWhile :: (Char -> Bool) -> Text -> Text
    takeWhile p t0 = takeWhile' t0
      where takeWhile' Empty        = Empty
            takeWhile' (Chunk t ts) =
              case T.findIndex (not . p) t of
                Just n | n > 0     -> Chunk (T.take n t) Empty
                       | otherwise -> Empty
                Nothing            -> Chunk t (takeWhile' ts)
    {-# INLINE [1] takeWhile #-}

    {-# RULES
    "LAZY TEXT takeWhile -> fused" [~1] forall p t.
        takeWhile p t = unstream (S.takeWhile p (stream t))
    "LAZY TEXT takeWhile -> unfused" [1] forall p t.
        unstream (S.takeWhile p (stream t)) = takeWhile p t
      #-}
    -- | /O(n)/ 'takeWhileEnd', applied to a predicate @p@ and a 'Text',
    -- returns the longest suffix (possibly empty) of elements that
    -- satisfy @p@.
    -- Examples:
    --
    -- > takeWhileEnd (=='o') "foo" == "oo"
    --
    -- @since 1.2.2.0
    takeWhileEnd :: (Char -> Bool) -> Text -> Text
    takeWhileEnd p = takeChunk empty . L.reverse . toChunks
      where takeChunk acc []     = acc
            takeChunk acc (t:ts)
              | T.lengthWord16 t' < T.lengthWord16 t
                                 = chunk t' acc
              | otherwise        = takeChunk (Chunk t' acc) ts
              where t' = T.takeWhileEnd p t
    {-# INLINE takeWhileEnd #-}

    -- | /O(n)/ 'dropWhile' @p@ @t@ returns the suffix remaining after
    -- 'takeWhile' @p@ @t@.  Subject to fusion.
    dropWhile :: (Char -> Bool) -> Text -> Text
    dropWhile p t0 = dropWhile' t0
      where dropWhile' Empty        = Empty
            dropWhile' (Chunk t ts) =
              case T.findIndex (not . p) t of
                Just n  -> Chunk (T.drop n t) ts
                Nothing -> dropWhile' ts
    {-# INLINE [1] dropWhile #-}

    {-# RULES
    "LAZY TEXT dropWhile -> fused" [~1] forall p t.
        dropWhile p t = unstream (S.dropWhile p (stream t))
    "LAZY TEXT dropWhile -> unfused" [1] forall p t.
        unstream (S.dropWhile p (stream t)) = dropWhile p t
      #-}

    -- | /O(n)/ 'dropWhileEnd' @p@ @t@ returns the prefix remaining after
    -- dropping characters that satisfy the predicate @p@ from the end of
    -- @t@.
    --
    -- Examples:
    --
    -- > dropWhileEnd (=='.') "foo..." == "foo"
    dropWhileEnd :: (Char -> Bool) -> Text -> Text
    dropWhileEnd p = go
      where go Empty = Empty
            go (Chunk t Empty) = if T.null t'
                                 then Empty
                                 else Chunk t' Empty
                where t' = T.dropWhileEnd p t
            go (Chunk t ts) = case go ts of
                                Empty -> go (Chunk t Empty)
                                ts' -> Chunk t ts'
    {-# INLINE dropWhileEnd #-}

    -- | /O(n)/ 'dropAround' @p@ @t@ returns the substring remaining after
    -- dropping characters that satisfy the predicate @p@ from both the
    -- beginning and end of @t@.
    dropAround :: (Char -> Bool) -> Text -> Text
    dropAround p = dropWhile p . dropWhileEnd p
    {-# INLINE [1] dropAround #-}

    -- | /O(n)/ Remove leading white space from a string.  Equivalent to:
    --
    -- > dropWhile isSpace
    stripStart :: Text -> Text
    stripStart = dropWhile isSpace
    {-# INLINE stripStart #-}

    -- | /O(n)/ Remove trailing white space from a string.  Equivalent to:
    --
    -- > dropWhileEnd isSpace
    stripEnd :: Text -> Text
    stripEnd = dropWhileEnd isSpace
    {-# INLINE [1] stripEnd #-}

    -- | /O(n)/ Remove leading and trailing white space from a string.
    -- Equivalent to:
    --
    -- > dropAround isSpace
    strip :: Text -> Text
    strip = dropAround isSpace
    {-# INLINE [1] strip #-}

    -- | /O(n)/ 'splitAt' @n t@ returns a pair whose first element is a
    -- prefix of @t@ of length @n@, and whose second is the remainder of
    -- the string. It is equivalent to @('take' n t, 'drop' n t)@.
    splitAt :: Int64 -> Text -> (Text, Text)
    splitAt = loop
      where loop _ Empty      = (empty, empty)
            loop n t | n <= 0 = (empty, t)
            loop n (Chunk t ts)
                 | n < len   = let (t',t'') = T.splitAt (fromIntegral n) t
                               in (Chunk t' Empty, Chunk t'' ts)
                 | otherwise = let (ts',ts'') = loop (n - len) ts
                               in (Chunk t ts', ts'')
                 where len = fromIntegral (T.length t)

    -- | /O(n)/ 'splitAtWord' @n t@ returns a strict pair whose first
    -- element is a prefix of @t@ whose chunks contain @n@ 'Word16'
    -- values, and whose second is the remainder of the string.
    splitAtWord :: Int64 -> Text -> PairS Text Text
    splitAtWord _ Empty = empty :*: empty
    splitAtWord x (Chunk c@(T.Text arr off len) cs)
        | y >= len  = let h :*: t = splitAtWord (x-fromIntegral len) cs
                      in  Chunk c h :*: t
        | otherwise = chunk (text arr off y) empty :*:
                      chunk (text arr (off+y) (len-y)) cs
        where y = fromIntegral x

    -- | /O(n+m)/ Find the first instance of @needle@ (which must be
    -- non-'null') in @haystack@.  The first element of the returned tuple
    -- is the prefix of @haystack@ before @needle@ is matched.  The second
    -- is the remainder of @haystack@, starting with the match.
    --
    -- Examples:
    --
    -- > breakOn "::" "a::b::c" ==> ("a", "::b::c")
    -- > breakOn "/" "foobar"   ==> ("foobar", "")
    --
    -- Laws:
    --
    -- > append prefix match == haystack
    -- >   where (prefix, match) = breakOn needle haystack
    --
    -- If you need to break a string by a substring repeatedly (e.g. you
    -- want to break on every instance of a substring), use 'breakOnAll'
    -- instead, as it has lower startup overhead.
    --
    -- This function is strict in its first argument, and lazy in its
    -- second.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    breakOn :: Text -> Text -> (Text, Text)
    breakOn pat src
        | null pat  = emptyError "breakOn"
        | otherwise = case indices pat src of
                        []    -> (src, empty)
                        (x:_) -> let h :*: t = splitAtWord x src
                                 in  (h, t)

    -- | /O(n+m)/ Similar to 'breakOn', but searches from the end of the string.
    --
    -- The first element of the returned tuple is the prefix of @haystack@
    -- up to and including the last match of @needle@.  The second is the
    -- remainder of @haystack@, following the match.
    --
    -- > breakOnEnd "::" "a::b::c" ==> ("a::b::", "c")
    breakOnEnd :: Text -> Text -> (Text, Text)
    breakOnEnd pat src = let (a,b) = breakOn (reverse pat) (reverse src)
                       in  (reverse b, reverse a)
    {-# INLINE breakOnEnd #-}

    -- | /O(n+m)/ Find all non-overlapping instances of @needle@ in
    -- @haystack@.  Each element of the returned list consists of a pair:
    --
    -- * The entire string prior to the /k/th match (i.e. the prefix)
    --
    -- * The /k/th match, followed by the remainder of the string
    --
    -- Examples:
    --
    -- > breakOnAll "::" ""
    -- > ==> []
    -- > breakOnAll "/" "a/b/c/"
    -- > ==> [("a", "/b/c/"), ("a/b", "/c/"), ("a/b/c", "/")]
    --
    -- This function is strict in its first argument, and lazy in its
    -- second.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    --
    -- The @needle@ parameter may not be empty.
    breakOnAll :: Text              -- ^ @needle@ to search for
               -> Text              -- ^ @haystack@ in which to search
               -> [(Text, Text)]
    breakOnAll pat src
        | null pat  = emptyError "breakOnAll"
        | otherwise = go 0 empty src (indices pat src)
      where
        go !n p s (x:xs) = let h :*: t = splitAtWord (x-n) s
                               h'      = append p h
                           in (h',t) : go x h' t xs
        go _  _ _ _      = []

    -- | /O(n)/ 'break' is like 'span', but the prefix returned is over
    -- elements that fail the predicate @p@.
    break :: (Char -> Bool) -> Text -> (Text, Text)
    break p t0 = break' t0
      where break' Empty          = (empty, empty)
            break' c@(Chunk t ts) =
              case T.findIndex p t of
                Nothing      -> let (ts', ts'') = break' ts
                                in (Chunk t ts', ts'')
                Just n | n == 0    -> (Empty, c)
                       | otherwise -> let (a,b) = T.splitAt n t
                                      in (Chunk a Empty, Chunk b ts)

    -- | /O(n)/ 'span', applied to a predicate @p@ and text @t@, returns
    -- a pair whose first element is the longest prefix (possibly empty)
    -- of @t@ of elements that satisfy @p@, and whose second is the
    -- remainder of the list.
    span :: (Char -> Bool) -> Text -> (Text, Text)
    span p = break (not . p)
    {-# INLINE span #-}

    -- | The 'group' function takes a 'Text' and returns a list of 'Text's
    -- such that the concatenation of the result is equal to the argument.
    -- Moreover, each sublist in the result contains only equal elements.
    -- For example,
    --
    -- > group "Mississippi" = ["M","i","ss","i","ss","i","pp","i"]
    --
    -- It is a special case of 'groupBy', which allows the programmer to
    -- supply their own equality test.
    group :: Text -> [Text]
    group =  groupBy (==)
    {-# INLINE group #-}

    -- | The 'groupBy' function is the non-overloaded version of 'group'.
    groupBy :: (Char -> Char -> Bool) -> Text -> [Text]
    groupBy _  Empty        = []
    groupBy eq (Chunk t ts) = cons x ys : groupBy eq zs
                              where (ys,zs) = span (eq x) xs
                                    x  = T.unsafeHead t
                                    xs = chunk (T.unsafeTail t) ts

    -- | /O(n)/ Return all initial segments of the given 'Text',
    -- shortest first.
    inits :: Text -> [Text]
    inits = (Empty :) . inits'
      where inits' Empty        = []
            inits' (Chunk t ts) = L.map (\t' -> Chunk t' Empty) (L.tail (T.inits t))
                               ++ L.map (Chunk t) (inits' ts)

    -- | /O(n)/ Return all final segments of the given 'Text', longest
    -- first.
    tails :: Text -> [Text]
    tails Empty         = Empty : []
    tails ts@(Chunk t ts')
      | T.length t == 1 = ts : tails ts'
      | otherwise       = ts : tails (Chunk (T.unsafeTail t) ts')

    -- $split
    --
    -- Splitting functions in this library do not perform character-wise
    -- copies to create substrings; they just construct new 'Text's that
    -- are slices of the original.

    -- | /O(m+n)/ Break a 'Text' into pieces separated by the first 'Text'
    -- argument (which cannot be an empty string), consuming the
    -- delimiter. An empty delimiter is invalid, and will cause an error
    -- to be raised.
    --
    -- Examples:
    --
    -- > splitOn "\r\n" "a\r\nb\r\nd\r\ne" == ["a","b","d","e"]
    -- > splitOn "aaa"  "aaaXaaaXaaaXaaa"  == ["","X","X","X",""]
    -- > splitOn "x"    "x"                == ["",""]
    --
    -- and
    --
    -- > intercalate s . splitOn s         == id
    -- > splitOn (singleton c)             == split (==c)
    --
    -- (Note: the string @s@ to split on above cannot be empty.)
    --
    -- This function is strict in its first argument, and lazy in its
    -- second.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    splitOn :: Text
            -- ^ String to split on. If this string is empty, an error
            -- will occur.
            -> Text
            -- ^ Input text.
            -> [Text]
    splitOn pat src
        | null pat        = emptyError "splitOn"
        | isSingleton pat = split (== head pat) src
        | otherwise       = go 0 (indices pat src) src
      where
        go  _ []     cs = [cs]
        go !i (x:xs) cs = let h :*: t = splitAtWord (x-i) cs
                          in  h : go (x+l) xs (dropWords l t)
        l = foldlChunks (\a (T.Text _ _ b) -> a + fromIntegral b) 0 pat
    {-# INLINE [1] splitOn #-}

    {-# RULES
    "LAZY TEXT splitOn/singleton -> split/==" [~1] forall c t.
        splitOn (singleton c) t = split (==c) t
      #-}

    -- | /O(n)/ Splits a 'Text' into components delimited by separators,
    -- where the predicate returns True for a separator element.  The
    -- resulting components do not contain the separators.  Two adjacent
    -- separators result in an empty component in the output.  eg.
    --
    -- > split (=='a') "aabbaca" == ["","","bb","c",""]
    -- > split (=='a') []        == [""]
    split :: (Char -> Bool) -> Text -> [Text]
    split _ Empty = [Empty]
    split p (Chunk t0 ts0) = comb [] (T.split p t0) ts0
      where comb acc (s:[]) Empty        = revChunks (s:acc) : []
            comb acc (s:[]) (Chunk t ts) = comb (s:acc) (T.split p t) ts
            comb acc (s:ss) ts           = revChunks (s:acc) : comb [] ss ts
            comb _   []     _            = impossibleError "split"
    {-# INLINE split #-}

    -- | /O(n)/ Splits a 'Text' into components of length @k@.  The last
    -- element may be shorter than the other chunks, depending on the
    -- length of the input. Examples:
    --
    -- > chunksOf 3 "foobarbaz"   == ["foo","bar","baz"]
    -- > chunksOf 4 "haskell.org" == ["hask","ell.","org"]
    chunksOf :: Int64 -> Text -> [Text]
    chunksOf k = go
      where
        go t = case splitAt k t of
                 (a,b) | null a    -> []
                       | otherwise -> a : go b
    {-# INLINE chunksOf #-}

    -- | /O(n)/ Breaks a 'Text' up into a list of 'Text's at
    -- newline 'Char's. The resulting strings do not contain newlines.
    lines :: Text -> [Text]
    lines Empty = []
    lines t = let (l,t') = break ((==) '\n') t
              in l : if null t' then []
                     else lines (tail t')

    -- | /O(n)/ Breaks a 'Text' up into a list of words, delimited by 'Char's
    -- representing white space.
    words :: Text -> [Text]
    words = L.filter (not . null) . split isSpace
    {-# INLINE words #-}

    -- | /O(n)/ Joins lines, after appending a terminating newline to
    -- each.
    unlines :: [Text] -> Text
    unlines = concat . L.map (`snoc` '\n')
    {-# INLINE unlines #-}

    -- | /O(n)/ Joins words using single space characters.
    unwords :: [Text] -> Text
    unwords = intercalate (singleton ' ')
    {-# INLINE unwords #-}

    -- | /O(n)/ The 'isPrefixOf' function takes two 'Text's and returns
    -- 'True' iff the first is a prefix of the second.  Subject to fusion.
    isPrefixOf :: Text -> Text -> Bool
    isPrefixOf Empty _  = True
    isPrefixOf _ Empty  = False
    isPrefixOf (Chunk x xs) (Chunk y ys)
        | lx == ly  = x == y  && isPrefixOf xs ys
        | lx <  ly  = x == yh && isPrefixOf xs (Chunk yt ys)
        | otherwise = xh == y && isPrefixOf (Chunk xt xs) ys
      where (xh,xt) = T.splitAt ly x
            (yh,yt) = T.splitAt lx y
            lx = T.length x
            ly = T.length y
    {-# INLINE [1] isPrefixOf #-}

    {-# RULES
    "LAZY TEXT isPrefixOf -> fused" [~1] forall s t.
        isPrefixOf s t = S.isPrefixOf (stream s) (stream t)
    "LAZY TEXT isPrefixOf -> unfused" [1] forall s t.
        S.isPrefixOf (stream s) (stream t) = isPrefixOf s t
      #-}

    -- | /O(n)/ The 'isSuffixOf' function takes two 'Text's and returns
    -- 'True' iff the first is a suffix of the second.
    isSuffixOf :: Text -> Text -> Bool
    isSuffixOf x y = reverse x `isPrefixOf` reverse y
    {-# INLINE isSuffixOf #-}
    -- TODO: a better implementation

    -- | /O(n+m)/ The 'isInfixOf' function takes two 'Text's and returns
    -- 'True' iff the first is contained, wholly and intact, anywhere
    -- within the second.
    --
    -- This function is strict in its first argument, and lazy in its
    -- second.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    isInfixOf :: Text -> Text -> Bool
    isInfixOf needle haystack
        | null needle        = True
        | isSingleton needle = S.elem (head needle) . S.stream $ haystack
        | otherwise          = not . L.null . indices needle $ haystack
    {-# INLINE [1] isInfixOf #-}

    {-# RULES
    "LAZY TEXT isInfixOf/singleton -> S.elem/S.stream" [~1] forall n h.
        isInfixOf (singleton n) h = S.elem n (S.stream h)
      #-}

    -------------------------------------------------------------------------------
    -- * View patterns

    -- | /O(n)/ Return the suffix of the second string if its prefix
    -- matches the entire first string.
    --
    -- Examples:
    --
    -- > stripPrefix "foo" "foobar" == Just "bar"
    -- > stripPrefix ""    "baz"    == Just "baz"
    -- > stripPrefix "foo" "quux"   == Nothing
    --
    -- This is particularly useful with the @ViewPatterns@ extension to
    -- GHC, as follows:
    --
    -- > {-# LANGUAGE ViewPatterns #-}
    -- > import Data.Text.Lazy as T
    -- >
    -- > fnordLength :: Text -> Int
    -- > fnordLength (stripPrefix "fnord" -> Just suf) = T.length suf
    -- > fnordLength _                                 = -1
    stripPrefix :: Text -> Text -> Maybe Text
    stripPrefix p t
        | null p    = Just t
        | otherwise = case commonPrefixes p t of
                        Just (_,c,r) | null c -> Just r
                        _                     -> Nothing

    -- | /O(n)/ Find the longest non-empty common prefix of two strings
    -- and return it, along with the suffixes of each string at which they
    -- no longer match.
    --
    -- If the strings do not have a common prefix or either one is empty,
    -- this function returns 'Nothing'.
    --
    -- Examples:
    --
    -- > commonPrefixes "foobar" "fooquux" == Just ("foo","bar","quux")
    -- > commonPrefixes "veeble" "fetzer"  == Nothing
    -- > commonPrefixes "" "baz"           == Nothing
    commonPrefixes :: Text -> Text -> Maybe (Text,Text,Text)
    commonPrefixes Empty _ = Nothing
    commonPrefixes _ Empty = Nothing
    commonPrefixes a0 b0   = Just (go a0 b0 [])
      where
        go t0@(Chunk x xs) t1@(Chunk y ys) ps
            = case T.commonPrefixes x y of
                Just (p,a,b)
                  | T.null a  -> go xs (chunk b ys) (p:ps)
                  | T.null b  -> go (chunk a xs) ys (p:ps)
                  | otherwise -> (fromChunks (L.reverse (p:ps)),chunk a xs, chunk b ys)
                Nothing       -> (fromChunks (L.reverse ps),t0,t1)
        go t0 t1 ps = (fromChunks (L.reverse ps),t0,t1)

    -- | /O(n)/ Return the prefix of the second string if its suffix
    -- matches the entire first string.
    --
    -- Examples:
    --
    -- > stripSuffix "bar" "foobar" == Just "foo"
    -- > stripSuffix ""    "baz"    == Just "baz"
    -- > stripSuffix "foo" "quux"   == Nothing
    --
    -- This is particularly useful with the @ViewPatterns@ extension to
    -- GHC, as follows:
    --
    -- > {-# LANGUAGE ViewPatterns #-}
    -- > import Data.Text.Lazy as T
    -- >
    -- > quuxLength :: Text -> Int
    -- > quuxLength (stripSuffix "quux" -> Just pre) = T.length pre
    -- > quuxLength _                                = -1
    stripSuffix :: Text -> Text -> Maybe Text
    stripSuffix p t = reverse `fmap` stripPrefix (reverse p) (reverse t)

    -- | /O(n)/ 'filter', applied to a predicate and a 'Text',
    -- returns a 'Text' containing those characters that satisfy the
    -- predicate.
    filter :: (Char -> Bool) -> Text -> Text
    filter p t = unstream (S.filter p (stream t))
    {-# INLINE filter #-}

    -- | /O(n)/ The 'find' function takes a predicate and a 'Text', and
    -- returns the first element in matching the predicate, or 'Nothing'
    -- if there is no such element. Subject to fusion.
    find :: (Char -> Bool) -> Text -> Maybe Char
    find p t = S.findBy p (stream t)
    {-# INLINE find #-}

    -- | /O(n)/ The 'partition' function takes a predicate and a 'Text',
    -- and returns the pair of 'Text's with elements which do and do not
    -- satisfy the predicate, respectively; i.e.
    --
    -- > partition p t == (filter p t, filter (not . p) t)
    partition :: (Char -> Bool) -> Text -> (Text, Text)
    partition p t = (filter p t, filter (not . p) t)
    {-# INLINE partition #-}

    -- | /O(n)/ 'Text' index (subscript) operator, starting from 0.
    -- Subject to fusion.
    index :: Text -> Int64 -> Char
    index t n = S.index (stream t) n
    {-# INLINE index #-}

    -- | /O(n+m)/ The 'count' function returns the number of times the
    -- query string appears in the given 'Text'. An empty query string is
    -- invalid, and will cause an error to be raised.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    count :: Text -> Text -> Int64
    count pat src
        | null pat        = emptyError "count"
        | otherwise       = go 0 (indices pat src)
      where go !n []     = n
            go !n (_:xs) = go (n+1) xs
    {-# INLINE [1] count #-}

    {-# RULES
    "LAZY TEXT count/singleton -> countChar" [~1] forall c t.
        count (singleton c) t = countChar c t
      #-}

    -- | /O(n)/ The 'countChar' function returns the number of times the
    -- query element appears in the given 'Text'.  Subject to fusion.
    countChar :: Char -> Text -> Int64
    countChar c t = S.countChar c (stream t)

    -- | /O(n)/ 'zip' takes two 'Text's and returns a list of
    -- corresponding pairs of bytes. If one input 'Text' is short,
    -- excess elements of the longer 'Text' are discarded. This is
    -- equivalent to a pair of 'unpack' operations.
    zip :: Text -> Text -> [(Char,Char)]
    zip a b = S.unstreamList $ S.zipWith (,) (stream a) (stream b)
    {-# INLINE [0] zip #-}

    -- | /O(n)/ 'zipWith' generalises 'zip' by zipping with the function
    -- given as the first argument, instead of a tupling function.
    -- Performs replacement on invalid scalar values.
    zipWith :: (Char -> Char -> Char) -> Text -> Text -> Text
    zipWith f t1 t2 = unstream (S.zipWith g (stream t1) (stream t2))
        where g a b = safe (f a b)
    {-# INLINE [0] zipWith #-}

    revChunks :: [T.Text] -> Text
    revChunks = L.foldl' (flip chunk) Empty

    emptyError :: String -> a
    emptyError fun = P.error ("Data.Text.Lazy." ++ fun ++ ": empty input")

    impossibleError :: String -> a
    impossibleError fun = P.error ("Data.Text.Lazy." ++ fun ++ ": impossible case")